ImageOutput: Writing Images

Image Output Made Simple

Here is the simplest sequence required to write the pixels of a 2D image to a file:

#include <OpenImageIO/imageio.h>
using namespace OIIO;
...

const char *filename = "foo.jpg";
const int xres = 640, yres = 480;
const int channels = 3;  // RGB
unsigned char pixels[xres*yres*channels];

std::unique_ptr<ImageOutput> out = ImageOutput::create (filename);
if (! out)
    return;
ImageSpec spec (xres, yres, channels, TypeDesc::UINT8);
out->open (filename, spec);
out->write_image (TypeDesc::UINT8, pixels);
out->close ();

This little bit of code does a surprising amount of useful work:

  • Search for an ImageIO plugin that is capable of writing the file foo.jpg), deducing the format from the file extension. When it finds such a plugin, it creates a subclass instance of ImageOutput that writes the right kind of file format.

    std::unique_ptr<ImageOutput> out = ImageOutput::create (filename);
    
  • Open the file, write the correct headers, and in all other important ways prepare a file with the given dimensions (640 x 480), number of color channels (3), and data format (unsigned 8-bit integer).

    ImageSpec spec (xres, yres, channels, TypeDesc::UINT8);
    out->open (filename, spec);
    
  • Write the entire image, hiding all details of the encoding of image data in the file, whether the file is scanline- or tile-based, or what is the native format of data in the file (in this case, our in-memory data is unsigned 8-bit and we’ve requested the same format for disk storage, but if they had been different, write_image() would do all the conversions for us).

    out->write_image (TypeDesc::UINT8, &pixels);
    
  • Close the file.

    out->close ();
    

What happens when the file format doesn’t support the spec?

The open() call will fail (returning an empty pointer and set an appropriate error message) if the output format cannot accommodate what is requested by the ImageSpec. This includes:

  • Dimensions (width, height, or number of channels) exceeding the limits supported by the file format. 1

  • Volumetric (depth > 1) if the format does not support volumetric data.

  • Tile size >1 if the format does not support tiles.

  • Multiple subimages or MIP levels if not supported by the format.

1

One exception to the rule about number of channels is that a file format that supports only RGB, but not alpha, is permitted to silently drop the alpha channel without considering that to be an error.

However, several other mismatches between requested ImageSpec and file format capabilities will be silently ignored, allowing open() to succeed:

  • If the pixel data format is not supported (for example, a request for half pixels when writing a JPEG/JFIF file), the format writer may substitute another data format (generally, whichever commonly-used data format supported by the file type will result in the least reduction of precision or range).

  • If the ImageSpec requests different per-channel data formats, but the format supports only a single format for all channels, it may just choose the most precise format requested and use it for all channels.

  • If the file format does not support arbitrarily-named channels, the channel names may be lost when saving the file.

  • Any other metadata in the ImageSpec may be summarily dropped if not supported by the file format.

Advanced Image Output

Let’s walk through many of the most common things you might want to do, but that are more complex than the simple example above.

Writing individual scanlines, tiles, and rectangles

The simple example of Section Image Output Made Simple wrote an entire image with one call. But sometimes you are generating output a little at a time and do not wish to retain the entire image in memory until it is time to write the file. OpenImageIO allows you to write images one scanline at a time, one tile at a time, or by individual rectangles.

Writing individual scanlines

Individual scanlines may be written using the writescanline() API call:

...
unsigned char scanline[xres*channels];
out->open (filename, spec);
int z = 0;   // Always zero for 2D images
for (int y = 0;  y < yres;  ++y) {
    ... generate data in scanline[0..xres*channels-1] ...
    out->write_scanline (y, z, TypeDesc::UINT8, scanline);
}
out->close ();
...

The first two arguments to writescanline() specify which scanline is being written by its vertical (y) scanline number (beginning with 0) and, for volume images, its slice (z) number (the slice number should be 0 for 2D non-volume images). This is followed by a TypeDesc describing the data you are supplying, and a pointer to the pixel data itself. Additional optional arguments describe the data stride, which can be ignored for contiguous data (use of strides is explained in Section Data Strides).

All ImageOutput implementations will accept scanlines in strict order (starting with scanline 0, then 1, up to yres-1, without skipping any). See Section Random access and repeated transmission of pixels for details on out-of-order or repeated scanlines.

The full description of the writescanline() function may be found in Section ImageOutput Class Reference.

Writing individual tiles

Not all image formats (and therefore not all ImageOutput implementations) support tiled images. If the format does not support tiles, then writetile() will fail. An application using OpenImageIO should gracefully handle the case that tiled output is not available for the chosen format.

Once you create() an ImageOutput, you can ask if it is capable of writing a tiled image by using the supports("tiles") query:

...
std::unique_ptr<ImageOutput> out = ImageOutput::create (filename);
if (! out->supports ("tiles")) {
    // Tiles are not supported}

Assuming that the ImageOutput supports tiled images, you need to specifically request a tiled image when you open() the file. This is done by setting the tile size in the ImageSpec passed to open(). If the tile dimensions are not set, they will default to zero, which indicates that scanline output should be used rather than tiled output.

int tilesize = 64;
ImageSpec spec (xres, yres, channels, TypeDesc::UINT8);
spec.tile_width = tilesize;
spec.tile_height = tilesize;
out->open (filename, spec);
...

In this example, we have used square tiles (the same number of pixels horizontally and vertically), but this is not a requirement of OpenImageIO. However, it is possible that some image formats may only support square tiles, or only certain tile sizes (such as restricting tile sizes to powers of two). Such restrictions should be documented by each individual plugin.

unsigned char tile[tilesize*tilesize*channels];
int z = 0;   // Always zero for 2D images
for (int y = 0;  y < yres;  y += tilesize) {
    for (int x = 0;  x < xres;  x += tilesize) {
        ... generate data in tile[] ..
        out->write_tile (x, y, z, TypeDesc::UINT8, tile);
    }
}
out->close ();
...

The first three arguments to writetile() specify which tile is being written by the pixel coordinates of any pixel contained in the tile: x (column), y (scanline), and z (slice, which should always be 0 for 2D non-volume images). This is followed by a TypeDesc describing the data you are supplying, and a pointer to the tile’s pixel data itself, which should be ordered by increasing slice, increasing scanline within each slice, and increasing column within each scanline. Additional optional arguments describe the data stride, which can be ignored for contiguous data (use of strides is explained in Section Data Strides).

All ImageOutput implementations that support tiles will accept tiles in strict order of increasing y rows, and within each row, increasing x column, without missing any tiles. See

The full description of the writetile() function may be found in Section ImageOutput Class Reference.

Writing arbitrary rectangles

Some ImageOutput implementations — such as those implementing an interactive image display, but probably not any that are outputting directly to a file — may allow you to send arbitrary rectangular pixel regions. Once you create() an ImageOutput, you can ask if it is capable of accepting arbitrary rectangles by using the supports("rectangles") query:

...
std::unique_ptr<ImageOutput> out = ImageOutput::create (filename);
if (! out->supports ("rectangles")) {
    // Rectangles are not supported
}

If rectangular regions are supported, they may be sent using the write_rectangle() API call:

unsigned int rect[...];
... generate data in rect[] ..
out->write_rectangle (xbegin, xend, ybegin, yend, zbegin, zend,
                      TypeDesc::UINT8, rect);

The first six arguments to write_rectangle() specify the region of pixels that is being transmitted by supplying the minimum and one-past-maximum pixel indices in x (column), y (scanline), and z (slice, always 0 for 2D non-volume images).

Note

OpenImageIO nearly always follows the C++ STL convention of specifying ranges as [begin,end), that is, begin, begin+1, ..., end-1.

The total number of pixels being transmitted is therefore:

(xend-xbegin) * (yend-ybegin) * (zend-zbegin)

This is followed by a TypeDesc describing the data you are supplying, and a pointer to the rectangle’s pixel data itself, which should be ordered by increasing slice, increasing scanline within each slice, and increasing column within each scanline. Additional optional arguments describe the data stride, which can be ignored for contiguous data (use of strides is explained in Section Data Strides).

Converting pixel data types

The code examples of the previous sections all assumed that your internal pixel data is stored as unsigned 8-bit integers (i.e., 0-255 range). But OpenImageIO is significantly more flexible.

You may request that the output image pixels be stored in any of several data types. This is done by setting the format field of the ImageSpec prior to calling open. You can do this upon construction of the ImageSpec, as in the following example that requests a spec that stores pixel values as 16-bit unsigned integers:

ImageSpec spec (xres, yres, channels, TypeDesc::UINT16);

Or, for an ImageSpec that has already been constructed, you may reset its format using the set_format() method.

ImageSpec spec (...);
spec.set_format (TypeDesc::UINT16);

Note that resetting the pixel data type must be done before passing the spec to open(), or it will have no effect on the file.

Individual file formats, and therefore ImageOutput implementations, may only support a subset of the pixel data types understood by the OpenImageIO library. Each ImageOutput plugin implementation should document which data formats it supports. An individual ImageOutput implementation is expected to always succeed, but if the file format does not support the requested pixel data type, it is expected to choose a data type that is supported, usually the data type that best preserves the precision and range of the originally-requested data type.

The conversion from floating-point formats to integer formats (or from higher to lower integer, which is done by first converting to float) is always done by rescaling the value so that 0.0 maps to integer 0 and 1.0 to the maximum value representable by the integer type, then rounded to an integer value for final output. Here is the code that implements this transformation (T is the final output integer type):

float value = quant_max * input;
T output = (T) clamp ((int)(value + 0.5), quant_min, quant_max);

Quantization limits for each integer type is as follows:

Data Format

min

max

UINT8

0

255

INT8

-128

127

UINT16

0

65535

INT16

-32768

32767

UINT

0

4294967295

INT

-2147483648

2147483647

Note that the default is to use the entire positive range of each integer type to represent the floating-point (0.0 - 1.0) range. Floating-point types do not attempt to remap values, and do not clamp (except to their full floating-point range).

It is not required that the pixel data passed to writeimage(), writescanline(), writetile(), or write_rectangle() actually be in the same data type as that requested as the native pixel data type of the file. You can fully mix and match data you pass to the various “write” routines and OpenImageIO will automatically convert from the internal format to the native file format. For example, the following code will open a TIFF file that stores pixel data as 16-bit unsigned integers (values ranging from 0 to 65535), compute internal pixel values as floating-point values, with writeimage() performing the conversion automatically:

std::unique_ptr<ImageOutput> out = ImageOutput::create ("myfile.tif");
ImageSpec spec (xres, yres, channels, TypeDesc::UINT16);
out->open (filename, spec);
...
float pixels [xres*yres*channels];
...
out->write_image (TypeDesc::FLOAT, pixels);

Note that writescanline(), writetile(), and write_rectangle() have a parameter that works in a corresponding manner.

Data Strides

In the preceeding examples, we have assumed that the block of data being passed to the “write” functions are contiguous, that is:

  • each pixel in memory consists of a number of data values equal to the declared number of channels that are being written to the file;

  • successive column pixels within a row directly follow each other in memory, with the first channel of pixel x immediately following last channel of pixel x-1 of the same row;

  • for whole images, tiles or rectangles, the data for each row immediately follows the previous one in memory (the first pixel of row y immediately follows the last column of row y-1);

  • for 3D volumetric images, the first pixel of slice z immediately follows the last pixel of of slice z-1.

Please note that this implies that data passed to writetile() be contiguous in the shape of a single tile (not just an offset into a whole image worth of pixels), and that data passed to write_rectangle() be contiguous in the dimensions of the rectangle.

The writescanline() function takes an optional xstride argument, and the writeimage(), writetile(), and write_rectangle() functions take optional xstride, ystride, and zstride values that describe the distance, in bytes, between successive pixel columns, rows, and slices, respectively, of the data you are passing. For any of these values that are not supplied, or are given as the special constant AutoStride, contiguity will be assumed.

By passing different stride values, you can achieve some surprisingly flexible functionality. A few representative examples follow:

  • Flip an image vertically upon writing, by using negative y stride:

    unsigned char pixels[xres*yres*channels];
    int scanlinesize = xres * channels * sizeof(pixels[0]);
    ...
    out->write_image (TypeDesc::UINT8,
                      (char *)pixels+(yres-1)*scanlinesize, // offset to last
                      AutoStride,                  // default x stride
                      -scanlinesize,               // special y stride
                      AutoStride);                 // default z stride
    
  • Write a tile that is embedded within a whole image of pixel data, rather than having a one-tile-only memory layout:

    unsigned char pixels[xres*yres*channels];
    int pixelsize = channels * sizeof(pixels[0]);
    int scanlinesize = xres * pixelsize;
    ...
    out->write_tile (x, y, 0, TypeDesc::UINT8,
                     (char *)pixels + y*scanlinesize + x*pixelsize,
                     pixelsize,
                     scanlinesize);
    
  • Write only a subset of channels to disk. In this example, our internal data layout consists of 4 channels, but we write just channel 3 to disk as a one-channel image:

    // In-memory representation is 4 channel
    const int xres = 640, yres = 480;
    const int channels = 4;  // RGBA
    const int channelsize = sizeof(unsigned char);
    unsigned char pixels[xres*yres*channels];
    
    // File representation is 1 channel
    std::unique_ptr<ImageOutput> out = ImageOutput::create (filename);
    ImageSpec spec (xres, yres, 1, TypeDesc::UINT8);
    out->open (filename, spec);
    
    // Use strides to write out a one-channel "slice" of the image
    out->write_image (TypeDesc::UINT8,
                      (char *)pixels+3*channelsize, // offset to chan 3
                      channels*channelsize,         // 4 channel x stride
                      AutoStride,                   // default y stride
                      AutoStride);                  // default z stride
    ...
    

Please consult Section ImageOutput Class Reference for detailed descriptions of the stride parameters to each “write” function.

Writing a crop window or overscan region

The ImageSpec fields width, height, and depth describe the dimensions of the actual pixel data.

At times, it may be useful to also describe an abstract full or display image window, whose position and size may not correspond exactly to the data pixels. For example, a pixel data window that is a subset of the full display window might indicate a crop window; a pixel data window that is a superset of the full display window might indicate overscan regions (pixels defined outside the eventual viewport).

The ImageSpec fields full_width, full_height, and full_depth describe the dimensions of the full display window, and full_x, full_y, full_z describe its origin (upper left corner). The fields x, y, z describe the origin (upper left corner) of the pixel data.

These fields collectively describe an abstract full display image ranging from [full_xfull_x+full_width-1] horizontally, [full_yfull_y+full_height-1] vertically, and [full_zfull_z+full_depth-1] in depth (if it is a 3D volume), and actual pixel data over the pixel coordinate range [xx+width-1] horizontally, [yy+height-1] vertically, and [zz+depth-1] in depth (if it is a volume).

Not all image file formats have a way to describe display windows. An ImageOutput implementation that cannot express display windows will always write out the width * height pixel data, may upon writing lose information about offsets or crop windows.

Here is a code example that opens an image file that will contain a 32x32 pixel crop window within an abstract 640 x 480 full size image. Notice that the pixel indices (column, scanline, slice) passed to the “write” functions are the coordinates relative to the full image, not relative to the crop widow, but the data pointer passed to the “write” functions should point to the beginning of the actual pixel data being passed (not the the hypothetical start of the full data, if it was all present).

int fullwidth = 640, fulllength = 480; // Full display image size
int cropwidth = 16, croplength = 16;  // Crop window size
int xorigin = 32, yorigin = 128;      // Crop window position
unsigned char pixels [cropwidth * croplength * channels]; // Crop size!
...
std::unique_ptr<ImageOutput> out = ImageOutput::create (filename);
ImageSpec spec (cropwidth, croplength, channels, TypeDesc::UINT8);
spec.full_x = 0;
spec.full_y = 0;
spec.full_width = fullwidth;
spec.full_length = fulllength;
spec.x = xorigin;
spec.y = yorigin;
out->open (filename, spec);
...
int z = 0;   // Always zero for 2D images
for (int y = yorigin;  y < yorigin+croplength;  ++y) {
    out->write_scanline (y, z, TypeDesc::UINT8,
                         (y-yorigin)*cropwidth*channels);
}
out->close ();

Writing metadata

The ImageSpec passed to open() can specify all the common required properties that describe an image: data format, dimensions, number of channels, tiling. However, there may be a variety of additional metadata that should be carried along with the image or saved in the file.

Note

Metadata refers to data about data, in this case, data about the image that goes beyond the pixel values and description thereof.

The remainder of this section explains how to store additional metadata in the ImageSpec. It is up to the ImageOutput to store these in the file, if indeed the file format is able to accept the data. Individual ImageOutput implementations should document which metadata they respect.

Channel names

In addition to specifying the number of color channels, it is also possible to name those channels. Only a few ImageOutput implementations have a way of saving this in the file, but some do, so you may as well do it if you have information about what the channels represent.

By convention, channel names for red, green, blue, and alpha (or a main image) should be named "R", "G", "B", and "A", respectively. Beyond this guideline, however, you can use any names you want.

The ImageSpec has a vector of strings called channelnames. Upon construction, it starts out with reasonable default values. If you use it at all, you should make sure that it contains the same number of strings as the number of color channels in your image. Here is an example:

int channels = 4;
ImageSpec spec (width, length, channels, TypeDesc::UINT8);
spec.channelnames.clear ();
spec.channelnames.push_back ("R");
spec.channelnames.push_back ("G");
spec.channelnames.push_back ("B");
spec.channelnames.push_back ("A");

Here is another example in which custom channel names are used to label the channels in an 8-channel image containing beauty pass RGB, per-channel opacity, and texture s,t coordinates for each pixel.

int channels = 8;
ImageSpec spec (width, length, channels, TypeDesc::UINT8);
spec.channelnames.clear ();
spec.channelnames.push_back ("R");
spec.channelnames.push_back ("G");
spec.channelnames.push_back ("B");
spec.channelnames.push_back ("opacityR");
spec.channelnames.push_back ("opacityG");
spec.channelnames.push_back ("opacityB");
spec.channelnames.push_back ("texture_s");
spec.channelnames.push_back ("texture_t");

The main advantage to naming color channels is that if you are saving to a file format that supports channel names, then any application that uses OpenImageIO to read the image back has the option to retain those names and use them for helpful purposes. For example, the iv image viewer will display the channel names when viewing individual channels or displaying numeric pixel values in “pixel view” mode.

Specially-designated channels

The ImageSpec contains two fields, alpha_channel and z_channel, which can be used to designate which channel indices are used for alpha and z depth, if any. Upon construction, these are both set to -1, indicating that it is not known which channels are alpha or depth. Here is an example of setting up a 5-channel output that represents RGBAZ:

int channels = 5;
ImageSpec spec (width, length, channels, format);
spec.channelnames.clear();
spec.channelnames.push_back ("R");
spec.channelnames.push_back ("G");
spec.channelnames.push_back ("B");
spec.channelnames.push_back ("A");
spec.channelnames.push_back ("Z");
spec.alpha_channel = 3;
spec.z_channel = 4;

There are advantages to designating the alpha and depth channels in this manner: Some file formats may require that these channels be stored in a particular order, with a particular precision, or the ImageOutput may in some other way need to know about these special channels.

Arbitrary metadata

For all other metadata that you wish to save in the file, you can attach the data to the ImageSpec using the attribute() methods. These come in polymorphic varieties that allow you to attach an attribute name and a value consisting of a single int, unsigned int, float, char*, or std::string, as shown in the following examples:

ImageSpec spec (...);
...

unsigned int u = 1;
spec.attribute ("Orientation", u);

float x = 72.0;
spec.attribute ("dotsize", f);

std::string s = "Fabulous image writer 1.0";
spec.attribute ("Software", s);

These are convenience routines for metadata that consist of a single value of one of these common types. For other data types, or more complex arrangements, you can use the more general form of attribute(), which takes arguments giving the name, type (as a TypeDesc), number of values (1 for a single value, >1 for an array), and then a pointer to the data values. For example,

ImageSpec spec (...);

// Attach a 4x4 matrix to describe the camera coordinates
float mymatrix[16] = { ... };
spec.attribute ("worldtocamera", TypeMatrix, &mymatrix);

// Attach an array of two floats giving the CIE neutral color
float neutral[2] = { ... };
spec.attribute ("adoptedNeutral", TypeDesc(TypeDesc::FLOAT, 2), &neutral);

In general, most image file formats (and therefore most ImageOutput implementations) are aware of only a small number of name/value pairs that they predefine and will recognize. Some file formats (OpenEXR, notably) do accept arbitrary user data and save it in the image file. If an ImageOutput does not recognize your metadata and does not support arbitrary metadata, that metadatum will be silently ignored and will not be saved with the file.

Each individual ImageOutput implementation should document the names, types, and meanings of all metadata attributes that they understand.

Color space hints

We certainly hope that you are using only modern file formats that support high precision and extended range pixels (such as OpenEXR) and keeping all your images in a linear color space. But you may have to work with file formats that dictate the use of nonlinear color values. This is prevalent in formats that store pixels only as 8-bit values, since 256 values are not enough to linearly represent colors without banding artifacts in the dim values.

Since this can (and probably will) happen, we have a convention for explaining what color space your image pixels are in. Each individual ImageOutput should document how it uses this (or not).

The ImageSpec::extra_attribs field should store metadata that reveals the color space of the pixels you are sending the ImageOutput (see Section Color information metadata for explanations of particular values).

The color space hints only describe color channels. You should always pass alpha, depth, or other non-color channels with linear values.

Here is a simple example of setting up the ImageSpec when you know that the pixel values you are writing are linear:

ImageSpec spec (width, length, channels, format);
spec.attribute ("oiio:ColorSpace", "Linear");
...

If a particular ImageOutput implementation is required (by the rules of the file format it writes) to have pixels in a particular color space, then it should try to convert the color values of your image to the right color space if it is not already in that space. For example, JPEG images must be in sRGB space, so if you declare your pixels to be "Linear", the JPEG ImageOutput will convert to sRGB.

If you leave the "oiio:ColorSpace" unset, the values will not be transformed, since the plugin can’t be sure that it’s not in the correct space to begin with.

Random access and repeated transmission of pixels

All ImageOutput implementations that support scanlines and tiles should write pixels in strict order of increasing z slice, increasing y scanlines/rows within each slice, and increasing x column within each row. It is generally not safe to skip scanlines or tiles, or transmit them out of order, unless the plugin specifically advertises that it supports random access or rewrites, which may be queried using:

std::unique_ptr<ImageOutput> out = ImageOutput::create (filename);
if (out->supports ("random_access"))
    ...

Similarly, you should assume the plugin will not correctly handle repeated transmissions of a scanline or tile that has already been sent, unless it advertises that it supports rewrites, which may be queried using:

if (out->supports ("rewrite"))
    ...

Multi-image files

Some image file formats support storing multiple images within a single file. Given a created ImageOutput, you can query whether multiple images may be stored in the file:

std::unique_ptr<ImageOutput> out = ImageOutput::create (filename);
if (out->supports ("multiimage"))
    ...

Some image formats allow you to do the initial open() call without declaring the specifics of the subimages, and simply append subimages as you go. You can detect this by checking

if (out->supports ("appendsubimage"))
    ...

In this case, all you have to do is, after writing all the pixels of one image but before calling close(), call open() again for the next subimage and pass AppendSubimage as the value for the mode argument (see Section ImageOutput Class Reference for the full technical description of the arguments to open()). The close() routine is called just once, after all subimages are completed. Here is an example:

const char *filename = "foo.tif";
int nsubimages;     // assume this is set
ImageSpec specs[];  // assume these are set for each subimage
unsigned char *pixels[]; // assume a buffer for each subimage

// Create the ImageOutput
std::unique_ptr<ImageOutput> out = ImageOutput::create (filename);

// Be sure we can support subimages
if (subimages > 1 &&  (! out->supports("multiimage") ||
                       ! out->supports("appendsubimage"))) {
    std::cerr << "Does not support appending of subimages\n";
    return;
}

// Use Create mode for the first level.
ImageOutput::OpenMode appendmode = ImageOutput::Create;

// Write the individual subimages
for (int s = 0;  s < nsubimages;  ++s) {
    out->open (filename, specs[s], appendmode);
    out->write_image (TypeDesc::UINT8, pixels[s]);
    // Use AppendSubimage mode for subsequent levels
    appendmode = ImageOutput::AppendSubimage;
}
out->close ();

On the other hand, if out->supports("appendsubimage") returns false, then you must use a different open() variety that allows you to declare the number of subimages and their specifications up front.

Below is an example of how to write a multi-subimage file, assuming that you know all the image specifications ahead of time. This should be safe for any file format that supports multiple subimages, regardless of whether it supports appending, and thus is the preferred method for writing subimages, assuming that you are able to know the number and specification of the subimages at the time you first open the file.

const char *filename = "foo.tif";
int nsubimages;     // assume this is set
ImageSpec specs[];  // assume these are set for each subimage
unsigned char *pixels[]; // assume a buffer for each subimage

// Create the ImageOutput
std::unique_ptr<ImageOutput> out = ImageOutput::create (filename);

// Be sure we can support subimages
if (subimages > 1 && ! out->supports ("multiimage")) {
    std::cerr << "Cannot write multiple subimages\n";
    return;
}

// Open and declare all subimages
out->open (filename, nsubimages, specs);

// Write the individual subimages
for (int s = 0;  s < nsubimages;  ++s) {
    if (s > 0)  // Not needed for the first, which is already open
        out->open (filename, specs[s], ImageInput::AppendSubimage);
    out->write_image (TypeDesc::UINT8, pixels[s]);
}
out->close ();

In both of these examples, we have used writeimage(), but of course writescanline(), writetile(), and write_rectangle() work as you would expect, on the current subimage.

MIP-maps

Some image file formats support multiple copies of an image at successively lower resolutions (MIP-map levels, or an “image pyramid”). Given a created ImageOutput, you can query whether MIP-maps may be stored in the file:

std::unique_ptr<ImageOutput> out = ImageOutput::create (filename);
if (out->supports ("mipmap"))
    ...

If you are working with an ImageOutput that supports MIP-map levels, it is easy to write these levels. After writing all the pixels of one MIP-map level, call open() again for the next MIP level and pass ImageInput::AppendMIPLevel as the value for the mode argument, and then write the pixels of the subsequent MIP level. (See Section ImageOutput Class Reference for the full technical description of the arguments to open().) The close() routine is called just once, after all subimages and MIP levels are completed.

Below is pseudocode for writing a MIP-map (a multi-resolution image used for texture mapping):

const char *filename = "foo.tif";
const int xres = 512, yres = 512;
const int channels = 3;  // RGB
unsigned char *pixels = new unsigned char [xres*yres*channels];

// Create the ImageOutput
std::unique_ptr<ImageOutput> out = ImageOutput::create (filename);

// Be sure we can support either mipmaps or subimages
if (! out->supports ("mipmap") && ! out->supports ("multiimage")) {
    std::cerr << "Cannot write a MIP-map\n";
    return;
}
// Set up spec for the highest resolution
ImageSpec spec (xres, yres, channels, TypeDesc::UINT8);

// Use Create mode for the first level.
ImageOutput::OpenMode appendmode = ImageOutput::Create;

// Write images, halving every time, until we're down to
// 1 pixel in either dimension
while (spec.width >= 1 && spec.height >= 1) {
    out->open (filename, spec, appendmode);
    out->write_image (TypeDesc::UINT8, pixels);
    // Assume halve() resamples the image to half resolution
    halve (pixels, spec.width, spec.height);
    // Don't forget to change spec for the next iteration
    spec.width /= 2;
    spec.height /= 2;

    // For subsequent levels, change the mode argument to
    // open().  If the format doesn't support MIPmaps directly,
    // try to emulate it with subimages.
    if (out->supports("mipmap"))
        appendmode = ImageOutput::AppendMIPLevel;
    else
        appendmode = ImageOutput::AppendSubimage;
}
out->close ();

In this example, we have used writeimage(), but of course writescanline(), writetile(), and write_rectangle() work as you would expect, on the current MIP level.

Per-channel formats

Some image formats allow separate per-channel data formats (for example, half data for colors and float data for depth). When this is desired, the following steps are necessary:

  1. Verify that the writer supports per-channel formats by checking supports ("channelformats").

  2. The ImageSpec passed to open() should have its channelformats vector filled with the types for each channel.

  3. The call to write_scanline(), read_scanlines(), write_tile(), write_tiles(), or write_image() should pass a data pointer to the raw data, already in the native per-channel format of the file and contiguously packed, and specify that the data is of type TypeUnknown.

For example, the following code fragment will write a 5-channel image to an OpenEXR file, consisting of R/G/B/A channels in half and a Z channel in float:

// Mixed data type for the pixel
struct Pixel { half r,g,b,a; float z; };
Pixel pixels[xres*yres];

std::unique_ptr<ImageOutput> out = ImageOutput::create ("foo.exr");

// Double check that this format accepts per-channel formats
if (! out->supports("channelformats")) {
    return;
}

// Prepare an ImageSpec with per-channel formats
ImageSpec spec (xres, yres, 5, TypeDesc::FLOAT);
spec.channelformats.push_back (TypeDesc::HALF);
spec.channelformats.push_back (TypeDesc::HALF);
spec.channelformats.push_back (TypeDesc::HALF);
spec.channelformats.push_back (TypeDesc::HALF);
spec.channelformats.push_back (TypeDesc::FLOAT);
spec.channelnames.clear ();
spec.channelnames.push_back ("R");
spec.channelnames.push_back ("G");
spec.channelnames.push_back ("B");
spec.channelnames.push_back ("A");
spec.channelnames.push_back ("Z");

out->open (filename, spec);
out->write_image (TypeDesc::UNKNOWN, /* use channel formats */
                  pixels,            /* data buffer */
                  sizeof(Pixel));    /* pixel stride */

Writing “deep” data

Some image file formats (OpenEXR only, at this time) support the concept of “deep” pixels – those containing multiple samples per pixel (and a potentially differing number of them in each pixel). You can tell if a format supports deep images by checking supports("deepdata"), and you can specify a deep data in an ImageSpec by setting its deep field will be true.

Deep files cannot be written with the usual write_scanline(), write_scanlines(), write_tile(), write_tiles(), write_image() functions, due to the nature of their variable number of samples per pixel. Instead, ImageOutput has three special member functions used only for writing deep data:

bool write_deep_scanlines (int ybegin, int yend, int z,
                           const DeepData &deepdata);

bool write_deep_tiles (int xbegin, int xend, int ybegin, int yend,
                       int zbegin, int zend, const DeepData &deepdata);

bool write_deep_image (const DeepData &deepdata);

It is only possible to write “native” data types to deep files; that is, there is no automatic translation into arbitrary data types as there is for ordinary images. All three of these functions are passed deep data in a special DeepData structure, described in detail in Section “Deep” pixel data: DeepData.

Here is an example of using these methods to write a deep image:

// Prepare the spec for 'half' RGBA, 'float' z
int nchannels = 5;
ImageSpec spec (xres, yres, nchannels);
TypeDesc channeltypes[] = { TypeDesc::HALF, TypeDesc::HALF,
      TypeDesc::HALF, TypeDesc::HALF, TypeDesc::FLOAT };
spec.z_channel = 4;
spec.channelnames[spec.z_channel] = "Z";
spec.channeltypes.assign (channeltypes+0, channeltypes+nchannels);
spec.deep = true;

// Prepare the data (sorry, complicated, but need to show the gist)
DeepData deepdata;
deepdata.init (spec);
for (int y = 0;  y < yres;  ++y)
    for (int x = 0;  x < xres;  ++x)
        deepdata.set_samples(y*xres+x, ...samples for that pixel...);
deepdata.alloc ();  // allocate pointers and data
int pixel = 0;
for (int y = 0;  y < yres;  ++y)
    for (int x = 0;  x < xres;  ++x, ++pixel)
        for (int chan = 0;  chan < nchannels;  ++chan)
            for (int samp = 0; samp < deepdata.samples(pixel); ++samp)
                deepdata.set_deep_value (pixel, chan, samp, ...value...);


// Create the output
std::unique_ptr<ImageOutput> out = ImageOutput::create (filename);
if (! out)
    return;
// Make sure the format can handle deep data and per-channel formats
if (! out->supports("deepdata") || ! out->supports("channelformats"))
    return;

// Do the I/O (this is the easy part!)
out->open (filename, spec);
out->write_deep_image (deepdata);
out->close ();

Copying an entire image

Suppose you want to copy an image, perhaps with alterations to the metadata but not to the pixels. You could open an ImageInput and perform a read_image(), and open another ImageOutput and call write_image() to output the pixels from the input image. However, for compressed images, this may be inefficient due to the unnecessary decompression and subsequent re-compression. In addition, if the compression is lossy, the output image may not contain pixel values identical to the original input.

A special copy_image() method of ImageOutput is available that attempts to copy an image from an open ImageInput (of the same format) to the output as efficiently as possible with without altering pixel values, if at all possible.

Not all format plugins will provide an implementation of copy_image() (in fact, most will not), but the default implemenatation simply copies pixels one scanline or tile at a time (with decompression/recompression) so it’s still safe to call. Furthermore, even a provided copy_image() is expected to fall back on the default implementation if the input and output are not able to do an efficient copy. Nevertheless, this method is recommended for copying images so that maximal advantage will be taken in cases where savings can be had.

The following is an example use of copy_image() to transfer pixels without alteration while modifying the image description metadata:

// Open the input file
const char *input = "input.jpg";
std::unique_ptr<ImageInput> in = ImageInput::open (input);

// Make an output spec, identical to the input except for metadata
ImageSpec out_spec = in->spec();
out_spec.attribute ("ImageDescription", "My Title");

// Create the output file and copy the image
const char *output = "output.jpg";
std::unique_ptr<ImageOutput> out = ImageOutput::create (output);
out->open (output, out_spec);
out->copy_image (in);

// Clean up
out->close ();
in->close ();

Custom I/O proxies (and writing the file to a memory buffer)

Some file format writers allow you to supply a custom I/O proxy object that can allow bypassing the usual file I/O with custom behavior, including the ability to fill an in-memory buffer with a byte-for-byte representation of the correctly formatted file that would have been written to disk.

Only some output format writers support this feature. To find out if a particular file format supports this feature, you can create an ImageOutput of the right type, and check if it supports the feature name "ioproxy":

auto out = ImageOutput::create (filename);
if (! out  ||  ! out->supports ("ioproxy")) {
    return;
}

ImageOutput writers that support "ioproxy" will respond to a special attribute, "oiio:ioproxy", which passes a pointer to a Filesystem::IOProxy* (see OpenImageIO’s filesystem.h for this type and its subclasses). IOProxy is an abstract type, and concrete subclasses include IOFile (which wraps I/O to an open FILE*) and IOVecOutput (which sends output to a std::vector<unsigned char>).

Here is an example of using a proxy that writes the “file” to a std::vector<unsigned char>:

// ImageSpec describing the image we want to write.
ImageSpec spec (xres, yres, channels, TypeDesc::UINT8);

std::vector<unsigned char> file_buffer;  // bytes will go here
Filesystem::IOVecOutput vecout (file_buffer);  // I/O proxy object

auto out = ImageOutput::create ("out.exr", &vecout);
out->open ("out.exr", spec);
out->write_image (...);
out->close ();

// At this point, file_buffer will contain the "file"

Custom search paths for plugins

When you call ImageOutput::create(), the OpenImageIO library will try to find a plugin that is able to write the format implied by your filename. These plugins are alternately known as DLL’s on Windows (with the .dll extension), DSO’s on Linux (with the .so extension), and dynamic libraries on Mac OS X (with the .dylib extension).

OpenImageIO will look for matching plugins according to search paths, which are strings giving a list of directories to search, with each directory separated by a colon :. Within a search path, any substrings of the form {$FOO} will be replaced by the value of environment variable FOO. For example, the searchpath "${HOME}/plugins:/shared/plugins" will first check the directory /home/tom/plugins (assuming the user’s home directory is /home/tom), and if not found there, will then check the directory /shared/plugins.

The first search path it will check is that stored in the environment variable OIIO_LIBRARY_PATH. It will check each directory in turn, in the order that they are listed in the variable. If no adequate plugin is found in any of the directories listed in this environment variable, then it will check the custom searchpath passed as the optional second argument to ImageOutput::create(), searching in the order that the directories are listed. Here is an example:

char *mysearch = "/usr/myapp/lib:${HOME}/plugins";
std::unique_ptr<ImageOutput> out = ImageOutput::create (filename, mysearch);
...

Error checking

Nearly every ImageOutput API function returns a bool indicating whether the operation succeeded (true) or failed (false). In the case of a failure, the ImageOutput will have saved an error message describing in more detail what went wrong, and the latest error message is accessible using the ImageOutput method geterror(), which returns the message as a std::string.

The exception to this rule is ImageOutput::create(), which returns NULL if it could not create an appropriate ImageOutput. And in this case, since no ImageOutput exists for which you can call its geterror() function, there exists a global geterror() function (in the OpenImageIO namespace) that retrieves the latest error message resulting from a call to create().

Here is another version of the simple image writing code from Section Image Output Made Simple, but this time it is fully elaborated with error checking and reporting:

#include <OpenImageIO/imageio.h>
using namespace OIIO;
...

const char *filename = "foo.jpg";
const int xres = 640, yres = 480;
const int channels = 3;  // RGB
unsigned char pixels[xres*yres*channels];

std::unique_ptr<ImageOutput> out = ImageOutput::create (filename);
if (! out) {
    std::cerr << "Could not create an ImageOutput for "
              << filename << ", error = "
              << OpenImageIO::geterror() << "\n";
    return;
}
ImageSpec spec (xres, yres, channels, TypeDesc::UINT8);

if (! out->open (filename, spec)) {
    std::cerr << "Could not open " << filename
              << ", error = " << out->geterror() << "\n";
    return;
}

if (! out->write_image (TypeDesc::UINT8, pixels)) {
    std::cerr << "Could not write pixels to " << filename
              << ", error = " << out->geterror() << "\n";
    return;
}

if (! out->close ()) {
    std::cerr << "Error closing " << filename
              << ", error = " << out->geterror() << "\n";
    return;
}

ImageOutput Class Reference

class OIIO::ImageOutput

ImageOutput abstracts the writing of an image file in a file format-agnostic manner.

Users don’t directly declare these. Instead, you call the create() static method, which will return a unique_ptr holding a subclass of ImageOutput that implements writing the particular format.

Creating an ImageOutput

static unique_ptr create(string_view filename, Filesystem::IOProxy *ioproxy = nullptr, string_view plugin_searchpath = "")

Create an ImageOutput that can be used to write an image file. The type of image file (and hence, the particular subclass of ImageOutput returned, and the plugin that contains its methods) is inferred from the name, if it appears to be a full filename, or it may also name the format.

Return

A unique_ptr that will close and free the ImageOutput when it exits scope or is reset. The pointer will be empty if the required writer was not able to be created.

Parameters
  • filename: The name of the file format (e.g., “openexr”), a file extension (e.g., “exr”), or a filename from which the the file format can be inferred from its extension (e.g., “hawaii.exr”).

  • plugin_searchpath: An optional colon-separated list of directories to search for OpenImageIO plugin DSO/DLL’s.

  • ioproxy: Optional pointer to an IOProxy to use (not supported by all formats, see supports("ioproxy")). The caller retains ownership of the proxy.

Opening and closing files for output

enum OpenMode

Modes passed to the open() call.

Values:

enumerator Create
enumerator AppendSubimage
enumerator AppendMIPLevel
inline virtual int supports(string_view feature) const

Given the name of a “feature”, return whether this ImageOutput supports output of images with the given properties. Most queries will simply return 0 for “doesn’t support” and 1 for “supports it,” but it is acceptable to have queries return other nonzero integers to indicate varying degrees of support or limits (but should be clearly documented as such).

Feature names that ImageOutput implementations are expected to recognize include:

  • "tiles" : Is this format writer able to write tiled images?

  • "rectangles" : Does this writer accept arbitrary rectangular pixel regions (via write_rectangle())? Returning 0 indicates that pixels must be transmitted via write_scanline() (if scanline-oriented) or write_tile() (if tile-oriented, and only if supports("tiles") returns true).

  • "random_access" : May tiles or scanlines be written in any order (0 indicates that they must be in successive order)?

  • "multiimage" : Does this format support multiple subimages within a file?

  • "appendsubimage" : Does this format support multiple subimages that can be successively appended at will via open(name,spec,AppendSubimage)? A value of 0 means that the format requires pre-declaring the number and specifications of the subimages when the file is first opened, with open(name,subimages,specs).

  • "mipmap" : Does this format support multiple resolutions for an image/subimage?

  • "volumes" : Does this format support “3D” pixel arrays (a.k.a. volume images)?

  • "alpha" : Can this format support an alpha channel?

  • "nchannels" : Can this format support arbitrary number of channels (beyond RGBA)?

  • "rewrite" : May the same scanline or tile be sent more than once? Generally, this is true for plugins that implement interactive display, rather than a saved image file.

  • "empty" : Does this plugin support passing a NULL data pointer to the various write routines to indicate that the entire data block is composed of pixels with value zero? Plugins that support this achieve a speedup when passing blank scanlines or tiles (since no actual data needs to be transmitted or converted).

  • "channelformats" : Does this format writer support per-channel data formats, respecting the ImageSpec’s channelformats field? If not, it only accepts a single data format for all channels and will ignore the channelformats field of the spec.

  • "displaywindow" : Does the format support display (“full”) windows distinct from the pixel data window?

  • "origin" : Does the image format support specifying a pixel window origin (i.e., nonzero ImageSpec x, y, z)?

  • "negativeorigin" : Does the image format allow data and display window origins (i.e., ImageSpec x, y, z, full_x, full_y, full_z) to have negative values?

  • "deepdata" : Does the image format allow “deep” data consisting of multiple values per pixel (and potentially a differing number of values from pixel to pixel)?

  • "arbitrary_metadata" : Does the image file format allow metadata with arbitrary names (and either arbitrary, or a reasonable set of, data types)? (Versus the file format supporting only a fixed list of specific metadata names/values.)

  • "exif" Does the image file format support Exif camera data (either specifically, or via arbitrary named metadata)?

  • "iptc" Does the image file format support IPTC data (either specifically, or via arbitrary named metadata)?

  • "ioproxy" Does the image file format support writing to an IOProxy?

"procedural" : Is this a purely procedural output that doesn’t write an actual file?

  • "thumbnail" : Does this format writer support adding a reduced resolution copy of the image via the thumbnail() method?

  • "thumbnail_after_write" : Does this format writer support calling thumbnail() after the scanlines or tiles have been specified? (Supporting "thumbnail" but not "thumbnail_after_write" means that any thumbnail must be supplied immediately after open(), prior to any of the write_*() calls.)

This list of queries may be extended in future releases. Since this can be done simply by recognizing new query strings, and does not require any new API entry points, addition of support for new queries does not break ``link compatibility’’ with previously-compiled plugins.

virtual bool open(const std::string &name, const ImageSpec &newspec, OpenMode mode = Create) = 0

Open the file with given name, with resolution and other format data as given in newspec. It is legal to call open multiple times on the same file without a call to close(), if it supports multiimage and mode is AppendSubimage, or if it supports MIP-maps and mode is AppendMIPLevel this is interpreted as appending a subimage, or a MIP level to the current subimage, respectively.

Return

true upon success, or false upon failure.

Parameters
  • name: The name of the image file to open.

  • newspec: The ImageSpec describing the resolution, data types, etc.

  • mode: Specifies whether the purpose of the open is to create/truncate the file (default: Create), append another subimage (AppendSubimage), or append another MIP level (AppendMIPLevel).

inline virtual bool open(const std::string &name, int subimages, const ImageSpec *specs)

Open a multi-subimage file with given name and specifications for each of the subimages. Upon success, the first subimage will be open and ready for transmission of pixels. Subsequent subimages will be denoted with the usual call of open(name,spec,AppendSubimage) (and MIP levels by open(name,spec,AppendMIPLevel)).

The purpose of this call is to accommodate format-writing libraries that must know the number and specifications of the subimages upon first opening the file; such formats can be detected by:: supports(“multiimage”) && !supports(“appendsubimage”) The individual specs passed to the appending open() calls for subsequent subimages must match the ones originally passed.

Return

true upon success, or false upon failure.

Parameters
  • name: The name of the image file to open.

  • subimages: The number of subimages (and therefore the length of the specs[] array.

  • specs[]: Pointer to an array of ImageSpec objects describing each of the expected subimages.

inline const ImageSpec &spec(void) const

Return a reference to the image format specification of the current subimage. Note that the contents of the spec are invalid before open() or after close().

virtual bool close() = 0

Closes the currently open file associated with this ImageOutput and frees any memory or resources associated with it.

Writing pixels

Common features of all the write methods:

  • The format parameter describes the data type of the data[]. The write methods automatically convert the data from the specified format to the actual output data type of the file (as was specified by the ImageSpec passed to open()). If format is TypeUnknown, then rather than converting from format, it will just copy pixels assumed to already be in the file’s native data layout (including, possibly, per-channel data formats as specified by the ImageSpec’s channelfomats field).

  • The stride values describe the layout of the data buffer: xstride is the distance in bytes between successive pixels within each scanline. ystride is the distance in bytes between successive scanlines. For volumetric images zstride is the distance in bytes between successive “volumetric planes”. Strides set to the special value AutoStride imply contiguous data, i.e.,

    xstride = format.size() * nchannels
    ystride = xstride * width
    zstride = ystride * height
    

  • Any range parameters (such as ybegin and yend) describe a “half open interval”, meaning that begin is the first item and end is one past the last item. That means that the number of items is end - begin.

  • For ordinary 2D (non-volumetric) images, any z or zbegin coordinates should be 0 and any zend should be 1, indicating that only a single image “plane” exists.

  • Scanlines or tiles must be written in successive increasing coordinate order, unless the particular output file driver allows random access (indicated by supports("random_access")).

  • All write functions return true for success, false for failure (after which a call to geterror() may retrieve a specific error message).

virtual bool write_scanline(int y, int z, TypeDesc format, const void *data, stride_t xstride = AutoStride)

Write the full scanline that includes pixels (*,y,z). For 2D non-volume images, z should be 0. The xstride value gives the distance between successive pixels (in bytes). Strides set to AutoStride imply “contiguous” data.

Return

true upon success, or false upon failure.

Parameters
  • y/z: The y & z coordinates of the scanline.

  • format: A TypeDesc describing the type of data.

  • data: Pointer to the pixel data.

  • xstride: The distance in bytes between successive pixels in data (or AutoStride).

virtual bool write_scanlines(int ybegin, int yend, int z, TypeDesc format, const void *data, stride_t xstride = AutoStride, stride_t ystride = AutoStride)

Write multiple scanlines that include pixels (*,y,z) for all ybegin <= y < yend, from data. This is analogous to write_scanline(y,z,format,data,xstride) repeatedly for each of the scanlines in turn (the advantage, though, is that some image file types may be able to write multiple scanlines more efficiently or in parallel, than it could with one scanline at a time).

Return

true upon success, or false upon failure.

Parameters
  • ybegin/yend: The y range of the scanlines being passed.

  • z: The z coordinate of the scanline.

  • format: A TypeDesc describing the type of data.

  • data: Pointer to the pixel data.

  • xstride/ystride: The distance in bytes between successive pixels and scanlines (or AutoStride).

virtual bool write_tile(int x, int y, int z, TypeDesc format, const void *data, stride_t xstride = AutoStride, stride_t ystride = AutoStride, stride_t zstride = AutoStride)

Write the tile with (x,y,z) as the upper left corner. The three stride values give the distance (in bytes) between successive pixels, scanlines, and volumetric slices, respectively. Strides set to AutoStride imply ‘contiguous’ data in the shape of a full tile, i.e.,

xstride = format.size() * spec.nchannels
ystride = xstride * spec.tile_width
zstride = ystride * spec.tile_height

Return

true upon success, or false upon failure.

Note

This call will fail if the image is not tiled, or if (x,y,z) is not the upper left corner coordinates of a tile.

Parameters
  • x/y/z: The upper left coordinate of the tile being passed.

  • format: A TypeDesc describing the type of data.

  • data: Pointer to the pixel data.

  • xstride/ystride/zstride: The distance in bytes between successive pixels, scanlines, and image planes (or AutoStride to indicate a “contiguous” single tile).

virtual bool write_tiles(int xbegin, int xend, int ybegin, int yend, int zbegin, int zend, TypeDesc format, const void *data, stride_t xstride = AutoStride, stride_t ystride = AutoStride, stride_t zstride = AutoStride)

Write the block of multiple tiles that include all pixels in

[xbegin,xend) X [ybegin,yend) X [zbegin,zend)

This is analogous to calling write_tile(x,y,z,...) for each tile in turn (but for some file formats, passing multiple tiles may allow it to write more efficiently or in parallel).

The begin/end pairs must correctly delineate tile boundaries, with the exception that it may also be the end of the image data if the image resolution is not a whole multiple of the tile size. The stride values give the data spacing of adjacent pixels, scanlines, and volumetric slices (measured in bytes). Strides set to AutoStride imply contiguous data in the shape of the [begin,end) region, i.e.,

xstride = format.size() * spec.nchannels
ystride = xstride * (xend-xbegin)
zstride = ystride * (yend-ybegin)

Return

true upon success, or false upon failure.

Note

The call will fail if the image is not tiled, or if the pixel ranges do not fall along tile (or image) boundaries, or if it is not a valid tile range.

Parameters
  • xbegin/xend: The x range of the pixels covered by the group of tiles passed.

  • ybegin/yend: The y range of the pixels covered by the tiles.

  • zbegin/zend: The z range of the pixels covered by the tiles (for a 2D image, zbegin=0 and zend=1).

  • format: A TypeDesc describing the type of data.

  • data: Pointer to the pixel data.

  • xstride/ystride/zstride: The distance in bytes between successive pixels, scanlines, and image planes (or AutoStride).

virtual bool write_rectangle(int xbegin, int xend, int ybegin, int yend, int zbegin, int zend, TypeDesc format, const void *data, stride_t xstride = AutoStride, stride_t ystride = AutoStride, stride_t zstride = AutoStride)

Write a rectangle of pixels given by the range

[xbegin,xend) X [ybegin,yend) X [zbegin,zend)

The stride values give the data spacing of adjacent pixels, scanlines, and volumetric slices (measured in bytes). Strides set to AutoStride imply contiguous data in the shape of the [begin,end) region, i.e.,

xstride = format.size() * spec.nchannels
ystride = xstride * (xend-xbegin)
zstride = ystride * (yend-ybegin)

Return

true upon success, or false upon failure.

Note

The call will fail for a format plugin that does not return true for supports("rectangles").

Parameters
  • xbegin/xend: The x range of the pixels being passed.

  • ybegin/yend: The y range of the pixels being passed.

  • zbegin/zend: The z range of the pixels being passed (for a 2D image, zbegin=0 and zend=1).

  • format: A TypeDesc describing the type of data.

  • data: Pointer to the pixel data.

  • xstride/ystride/zstride: The distance in bytes between successive pixels, scanlines, and image planes (or AutoStride).

virtual bool write_image(TypeDesc format, const void *data, stride_t xstride = AutoStride, stride_t ystride = AutoStride, stride_t zstride = AutoStride, ProgressCallback progress_callback = nullptr, void *progress_callback_data = nullptr)

Write the entire image of spec.width x spec.height x spec.depth pixels, from a buffer with the given strides and in the desired format.

Depending on the spec, this will write either all tiles or all scanlines. Assume that data points to a layout in row-major order.

Because this may be an expensive operation, a progress callback may be passed. Periodically, it will be called as follows:

progress_callback (progress_callback_data, float done);

where done gives the portion of the image (between 0.0 and 1.0) that has been written thus far.

Return

true upon success, or false upon failure.

Parameters
  • format: A TypeDesc describing the type of data.

  • data: Pointer to the pixel data.

  • xstride/ystride/zstride: The distance in bytes between successive pixels, scanlines, and image planes (or AutoStride).

  • progress_callback/progress_callback_data: Optional progress callback.

virtual bool write_deep_scanlines(int ybegin, int yend, int z, const DeepData &deepdata)

Write deep scanlines containing pixels (*,y,z), for all y in the range [ybegin,yend), to a deep file. This will fail if it is not a deep file.

Return

true upon success, or false upon failure.

Parameters
  • ybegin/yend: The y range of the scanlines being passed.

  • z: The z coordinate of the scanline.

  • deepdata: A DeepData object with the data for these scanlines.

virtual bool write_deep_tiles(int xbegin, int xend, int ybegin, int yend, int zbegin, int zend, const DeepData &deepdata)

Write the block of deep tiles that include all pixels in the range

[xbegin,xend) X [ybegin,yend) X [zbegin,zend)

The begin/end pairs must correctly delineate tile boundaries, with the exception that it may also be the end of the image data if the image resolution is not a whole multiple of the tile size.

Return

true upon success, or false upon failure.

Note

The call will fail if the image is not tiled, or if the pixel ranges do not fall along tile (or image) boundaries, or if it is not a valid tile range.

Parameters
  • xbegin/xend: The x range of the pixels covered by the group of tiles passed.

  • ybegin/yend: The y range of the pixels covered by the tiles.

  • zbegin/zend: The z range of the pixels covered by the tiles (for a 2D image, zbegin=0 and zend=1).

  • deepdata: A DeepData object with the data for the tiles.

virtual bool write_deep_image(const DeepData &deepdata)

Write the entire deep image described by deepdata. Depending on the spec, this will write either all tiles or all scanlines.

Return

true upon success, or false upon failure.

Parameters
  • deepdata: A DeepData object with the data for the image.

inline virtual bool set_thumbnail(const ImageBuf &thumb)

Specify a reduced-resolution (“thumbnail”) version of the image. Note that many image formats may require the thumbnail to be specified prior to writing the pixels.

Return

true upon success, false if it was not possible to write the thumbnail, or if this file format (or writer) does not support thumbnails.

Note

This method was added to OpenImageIO 2.3.

Parameters
  • thumb: A reference to an ImageBuf containing the thumbnail image.

Public Types

using unique_ptr = std::unique_ptr<ImageOutput>

unique_ptr to an ImageOutput.

typedef ImageOutput *(*Creator)()

Call signature of a function that creates and returns an ImageOutput*.

Public Functions

virtual const char *format_name(void) const = 0

Return the name of the format implemented by this class.

virtual bool copy_image(ImageInput *in)

Read the pixels of the current subimage of in, and write it as the next subimage of *this, in a way that is efficient and does not alter pixel values, if at all possible. Both in and this must be a properly-opened ImageInput and ImageOutput, respectively, and their current images must match in size and number of channels.

If a particular ImageOutput implementation does not supply a copy_image method, it will inherit the default implementation, which is to simply read scanlines or tiles from in and write them to *this. However, some file format implementations may have a special technique for directly copying raw pixel data from the input to the output, when both are the same file type and the same pixel data type. This can be more efficient than in->read_image() followed by out->write_image(), and avoids any unintended pixel alterations, especially for formats that use lossy compression.

Note: this method is NOT thread-safe, since it depends on persistent state in the ImageInput.

Return

true upon success, or false upon failure.

Parameters
  • in: A pointer to the open ImageInput to read from.

inline virtual bool set_ioproxy(Filesystem::IOProxy *ioproxy)

Set an IOProxy for this writer. This must be called prior to open(), and only for writers that support them (supports("ioproxy")). The caller retains ownership of the proxy.

Return

true for success, false for failure.

bool has_error() const

Is there a pending error message waiting to be retrieved, that resulted from an ImageOutput API call made by the this thread?

Note that any error() calls issued are thread-specific, and the geterror()/has_error() are expected to be called by the same thread that called whichever API function encountered an error.

std::string geterror(bool clear = true) const

Return the text of all pending error messages issued against this ImageOutput by the calling thread, and clear the pending error message unless clear is false. If no error message is pending, it will return an empty string.

Note that any error() calls issued are thread-specific, and the geterror()/has_error() are expected to be called by the same thread that called whichever API function encountered an error.

template<typename ...Args>
inline void error(const char *fmt, const Args&... args) const

Error reporting for the plugin implementation: call this with Strutil::format-like arguments. It is not necessary to have the error message contain a trailing newline. Use with caution! Some day this will change to be fmt-like rather than printf-like.

template<typename ...Args>
inline void errorf(const char *fmt, const Args&... args) const

Error reporting for the plugin implementation: call this with printf-like arguments. It is not necessary to have the error message contain a trailing newline.

template<typename ...Args>
inline void errorfmt(const char *fmt, const Args&... args) const

Error reporting for the plugin implementation: call this with std::format-like arguments. It is not necessary to have the error message contain a trailing newline.

void threads(int n)

Set the threading policy for this ImageOutput, controlling the maximum amount of parallelizing thread “fan-out” that might occur during large write operations. The default of 0 means that the global attribute("threads") value should be used (which itself defaults to using as many threads as cores; see Section Global Attributes_).

The main reason to change this value is to set it to 1 to indicate that the calling thread should do all the work rather than spawning new threads. That is probably the desired behavior in situations where the calling application has already spawned multiple worker threads.

int threads() const

Retrieve the current thread-spawning policy.

See

threads(int)