ImageInput: Reading Images

Image Input Made Simple

Here is the simplest sequence required to open an image file, find out its resolution, and read the pixels (converting them into 8-bit values in memory, even if that’s not the way they’re stored in the file):

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

auto in = ImageInput::open (filename);
if (! in)
    return;
const ImageSpec &spec = in->spec();
int xres = spec.width;
int yres = spec.height;
int channels = spec.nchannels;
std::vector<unsigned char> pixels (xres*yres*channels);
in->read_image (TypeDesc::UINT8, &pixels[0]);
in->close ();

Here is a breakdown of what work this code is doing:

  • Search for an ImageIO plugin that is capable of reading the file (foo.jpg), first by trying to deduce the correct plugin from the file extension, but if that fails, by opening every ImageIO plugin it can find until one will open the file without error. When it finds the right plugin, it creates a subclass instance of ImageInput that reads the right kind of file format, and tries to fully open the file. The open() method returns a std::unique_ptr<ImageInput> that will be automatically freed when it exits scope.

    auto in = ImageInput::open (filename);
    
  • The specification, accessible as in->spec(), contains vital information such as the dimensions of the image, number of color channels, and data type of the pixel values. This is enough to allow us to allocate enough space for the image.

    const ImageSpec &spec = in->spec();
    int xres = spec.width;
    int yres = spec.height;
    int channels = spec.nchannels;
    std::vector<unsigned char> pixels (xres*yres*channels);
    

    Note that in this example, we don’t care what data format is used for the pixel data in the file — we allocate enough space for unsigned 8-bit integer pixel values, and will rely on OpenImageIO’s ability to convert to our requested format from the native data format of the file.

  • Read 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 the data in the file (in this case, we request that it be automatically converted to unsigned 8-bit integers).

    in->read_image (TypeDesc::UINT8, &pixels[0]);
    
  • Close the file.

    in->close ();
    
  • When in exits its scope, the ImageInput will automatically be destroyed and any resources used by the plugin will be released.

Advanced Image Input

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

Reading individual scanlines and tiles

The simple example of Section Image Input Made Simple read an entire image with one call. But sometimes you want to read a large image a little at a time and do not wish to retain the entire image in memory as you process it. OpenImageIO allows you to read images one scanline at a time or one tile at a time.

Examining the ImageSpec reveals whether the file is scanline or tile-oriented: a scanline image will have spec.tile_width and spec.tile_height set to 0, whereas a tiled images will have nonzero values for the tile dimensions.

Reading scanlines

Individual scanlines may be read using the read_scanline() API call:

...
auto in = ImageInput::open (filename);
const ImageSpec &spec = in->spec();
if (spec.tile_width == 0) {
    std::vector<unsigned char> scanline (spec.width*spec.channels);
    for (int y = 0;  y < yres;  ++y) {
        in->read_scanline (y, 0, TypeDesc::UINT8, &scanline[0]);
        ... process data in scanline[0..width*channels-1] ...
    }
} else {
    ... handle tiles, or reject the file ...
}
in->close ();
...

The first two arguments to read_scanline() specify which scanline is being read 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 type of the pixel buffer you are supplying, and a pointer to the pixel buffer itself. Additional optional arguments describe the data stride, which can be ignored for contiguous data (use of strides is explained in Section Data Strides).

Nearly all ImageInput implementations will be most efficient reading scanlines in strict order (starting with scanline 0, then 1, up to yres-1, without skipping any). An ImageInput is required to accept read_scanline() requests in arbitrary order, but depending on the file format and reader implementation, out-of-order scanline reads may be inefficient.

There is also a read_scanlines() function that operates similarly, except that it takes a ybegin and yend that specify a range, reading all scanlines ybegin <= y < yend. For most image format readers, this is implemented as a loop over individual scanlines, but some image format readers may be able to read a contiguous block of scanlines more efficiently than reading each one individually.

The full descriptions of the read_scanline() and read_scanlines() functions may be found in Section ImageInput Class Reference.

Reading tiles

Once you open() an image file, you can find out if it is a tiled image (and the tile size) by examining the ImageSpec’s tile_width, tile_height, and tile_depth fields. If they are zero, it’s a scanline image and you should read pixels using read_scanline(), not read_tile().

...
auto in = ImageInput::open (filename);
const ImageSpec &spec = in->spec();
if (spec.tile_width == 0) {
    ... read by scanline ...
} else {
    // Tiles
    int tilesize = spec.tile_width * spec.tile_height;
    std::vector<unsigned char> tile (tilesize * spec.channels);
    for (int y = 0;  y < yres;  y += spec.tile_height) {
        for (int x = 0;  x < xres;  x += spec.tile_width) {
            in->read_tile (x, y, 0, TypeDesc::UINT8, &tile[0]);
            ... process the pixels in tile[] ..
        }
    }
}
in->close ();
...

The first three arguments to read_tile() specify which tile is being read 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 format of the pixel buffer you are supplying, and a pointer to the pixel buffer. Pixel data will be written to your buffer in order of 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 ImageInput implementations are required to support reading tiles in arbitrary order (i.e., not in strict order of increasing y rows, and within each row, increasing x column, without missing any tiles).

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

Converting formats

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 pixels be stored in any of several formats. This is done merely by passing the read function the data type of your pixel buffer, as one of the enumerated type TypeDesc.

It is not required that the pixel data buffer passed to read_image(), read_scanline(), or read_tile() actually be in the same data format as the data in the file being read. OpenImageIO will automatically convert from native data type of the file to the internal data format of your choice. For example, the following code will open a TIFF and read pixels into your internal buffer represented as float values. This will work regardless of whether the TIFF file itself is using 8-bit, 16-bit, or float values.

std::unique_ptr<ImageInput> in = ImageInput::open ("myfile.tif");
const ImageSpec &spec = in->spec();
...
int numpixels = spec.width * spec.height;
float pixels = new float [numpixels * channels];
...
in->read_image (TypeDesc::FLOAT, pixels);

Note that read_scanline() and read_tile() have a parameter that works in a corresponding manner.

You can, of course, find out the native type of the file simply by examining spec.format. If you wish, you may then allocate a buffer big enough for an image of that type and request the native type when reading, therefore eliminating any translation among types and seeing the actual numerical values in the file.

Data Strides

In the preceeding examples, we have assumed that the buffer passed to the read functions (i.e., the place where you want your pixels to be stored) is contiguous, that is:

  • each pixel in memory consists of a number of data values equal to the number of channels in 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 or tiles, 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 read_tile() will write pixel data into your buffer so that it is contiguous in the shape of a single tile, not just an offset into a whole image worth of pixels.

The read_scanline() function takes an optional xstride argument, and the read_image() and read_tile() functions take optional xstride, ystride, and zstride values that describe the distance, in bytes, between successive pixel columns, rows, and slices, respectively, of your pixel buffer. 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 reading, by using negative y stride:

    unsigned char pixels[spec.width * spec.height * spec.nchannels];
    int scanlinesize = spec.width * spec.nchannels * sizeof(pixels[0]);
    ...
    in->read_image (TypeDesc::UINT8,
                    (char *)pixels+(yres-1)*scanlinesize, // offset to last
                    AutoStride,                  // default x stride
                    -scanlinesize,               // special y stride
                    AutoStride);                 // default z stride
    
  • Read a tile into its spot in a buffer whose layout matches a whole image of pixel data, rather than having a one-tile-only memory layout:

    int pixelsize = spec.nchannels * sizeof(pixels[0]);
    int scanlinesize = xpec.width * pixelsize;
    ...
    in->read_tile (x, y, 0, TypeDesc::UINT8,
                   (char *)pixels + y*scanlinesize + x*pixelsize,
                   pixelsize,
                   scanlinesize);
    

Please consult Section ImageInput Class Reference for detailed descriptions of the stride parameters to each read function.

Reading channels to separate buffers

While specifying data strides allows writing entire pixels to buffers with arbitrary layouts, it is not possible to separate those pixels into multiple buffers (i.e. to write image data to a separate or planar memory layout: RRRRGGGGBBBB instead of the interleaved RGBRGBRGBRGB).

A workaround for this is to call read_scanlines, read_tiles or read_image repeatedly with arguments chbegin and chend of 0 <= chbegin < spec.nchannels and chend == chbegin + 1:

// one buffer for all three channels
unsigned char pixels[spec.width * spec.height * spec.nchannels];

for (int channel = 0; channel < spec.nchannels; ++channel) {
    file->read_image(
        // reading one channel at a time
        channel, channel + 1,
        TypeDesc::UINT8,
        // writing the data to offsets spaced `spec.width * spec.height`
        // apart
        &pixels[spec.width * spec.height * channel]);
}

For many formats, this is nearly as fast as reading the image with interleaved pixel data if the format stores the pixels in an interleaved layout and even slightly faster if the pixels are stored in separate planes in the file.

Reading metadata

The ImageSpec that is filled in by ImageInput::open() specifies all the common properties that describe an image: data format, dimensions, number of channels, tiling. However, there may be a variety of additional metadata that are present in the image file and could be queried by your application.

The remainder of this section explains how to query additional metadata in the ImageSpec. It is up to the ImageInput to read these from the file, if indeed the file format is able to carry additional data. Individual ImageInput implementations should document which metadata they read.

Channel names

In addition to specifying the number of color channels, the ImageSpec also stores the names of those channels in its channelnames field, which is a std::vector<std::string>. Its length should always be equal to the number of channels (it’s the responsibility of the ImageInput to ensure this).

Only a few file formats (and thus ImageInput implementations) have a way of specifying custom channel names, so most of the time you will see that the channel names follow the default convention of being named "R", "G", "B", and "A", for red, green, blue, and alpha, respectively.

Here is example code that prints the names of the channels in an image:

auto in = ImageInput::open (filename);
const ImageSpec &spec = in->spec();
for (int i = 0;  i < spec.nchannels;  ++i)
    std::cout << "Channel " << i << " is "
              << spec.channelnames[i] << "\n";

Specially-designated channels

The ImageSpec contains two fields, alpha_channel and z_channel, which designate which channel numbers represent alpha and z depth, if any. If either is set to -1, it indicates that it is not known which channel is used for that data.

If you are doing something special with alpha or depth, it is probably safer to respect the alpha_channel and z_channel designations (if not set to -1) rather than merely assuming that, for example, channel 3 is always the alpha channel.

Arbitrary metadata

All other metadata found in the file will be stored in the ImageSpec’s extra_attribs field, which is a ParamValueList, which is itself essentially a vector of ParamValue instances. Each ParamValue stores one meta-datum consisting of a name, type (specified by a TypeDesc), number of values, and data pointer.

If you know the name of a specific piece of metadata you want to use, you can find it using the ImageSpec::find_attribute() method, which returns a pointer to the matching ParamValue, or nullptr if no match was found. An optional TypeDesc argument can narrow the search to only parameters that match the specified type as well as the name. Below is an example that looks for orientation information, expecting it to consist of a single integer:

auto in = ImageInput::open (filename);
const ImageSpec &spec = in->spec();
...
ParamValue *p = spec.find_attribute ("Orientation", TypeInt);
if (p) {
    int orientation = * (int *) p->data();
} else {
    std::cout << "No integer orientation in the file\n";
}

By convention, ImageInput plugins will save all integer metadata as 32-bit integers (TypeDesc::INT or TypeDesc::UINT), even if the file format dictates that a particular item is stored in the file as a 8- or 16-bit integer. This is just to keep client applications from having to deal with all the types. Since there is relatively little metadata compared to pixel data, there’s no real memory waste of promoting all integer types to int32 metadata. Floating-point metadata and string metadata may also exist, of course.

For certain common types, there is an even simpler method for retrieving the metadata:

int i = spec.get_int_attribute ("Orientation", 0);
float f = spec.get_float_attribute ("PixelAspectRatio", 1.0f);
std::string s = spec.get_string_attribute ("ImageDescription", "");

This method simply returns the value. The second argument is the default value to use if the attribute named is not found. These versions will do automatic type conversion as well — for example, if you ask for a float and the attribute is really an int, it will return the proper float for it; or if the attribute is a UINT16 and you call get_int_attribute(), it will succeed, promoting to an int.

It is also possible to step through all the metadata, item by item. This can be accomplished using the technique of the following example:

for (size_t i = 0;  i < spec.extra_attribs.size();  ++i) {
    const ParamValue &p (spec.extra_attribs[i]);
    printf ("    %s: ", p.name.c_str());
    if (p.type() == TypeString)
        printf ("\"%s\"", *(const char **)p.data());
    else if (p.type() == TypeFloat)
        printf ("%g", *(const float *)p.data());
    else if (p.type() == TypeInt)
        printf ("%d", *(const int *)p.data());
    else if (p.type() == TypeDesc::UINT)
        printf ("%u", *(const unsigned int *)p.data());
    else if (p.type() == TypeMatrix) {
        const float *f = (const float *)p.data();
        printf ("%f %f %f %f %f %f %f %f "
                "%f %f %f %f %f %f %f %f",
                f[0], f[1], f[2], f[3], f[4], f[5], f[6], f[7],
                f[8], f[9], f[10], f[11], f[12], f[13], f[14], f[15]);
    }
    else
        printf (" <unknown data type> ");
    printf ("\n");
}

Each individual ImageInput 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.

The ImageSpec::extra_attribs field may store metadata that reveals the color space the image file in the "oiio:ColorSpace" attribute (see Section Color information metadata for explanations of particular values).

The ImageInput sets the "oiio:ColorSpace" metadata in a purely advisory capacity — the read will not convert pixel values among color spaces. Many image file formats only support nonlinear color spaces (for example, JPEG/JFIF dictates use of sRGB). So your application should intelligently deal with gamma-corrected and sRGB input, at the very least.

The color space hints only describe color channels. You should assume that alpha or depth (z) channels (designated by the alpha_channel and z_channel fields, respectively) always represent linear values and should never be transformed by your application.

Multi-image files and MIP-maps

Some image file formats support multiple discrete subimages to be stored in one file, and/or miltiple resolutions for each image to form a MIPmap. When you open() an ImageInput, it will by default point to the first (i.e., number 0) subimage in the file, and the highest resolution (level 0) MIP-map level. You can switch to viewing another subimage or MIP-map level using the seek_subimage() function:

auto in = ImageInput::open (filename);
...
int subimage = 1;
int miplevel = 0;
if (in->seek_subimage (subimage, miplevel)) {
    ...
} else {
    ... no such subimage/miplevel ...
}

The seek_subimage() function takes three arguments: the index of the subimage to switch to (starting with 0), the MIPmap level (starting with 0 for the highest-resolution level), and a reference to an ImageSpec, into which will be stored the spec of the new subimage/miplevel. The seek_subimage() function returns true upon success, and false if no such subimage or MIP level existed. It is legal to visit subimages and MIP levels out of order; the ImageInput is responsible for making it work properly. It is also possible to find out which subimage and MIP level is currently being viewed, using the current_subimage() and current_miplevel() functions, which return the index of the current subimage and MIP levels, respectively.

Below is pseudocode for reading all the levels of a MIP-map (a multi-resolution image used for texture mapping) that shows how to read multi-image files:

auto in = ImageInput::open (filename);
const ImageSpec &spec = in->spec();

int num_miplevels = 0;
while (in->seek_subimage (0, num_miplevels, spec)) {
    // Note: spec has the format of the current subimage/miplevel
    int npixels = spec.width * spec.height;
    int nchannels = spec.nchannels;
    unsigned char *pixels = new unsigned char [npixels * nchannels];
    in->read_image (TypeDesc::UINT8, pixels);

    ... do whatever you want with this level, in pixels ...

    delete [] pixels;
    ++num_miplevels;
}
// Note: we break out of the while loop when seek_subimage fails
// to find a next MIP level.

in->close ();

In this example, we have used read_image(), but of course read_scanline() and read_tile() work as you would expect, on the current subimage and 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). If you want to read the pixels in their true native per-channel formats, the following steps are necessary:

  1. Check the ImageSpec’s channelformats vector. If non-empty, the channels in the file do not all have the same format.

  2. When calling read_scanline, read_scanlines, read_tile, read_tiles, or read_image, pass a format of TypeUnknown to indicate that you would like the raw data in native per-channel format of the file written to your data buffer.

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

auto in = ImageInput::open (filename);
const ImageSpec &spec = in->spec();

// Allocate enough space
unsigned char *pixels = new unsigned char [spec.image_bytes(true)];

in->read_image (TypeDesc::UNKNOWN, /* use native channel formats */
                pixels);           /* data buffer */

if (spec.channelformats.size() > 0) {
    ... the buffer contains packed data in the native
        per-channel formats ...
} else {
    ... the buffer contains all data per spec.format ...
}

Reading “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 an image is “deep” from its ImageSpec: the deep field will be true.

Deep files cannot be read with the usual read_scanline(), read_scanlines(), read_tile(), read_tiles(), read_image() functions, due to the nature of their variable number of samples per pixel. Instead, ImageInput has three special member functions used only for reading deep data:

bool read_native_deep_scanlines (int subimage, int miplevel,
                                 int ybegin, int yend, int z,
                                 int chbegin, int chend,
                                 DeepData &deepdata);

bool read_native_deep_tiles (int subimage, int miplevel,
                             int xbegin, int xend, int ybegin int yend,
                             int zbegin, int zend,
                             int chbegin, int chend, DeepData &deepdata);

bool read_native_deep_image (int subimage, int miplevel,
                             DeepData &deepdata);

It is only possible to read “native” data types from deep files; that is, there is no automatic translation into arbitrary data types as there is for ordinary images. All three of these functions store the resulting deep data in a special DeepData structure, described in detail in Section Reading “deep” data.

Here is an example of using these methods to read a deep image from a file and print all its values:

auto in = ImageInput::open (filename);
if (! in)
    return;
const ImageSpec &spec = in->spec();
if (spec.deep) {
    DeepData deepdata;
    in->read_native_deep_image (0, 0, deepdata);
    int p = 0;  // absolute pixel number
    for (int y = 0; y < spec.height;  ++y) {
        for (int x = 0;  x < spec.width;  ++x, ++p) {
            std::cout << "Pixel " << x << "," << y << ":\n";
            if (deepdata.samples(p) == 0)
                std::cout << "  no samples\n";
            else
                for (int c = 0;  c < spec.nchannels;  ++c) {
                    TypeDesc type = deepdata.channeltype(c);
                    std::cout << "  " << spec.channelnames[c] << ": ";
                    void *ptr = deepdata.pointers[p*spec.nchannels+c]
                    for (int s = 0; s < deepdata.samples(p); ++s) {
                        if (type.basetype == TypeDesc::FLOAT ||
                            type.basetype == TypeDesc::HALF)
                            std::cout << deepdata.deep_value(p, c, s) << ' ';
                        else if (type.basetype == TypeDesc::UINT32)
                            std::cout << deepdata.deep_value_uint(p, c, s) << ' ';
                    }
                    std::cout << "\n";
                }
        }
    }
}
in->close ();

Custom I/O proxies (and reading the file from a memory buffer)

Some file format readers 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 read the file form an in-memory buffer rather than reading from disk.

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

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

ImageInput readers 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 IOMemReader (which reads input from a block of memory).

Here is an example of using a proxy that reads the “file” from a memory buffer:

const void *buf = ...;   // pointer to memory block
size_t size = ...;       // length of memory block
Filesystem::IOMemReader memreader (buf, size);  // I/O proxy object

auto in = ImageInput::open ("in.exr", nullptr, &memreader);
in->read_image (...);
in->close();

// That will have read the "file" from the memory buffer

Custom search paths for plugins

Please see Section Global Attributes for discussion about setting the plugin search path via the attribute() function. For example:

std::string mysearch = "/usr/myapp/lib:${HOME}/plugins";
OIIO::attribute ("plugin_searchpath", mysearch);
auto in = ImageInput::open (filename);
...

Error checking

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

The exceptions to this rule are static methods such as the static ImageInput::open() and ImageInput::create(), which return an empty pointer if it could not create an appropriate ImageInput (and open it, in the case of open(). In such a case, since no ImageInput is returned 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 static open() or create().

Here is another version of the simple image reading code from Section Image Input 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";
int xres, yres, channels;
std::vector<unsigned char> pixels;

auto in = ImageInput::open (filename);
if (! in) {
    std::cerr << "Could not open " << filename
              << ", error = " << OIIO::geterror() << "\n";
    return;
}
const ImageSpec &spec = in->spec();
xres = spec.width;
yres = spec.height;
channels = spec.nchannels;
pixels.resize (xres*yres*channels);

if (! in->read_image (TypeDesc::UINT8, &pixels[0])) {
    std::cerr << "Could not read pixels from " << filename
              << ", error = " << in->geterror() << "\n";
    return;
}

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

ImageInput Class Reference

class OIIO::ImageInput

ImageInput abstracts the reading of an image file in a file format-agnostic manner.

Creating an ImageIntput

static unique_ptr open(const std::string &filename, const ImageSpec *config = nullptr, Filesystem::IOProxy *ioproxy = nullptr)

Create an ImageInput subclass instance that is able to read the given file and open it, returning a unique_ptr to the ImageInput if successful. The unique_ptr is set up with an appropriate deleter so the ImageInput will be properly closed and deleted when the unique_ptr goes out of scope or is reset. If the open fails, return an empty unique_ptr and set an error that can be retrieved by OIIO::geterror().

The config, if not nullptr, points to an ImageSpec giving hints, requests, or special instructions. ImageInput implementations are free to not respond to any such requests, so the default implementation is just to ignore config.

open() will first try to make an ImageInput corresponding to the format implied by the file extension (for example, "foo.tif" will try the TIFF plugin), but if one is not found or if the inferred one does not open the file, every known ImageInput type will be tried until one is found that will open the file.

Return

A unique_ptr that will close and free the ImageInput 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 to open.

  • config: Optional pointer to an ImageSpec whose metadata contains “configuration hints.”

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

static unique_ptr create(string_view filename, bool do_open = false, const ImageSpec *config = nullptr, Filesystem::IOProxy *ioproxy = nullptr, string_view plugin_searchpath = "")

Create and return an ImageInput implementation that is able to read the given file or format. If do_open is true (and the filename is the name of a file, not just a format), fully open it if possible (using the optional config configuration spec, if supplied), otherwise just create the ImageInput but don’t open it. The plugin_searchpath parameter is an override of the searchpath. colon-separated list of directories to search for ImageIO plugin DSO/DLL’s (not a searchpath for the image itself!).

If the filename parameter is the name of a file format (such as “openexr”), it will create an ImageInput that reads that particular format. If the name is a file extension (such as “exr” or “.exr”), it will guess the file format from the extension and return that type of ImageInput.

If filename is a full file name (such as “hawaii.exr”), it will create an ImageInput that reads the format implied by the file extension (“.tif”) and try to open the file with that reader. If the file can be opened and appears to be of the correct type, then that ImageInput (after being closed) will be returned to the caller. But if it fails (say, because the file type does not match the extension), then every known kind of image reader will be tried in turn, until one can be found that succeeds in opening that file. The create() file will fail entirely only if no known image reader type succeeds.

If the caller intends to immediately open the file, then it is often simpler to call static ImageInput::open().

Return

A unique_ptr that will close and free the ImageInput 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 an image file, or a file extension, or the name of a file format.

  • do_open: If true, not only create but also open the file.

  • config: Optional pointer to an ImageSpec whose metadata contains “configuration hints” for the ImageInput implementation.

  • ioproxy: Optional pointer to an IOProxy to use (not supported by all formats, see supports("ioproxy")). The caller retains ownership of the proxy. If this is not supplied, it is still possible to set the proxy with a call to set_proxy() prior to open().

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

Reading pixels

Common features of all the read methods:

  • The format parameter describes the data type of the data[] buffer. The read methods automatically convert the data from the data type it is stored in the file into the format of the data buffer. If format is TypeUnknown it will just copy pixels of 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.

  • Some read methods take a channel range [chbegin,chend) to allow reading of a contiguous subset of channels (chbegin=0, chend=spec.nchannels reads all channels).

  • ImageInput readers are expected to give the appearance of random access in other words, if it can’t randomly seek to the given scanline or tile, it should transparently close, reopen, and sequentially read through prior scanlines.

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

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

Read the scanline that includes pixels (*,y,z) from the “current” subimage and MIP level. The xstride value gives the distance between successive pixels (in bytes). Strides set to AutoStride imply “contiguous” data.

Note

This variety of read_scanline is not re-entrant nor thread-safe. If you require concurrent reads to the same open ImageInput, you should use read_scanlines that has the subimage and miplevel passed explicitly.

Return

true upon success, or false upon failure.

Parameters
  • y/z: The y & z coordinates of the scanline. For 2D images, z should be 0.

  • format: A TypeDesc describing the type of data.

  • data: Pointer to the pixel data buffer.

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

inline bool read_scanline(int y, int z, float *data)

Simple read_scanline reads into contiguous float pixels.

virtual bool read_scanlines(int subimage, int miplevel, int ybegin, int yend, int z, int chbegin, int chend, TypeDesc format, void *data, stride_t xstride = AutoStride, stride_t ystride = AutoStride)

Read multiple scanlines that include pixels (*,y,z) for all ybegin <= y < yend in the specified subimage and mip level, into data, using the strides given and converting to the requested data format (TypeUnknown indicates no conversion, just copy native data types). Only channels [chbegin,chend) will be read/copied (chbegin=0, chend=spec.nchannels reads all channels, yielding equivalent behavior to the simpler variant of read_scanlines).

This version of read_scanlines, because it passes explicit subimage/miplevel, does not require a separate call to seek_subimage, and is guaranteed to be thread-safe against other concurrent calls to any of the read_* methods that take an explicit subimage/miplevel (but not against any other ImageInput methods).

Return

true upon success, or false upon failure.

Note

This call was changed for OpenImageIO 2.0 to include the explicit subimage and miplevel parameters. The previous versions, which lacked subimage and miplevel parameters (thus were dependent on a prior call to seek_subimage) are considered deprecated.

Parameters
  • subimage: The subimage to read from (starting with 0).

  • miplevel: The MIP level to read (0 is the highest resolution level).

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

  • z: The z coordinate of the scanline.

  • chbegin/chend: The channel range to read.

  • 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 read_tile(int x, int y, int z, TypeDesc format, void *data, stride_t xstride = AutoStride, stride_t ystride = AutoStride, stride_t zstride = AutoStride)

Read the tile whose upper-left origin is (x,y,z) into data[], converting if necessary from the native data format of the file into the format specified. 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 a full tile, i.e.,

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

Note

This variety of read_tile is not re-entrant nor thread-safe. If you require concurrent reads to the same open ImageInput, you should use read_tiles() that has the subimage and miplevel passed explicitly.

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).

inline bool read_tile(int x, int y, int z, float *data)

Simple read_tile reads into contiguous float pixels.

virtual bool read_tiles(int subimage, int miplevel, int xbegin, int xend, int ybegin, int yend, int zbegin, int zend, int chbegin, int chend, TypeDesc format, void *data, stride_t xstride = AutoStride, stride_t ystride = AutoStride, stride_t zstride = AutoStride)

Read the block of multiple tiles that include all pixels in

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

This is analogous to calling read_tile(x,y,z,...) for each tile in turn (but for some file formats, reading multiple tiles may allow it to read 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)

This version of read_tiles, because it passes explicit subimage and miplevel, does not require a separate call to seek_subimage, and is guaranteed to be thread-safe against other concurrent calls to any of the read_* methods that take an explicit subimage/miplevel (but not against any other ImageInput methods).

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
  • subimage: The subimage to read from (starting with 0).

  • miplevel: The MIP level to read (0 is the highest resolution level).

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

  • 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).

  • chbegin/chend: The channel range to read.

  • 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 read_image(int subimage, int miplevel, int chbegin, int chend, TypeDesc format, void *data, stride_t xstride = AutoStride, stride_t ystride = AutoStride, stride_t zstride = AutoStride, ProgressCallback progress_callback = NULL, void *progress_callback_data = NULL)

Read the entire image of spec.width x spec.height x spec.depth pixels into a buffer with the given strides and in the desired data format.

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

This version of read_image, because it passes explicit subimage and miplevel, does not require a separate call to seek_subimage, and is guaranteed to be thread-safe against other concurrent calls to any of the read_* methods that take an explicit subimage/miplevel (but not against any other ImageInput methods).

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
  • subimage: The subimage to read from (starting with 0).

  • miplevel: The MIP level to read (0 is the highest resolution level).

  • chbegin/chend: The channel range to read.

  • 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 read_native_deep_scanlines(int subimage, int miplevel, int ybegin, int yend, int z, int chbegin, int chend, DeepData &deepdata)

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

Return

true upon success, or false upon failure.

Parameters
  • subimage: The subimage to read from (starting with 0).

  • miplevel: The MIP level to read (0 is the highest resolution level).

  • chbegin/chend: The channel range to read.

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

  • z: The z coordinate of the scanline.

  • deepdata: A DeepData object into which the data for these scanlines will be placed.

virtual bool read_native_deep_tiles(int subimage, int miplevel, int xbegin, int xend, int ybegin, int yend, int zbegin, int zend, int chbegin, int chend, DeepData &deepdata)

Read into deepdata the block of native deep data tiles that include all pixels and channels specified by pixel range.

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
  • subimage: The subimage to read from (starting with 0).

  • miplevel: The MIP level to read (0 is the highest resolution level).

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

  • 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).

  • chbegin/chend: The channel range to read.

  • deepdata: A DeepData object into which the data for these tiles will be placed.

virtual bool read_native_deep_image(int subimage, int miplevel, DeepData &deepdata)

Read the entire deep data image of spec.width x spec.height x spec.depth pixels, all channels, into deepdata.

Return

true upon success, or false upon failure.

Parameters
  • subimage: The subimage to read from (starting with 0).

  • miplevel: The MIP level to read (0 is the highest resolution level).

  • deepdata: A DeepData object into which the data for the image will be placed.

Reading native pixels – implementation overloads

Note

read_native_* methods are usually not directly called by user code (except for read_native_deep_* varieties). These are the methods that are overloaded by the ImageInput subclasses that implement the individual file format readers.

virtual bool read_native_scanline(int subimage, int miplevel, int y, int z, void *data) = 0

Read a single scanline (all channels) of native data into contiguous memory.

virtual bool read_native_scanlines(int subimage, int miplevel, int ybegin, int yend, int z, void *data)

Read a range of scanlines (all channels) of native data into contiguous memory.

virtual bool read_native_scanlines(int subimage, int miplevel, int ybegin, int yend, int z, int chbegin, int chend, void *data)

Read a range of scanlines (with optionally a subset of channels) of native data into contiguous memory.

virtual bool read_native_tile(int subimage, int miplevel, int x, int y, int z, void *data)

Read a single tile (all channels) of native data into contiguous memory. The base class read_native_tile fails. A format reader that supports tiles MUST overload this virtual method that reads a single tile (all channels).

virtual bool read_native_tiles(int subimage, int miplevel, int xbegin, int xend, int ybegin, int yend, int zbegin, int zend, void *data)

Read multiple tiles (all channels) of native data into contigious memory. A format reader that supports reading multiple tiles at once (in a way that’s more efficient than reading the tiles one at a time) is advised (but not required) to overload this virtual method. If an ImageInput subclass does not overload this, the default implementation here is simply to loop over the tiles, calling the single-tile read_native_tile() for each one.

virtual bool read_native_tiles(int subimage, int miplevel, int xbegin, int xend, int ybegin, int yend, int zbegin, int zend, int chbegin, int chend, void *data)

Read multiple tiles (potentially a subset of channels) of native data into contigious memory. A format reader that supports reading multiple tiles at once, and can handle a channel subset while doing so, is advised (but not required) to overload this virtual method. If an ImageInput subclass does not overload this, the default implementation here is simply to loop over the tiles, calling the single-tile read_native_tile() for each one (and copying carefully to handle the channel subset issues).

Public Types

using unique_ptr = std::unique_ptr<ImageInput>

unique_ptr to an ImageInput

typedef std::lock_guard<const ImageInput&> lock_guard

The presence of lock() and unlock() establish an ImageInput itself as having the BasicLockable concept and therefore can be used by std::lock_guard.

typedef ImageInput *(*Creator)()

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

Public Functions

virtual const char *format_name(void) const = 0

Return the name of the format implemented by this class.

inline virtual int supports(string_view feature) const

Given the name of a “feature”, return whether this ImageInput 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 ImageInput implementations are expected to recognize include:

  • "arbitrary_metadata" : Does this format allow metadata with arbitrary names and types?

  • "exif" : Can this format store Exif camera data?

  • "ioproxy" : Does this format reader support reading from an IOProxy?

  • "iptc" : Can this format store IPTC data?

  • "procedural" : Can this format create images without reading from a disk file?

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

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 valid_file(const std::string &filename) const

Return true if the filename names a file of the type for this ImageInput. The implementation will try to determine this as efficiently as possible, in most cases much less expensively than doing a full open(). Note that there can be false positives: a file can appear to be of the right type (i.e., valid_file() returning true) but still fail a subsequent call to open(), such as if the contents of the file are truncated, nonsensical, or otherwise corrupted.

Return

true upon success, or false upon failure.

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

Opens the file with given name and seek to the first subimage in the file. Various file attributes are put in newspec and a copy is also saved internally to the ImageInput (retrievable via spec(). From examining newspec or spec(), you can discern the resolution, if it’s tiled, number of channels, native data format, and other metadata about the image.

Return

true if the file was found and opened successfully.

Parameters
  • name: Filename to open.

  • newspec: Reference to an ImageSpec in which to deposit a full description of the contents of the first subimage of the file.

inline virtual bool open(const std::string &name, ImageSpec &newspec, const ImageSpec &config)

Open file with given name, similar to open(name,newspec). The config is an ImageSpec giving requests or special instructions. ImageInput implementations are free to not respond to any such requests, so the default implementation is just to ignore config and call regular open(name,newspec).

Return

true if the file was found and opened successfully.

Parameters
  • name: Filename to open.

  • newspec: Reference to an ImageSpec in which to deposit a full description of the contents of the first subimage of the file.

  • config: An ImageSpec whose metadata contains “configuration hints” for the ImageInput implementation.

inline virtual const ImageSpec &spec(void) const

Return a reference to the image specification of the current subimage/MIPlevel. Note that the contents of the spec are invalid before open() or after close(), and may change with a call to seek_subimage(). It is thus not thread-safe, since the spec may change if another thread calls seek_subimage, or any of the read_*() functions that take explicit subimage/miplevel.

virtual ImageSpec spec(int subimage, int miplevel = 0)

Return a full copy of the ImageSpec of the designated subimage and MIPlevel. This method is thread-safe, but it is potentially expensive, due to the work that needs to be done to fully copy an ImageSpec if there is lots of named metadata to allocate and copy. See also the less expensive spec_dimensions(). Errors (such as having requested a nonexistent subimage) are indicated by returning an ImageSpec with format==TypeUnknown.

virtual ImageSpec spec_dimensions(int subimage, int miplevel = 0)

Return a copy of the ImageSpec of the designated subimage and miplevel, but only the dimension and type fields. Just as with a call to ImageSpec::copy_dimensions(), neither the channel names nor any of the arbitrary named metadata will be copied, thus this is a relatively inexpensive operation if you don’t need that information. It is guaranteed to be thread-safe. Errors (such as having requested a nonexistent subimage) are indicated by returning an ImageSpec with format==TypeUnknown.

inline virtual bool get_thumbnail(ImageBuf &thumb, int subimage)

Retrieve a reduced-resolution (“thumbnail”) version of the given subimage. It is guaranteed to be thread-safe.

Return

true upon success, false if no thumbnail was available, or if this file format (or reader) does not support thumbnails.

Note

This method was added to OpenImageIO 2.3.

Parameters
  • thumb: A reference to an ImageBuf which will be overwritten with the thumbnail image.

  • subimage: The index of the subimage in the file whose thumbnail is to be retrieved.

virtual bool close() = 0

Close an open ImageInput. The call to close() is not strictly necessary if the ImageInput is destroyed immediately afterwards, since it is required for the destructor to close if the file is still open.

Return

true upon success, or false upon failure.

inline virtual int current_subimage(void) const

Returns the index of the subimage that is currently being read. The first subimage (or the only subimage, if there is just one) is number 0.

inline virtual int current_miplevel(void) const

Returns the index of the MIPmap image that is currently being read. The highest-res MIP level (or the only level, if there is just one) is number 0.

inline virtual bool seek_subimage(int subimage, int miplevel)

Seek to the given subimage and MIP-map level within the open image file. The first subimage of the file has index 0, the highest- resolution MIP level has index 0. The new subimage’s vital statistics=may be retrieved by this->spec(). The reader is expected to give the appearance of random access to subimages and MIP levels in other words, if it can’t randomly seek to the given subimage/level, it should transparently close, reopen, and sequentially read through prior subimages and levels.

Return

true upon success, or false upon failure. A failure may indicate that no such subimage or MIP level exists in the file.

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

Set an IOProxy for this reader. This must be called prior to open(), and only for readers 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 ImageInput 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 ImageInput 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 ImageInput, controlling the maximum amount of parallelizing thread “fan-out” that might occur during large read 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)

void lock() const

There is a (hidden) internal recursive mutex to each ImageInput that can be used by the II to enforce thread safety. This is exposed via the obvious lock()/unlock()/try_lock() semantics.