Metadata conventions#

The ImageSpec class, described thoroughly in Image Specification: ImageSpec, provides the basic description of an image that are essential across all formats — resolution, number of channels, pixel data format, etc. Individual images may have additional data, stored as name/value pairs in the extra_attribs field. Though literally anything can be stored in extra_attribs — it’s specifically designed for format- and user-extensibility — this chapter establishes some guidelines and lays out all of the field names that OpenImageIO understands.

Description of the image#

"ImageDescription" : string#

The image description, title, caption, or comments.

"Keywords" : string#

Semicolon-separated keywords describing the contents of the image. (Semicolons are used rather than commas because of the common case of a comma being part of a keyword itself, e.g., “Kurt Vonnegut, Jr.” or “Washington, DC.””)

"Artist" : string#

The artist, creator, or owner of the image.

Any copyright notice or owner of the image.

"DateTime" : string#

The creation date of the image, in the following format: YYYY:MM:DD HH:MM:SS (exactly 19 characters long, not including a terminating NULL). For example, 7:30am on Dec 31, 2008 is encoded as "2008:12:31 07:30:00".

Usually this is simply the time that the image data was last modified. It may be wise to also store the "Exif:DateTimeOriginal" and "Exif:DateTimeDigitized" (see Section Exif metadata) to further distinguish the original image and its conversion to digital media.

"DocumentName" : string#

The name of an overall document that this image is a part of.

"Software" : string#

The software that was used to create the image.

"HostComputer" : string#

The name or identity of the computer that created the image.

Display hints#

"Orientation" : int#

By default, image pixels are ordered from the top of the display to the bottom, and within each scanline, from left to right (i.e., the same ordering as English text and scan progression on a CRT). But the "Orientation" field can suggest that it should be displayed with a different orientation, according to the TIFF/EXIF conventions:


normal (top to bottom, left to right)


flipped horizontally (top to bottom, right to left)


rotated \(180^\circ\) (bottom to top, right to left)


flipped vertically (bottom to top, left to right)


transposed (left to right, top to bottom)


rotated \(90^\circ\) clockwise (right to left, top to bottom)


transverse (right to left, bottom to top)


rotated \(90^\circ\) counter-clockwise (left to right, bottom to top)

"PixelAspectRatio" : float#

The aspect ratio (\(x/y\)) of the size of individual pixels, with square pixels being 1.0 (the default).

"XResolution" : float#
"YResolution" : float#
"ResolutionUnit" : string#

The number of horizontal (x) and vertical (y) pixels per resolution unit. This ties the image to a physical size (where applicable, such as with a scanned image, or an image that will eventually be printed).

Different file formats may dictate different resolution units. For example, the TIFF ImageIO plugin supports none, in, and cm.

"oiio:Movie" : int#

If nonzero, a hint that a multi-image file is meant to be interpreted as an animation (i.e., that the subimages are a time sequence).

"oiio:subimages" : int#

If nonzero, the number of subimages in the file. Not all image file formats can know this without reading the entire file, and in such cases, this attribute will not be set or will be 0. If the value is present and greater than zero, it can be trusted, but if not, nothing should be inferred and you will have to repeatedly seek to subimages to find out how many there are.

"FramesPerSecond" : rational#

For a multi-image file intended to be played back as an animation, the frame refresh rate. (It’s technically a rational, but it may be retrieved as a float also, if you are ok with imprecision.)

Color information#

"oiio:ColorSpace" : string#

The name of the color space of the color channels. Values include:

  • "scene_linear" : Color pixel values are known to be scene-linear and using facility-default color primaries as defined by the OpenColorIO configuration. Note that "linear" is treated as a synonym. (Note: when no color config is found, this are presumed to use sRGB/Rec709 color primaries when built against OpenColorIO 2.1 or earlier, or when no OCIO support is available, but is presumed to be ACEScg when built against OCIO 2.2 or higher and using its built-in config.)

  • "lin_srgb" : Color pixel values are known to be linear and using sRGB/Rec709 color primaries.

  • "sRGB" : Using standard sRGB response and primaries.

  • "Rec709" : Using standard Rec709 response and primaries.

  • "ACEScg" : ACEScg color space encoding.

  • "AdobeRGB" : Adobe RGB color space.

  • "KodakLog" : Kodak logarithmic color space.

  • "GammaX.Y" : Color values have been gamma corrected (raised to the power \(1/\gamma\)). The X.Y is the numeric value of the gamma exponent.

  • arbitrary : The name of any color space known to OpenColorIO (if OCIO support is present).

"oiio:Gamma" : float#

If the color space is “GammaX.Y”, this value is the gamma exponent. (Optional/deprecated; if present, it should match the suffix of the color space.)

"oiio:BorderColor" : float[nchannels]#

The color presumed to be filling any parts of the display/full image window that are not overlapping the pixel data window. If not supplied, the default is black (0 in all channels).

"ICCProfile" : uint8[]#
"ICCProfile:...various..." : ...various types...#

The ICC color profile takes the form of an array of bytes (unsigned 8 bit chars) whose contents and format is as dictated by the ICC specification. The length of the array is the length of the profile blob.

When reading images containing ICC profiles, there may be a number of other attributes extracted from the ICC profile (such as ICCProfile:device_class). These are for informational convenience only, as they replicate information which is also in the ICCProfile byte array. Be aware that setting these attributes will not result in modifying the ICC profile byte array ("ICCProfile"), which is considered the sole piece of durable ICC profile information.

Disk file format info/hints#

"oiio:BitsPerSample" : int#

Number of bits per sample in the file.

Note that this may not match the reported ImageSpec::format, if the plugin is translating from an unsupported format. For example, if a file stores 4 bit grayscale per channel, the "oiio:BitsPerSample" may be 4 but the format field may be TypeDesc::UINT8 (because the OpenImageIO APIs do not support fewer than 8 bits per sample).

"oiio:UnassociatedAlpha" : int#

Whether the data in the file stored alpha channels (if any) that were unassociated with the color (i.e., color not “premultiplied” by the alpha coverage value).

"planarconfig" : string#

contig indicates that the file has contiguous pixels (RGB RGB RGB…), whereas separate indicate that the file stores each channel separately (RRR…GGG…BBB…).

Note that only contiguous pixels are transmitted through the OpenImageIO APIs, but this metadata indicates how it is (or should be) stored in the file, if possible.

"compression" : string#

Indicates the type of compression the file uses. Supported compression modes will vary from file format to file format, and each reader/writer plugin should document the modes it supports. If ImageOutput::open is called with an ImageSpec that specifies an compression mode not supported by that ImageOutput, it will choose a reasonable default. As an example, the OpenEXR writer supports none, rle, zip, zips, piz, pxr24, b44, b44a, dwaa, or dwab.

he compression name is permitted to have a quality value to be appended fter a colon, for example dwaa:60. The exact meaning and range of he quality value can vary between different file formats and compression odes, and some don’t support quality values at all (it will be ignored if ot supported, or if out of range).

"CompressionQuality" : int#


This is a deprecated methods of separately specifying the compression quality. Indicates the quality of compression to use (0–100), for those plugins and compression methods that allow a variable amount of compression, with higher numbers indicating higher image fidelity.

Substituting an IOPRoxy for custom I/O overrides#

Format readers and writers that respond positively to supports("ioproxy") have the ability to read or write using an I/O proxy object. Among other things, this lets an ImageOutput write the file to a memory buffer rather than saving to disk, and for an ImageInput to read the file from a memory buffer. This behavior is controlled by a special attributes

"oiio:ioproxy" : pointer#

Pointer to a Filesystem::IOProxy that will handle the I/O.

An explanation of how this feature is used may be found in Sections Custom I/O proxies (and reading the file from a memory buffer) and Custom I/O proxies (and writing the file to a memory buffer).

In addition to communicating IOProxy information via this attribute, it is also allowed (and probably preferred) to directly supply the IOProxy information via calls to ImageInput::open(), create(), and set_ioproxy(), ImageOutput::create() and set_ioproxy(), and ImageBuf::ImageBuf(), reset(), and set_write_ioproxy().

Photographs or scanned images#

The following metadata items are specific to photos or captured images.

"Make" : string#

For captured or scanned image, the make of the camera or scanner.

"Model" : string#

For captured or scanned image, the model of the camera or scanner.

"ExposureTime" : float#

The exposure time (in seconds) of the captured image.

"FNumber" : float#

The f/stop of the camera when it captured the image.

Thumbnails / postage stamps / preview images#

Some image file formats allow for storing a thumbnail – a lower-resolution image suitable for an icon or other small preview. These are retrievable separately via ImageInput::get_thumbnail(), ImageBuf::thumbnail(), or ImageCache::get_thumbnail(). In addition, if a thumbnail is available, the ImageSpec should also contain the following metadata:

"thumbnail_width" : int#

The thumbnail width, in pixels.

"thumbnail_height" : int#

The thumbnail height, in pixels.

"thumbnail_nchannels" : int#

The number of color channels in the thumbnail image.

Texture Information#

Several standard metadata are very helpful for images that are intended to be used as textures (especially for OpenImageIO’s TextureSystem).

"textureformat" : string#

The kind of texture that this image is intended to be. We suggest the following names:

Plain Texture

Ordinary 2D texture

Volume Texture

3D volumetric texture


Ordinary z-depth shadow map

CubeFace Shadow

Cube-face shadow map

Volume Shadow

Volumetric (“deep”) shadow map

LatLong Environment

Latitude-longitude (rectangular) environment map

CubeFace Environment

Cube-face environment map

"wrapmodes" : string#

Give the intended texture wrap mode indicating what happens with texture coordinates outside the \([0...1]\) range. We suggest the following names: black, periodic, clamp, mirror. If the wrap mode is different in each direction, they should simply be separated by a comma. For example, black means black wrap in both directions, whereas clamp,periodic means to clamp in \(u\) and be periodic in \(v\).

"fovcot" : float#

The cotangent (\(x/y\)) of the field of view of the original image (which may not be the same as the aspect ratio of the pixels of the texture, which may have been resized).

"worldtocamera" : matrix44#

For shadow maps or rendered images this item (of type TypeDesc::PT_MATRIX) is the world-to-camera matrix describing the camera position.

"worldtoscreen" : matrix44#

For shadow maps or rendered images this item (of type TypeDesc::PT_MATRIX) is the world-to-screen matrix describing the full projection of the 3D view onto a \([-1...1] \times [-1...1]\) 2D domain.

"worldtoNDC" : matrix44#

For shadow maps or rendered images this item (of type TypeDesc::PT_MATRIX) is the world-to-NDC matrix describing the full projection of the 3D view onto a \([0...1] \times [0...1]\) 2D domain.

"oiio:updirection" : string#

For environment maps, indicates which direction is “up” (valid values are y or z), to disambiguate conventions for environment map orientation.

"oiio:sampleborder" : int#

If not present or 0 (the default), the conversion from pixel integer coordinates \((i,j)\) to texture coordinates \((s,t)\) follows the usual convention of \(s = (i+0.5)/\mathit{xres}\) and \(t = (j+0.5)/\mathit{yres}\). However, if this attribute is present and nonzero, the first and last row/column of pixels lie exactly at the \(s\) or \(t = 0\) or \(1\) boundaries, i.e., \(s = i/(\mathit{xres}-1)\) and \(t = j/(\mathit{yres}-1)\).

"oiio:ConstantColor" : string#

If present, is a hint that the texture has the same value in all pixels, and the metadata value is a string containing the channel values as a comma-separated list (no spaces, for example: 0.73,0.9,0.11,1.0).

"oiio:AverageColor" : string#

If present, is a hint giving the average value of all pixels in the texture, as a string containing a comma-separated list of the channel values (no spaces, for example: 0.73,0.9,0.11,1.0).

"oiio:SHA-1" : string#

f present, is a 40-byte SHA-1 hash of the input image (possibly salted with arious maketx options) that can serve to quickly compare two separate extures to know if they contain the same pixels. While it’s not, echnically, 100% guaranteed that no separate textures will match, it’s so stronomically unlikely that we discount the possibility (you’d be rendering ovies for centuries before finding a single match).

Exif metadata#

The following Exif metadata tags correspond to items in the “standard” set of metadata.

Exif tag

OpenImageIO metadata convention


(reflected in “oiio:ColorSpace”)





The other remaining Exif metadata tags all include the Exif: prefix to keep it from clashing with other names that may be used for other purposes.

"Exif:ExposureProgram" : int#

The exposure program used to set exposure when the picture was taken:






normal program


aperture priority


shutter priority


Creative program (biased toward depth of field)


Action program (biased toward fast shutter speed)


Portrait mode (closeup photo with background out of focus)


Landscape mode (background in focus)

"Exif:SpectralSensitivity" : string#

The camera’s spectral sensitivity, using the ASTM conventions.

"Exif:ISOSpeedRatings" : int#

The ISO speed and ISO latitude of the camera as specified in ISO 12232.

"Exif:DateTimeOriginal" : string#
"Exif:DateTimeDigitized" : string#

Date and time that the original image data was generated or captured, and the time/time that the image was stored as digital data. Both are in YYYY:MM:DD HH:MM:SS format.

To clarify the role of these (and also with respect to the standard DateTime metadata), consider an analog photograph taken in 1960 (Exif:DateTimeOriginal), which was scanned to a digital image in 2010 (Exif:DateTimeDigitized), and had color corrections or other alterations performed in 2015 (DateTime).

"Exif:CompressedBitsPerPixel" : float#

The compression mode used, measured in compressed bits per pixel.

"Exif:ShutterSpeedValue" : float#

Shutter speed, in APEX units: \(-\log_2(\mathit{exposure time})\)

"Exif:ApertureValue" : float#

Aperture, in APEX units: \(2 \log_2 (\mathit{fnumber})\)

"Exif:BrightnessValue" : float#

Brightness value, assumed to be in the range of \(-99.99\)\(99.99\).

"Exif:ExposureBiasValue" : float#

Exposure bias, assumed to be in the range of \(-99.99\)\(99.99\).

"Exif:MaxApertureValue" : float#

Smallest F number of the lens, in APEX units: \(2 \log_2 (\mathit{fnumber})\)

"Exif:SubjectDistance" : float#

Distance to the subject, in meters.

"Exif:MeteringMode" : int#

The metering mode:






center-weighted average











"Exif:LightSource" : int#

The kind of light source:






tungsten (incandescent light)




fine weather


cloudy weather




daylight fluorescent (D 5700-7100K)


day white fluorescent (N 4600-5400K)


cool white fuorescent (W 3900 - 4500K)


white fluorescent (WW 3200 - 3700K)


standard light A


standard light B


standard light C










ISO studio tungsten


other light source

"Exif:Flash" int}#

A sum of:


if the flash fired


no strobe return detection function


strobe return light was not detected


strobe return light was detected


compulsory flash firing


compulsory flash suppression


auto-flash mode


no flash function (0 if flash function present)


red-eye reduction supported (0 if no red-eye reduction mode)

"Exif:FocalLength" : float#

Actual focal length of the lens, in mm.

"Exif:SecurityClassification" : string#

Security classification of the image: C = confidential, R = restricted, S = secret, T = top secret, U = unclassified.

"Exif:ImageHistory" : string#

Image history.

"Exif:SubsecTime" : string#

Fractions of a second to augment the "DateTime" (expressed as text of the digits to the right of the decimal).

"Exif:SubsecTimeOriginal" : string#

Fractions of a second to augment the Exif:DateTimeOriginal (expressed as text of the digits to the right of the decimal).

"Exif:SubsecTimeDigitized" : string#

Fractions of a second to augment the Exif:DateTimeDigital (expressed as text of the digits to the right of the decimal).

"Exif:PixelXDimension" : int#
"Exif:PixelYDimension" : int#

The x and y dimensions of the valid pixel area.

"Exif:FlashEnergy" : float#

Strobe energy when the image was captures, measured in Beam Candle Power Seconds (BCPS).

"Exif:FocalPlaneXResolution" : float#
"Exif:FocalPlaneYResolution" : float#
"Exif:FocalPlaneResolutionUnit" : int#

The number of pixels in the x and y dimension, per resolution unit. The codes for resolution units are:










\(\mu m\)

"Exif:ExposureIndex" : float#

The exposure index selected on the camera.

"Exif:SensingMethod" : int#

The image sensor type on the camra:




one-chip color area sensor


two-chip color area sensor


three-chip color area sensor


color sequential area sensor


trilinear sensor


color trilinear sensor

"Exif:FileSource" : int#

The source type of the scanned image, if known:


film scanner


reflection print scanner


digital camera

"Exif:SceneType" : int#

Set to 1, if a directly-photographed image, otherwise it should not be present.

"Exif:CustomRendered" : int#

Set to 0 for a normal process, 1 if some custom processing has been performed on the image data.

"Exif:ExposureMode" : int#

The exposure mode:







"Exif:WhiteBalance" : int#

Set to 0 for auto white balance, 1 for manual white balance.

"Exif:DigitalZoomRatio" : float#

The digital zoom ratio used when the image was shot.

"Exif:FocalLengthIn35mmFilm" : int#

The equivalent focal length of a 35mm camera, in mm.

"Exif:SceneCaptureType" : int#

The type of scene that was shot:








night scene

"Exif:GainControl" : float#

The degree of overall gain adjustment:




low gain up


high gain up


low gain down


high gain down

"Exif:Contrast" : int#

The direction of contrast processing applied by the camera:







"Exif:Saturation" : int#

The direction of saturation processing applied by the camera:




low saturation


high saturation

"Exif:Sharpness" : int#

The direction of sharpness processing applied by the camera:







"Exif:SubjectDistanceRange" : int#

The distance to the subject:









"Exif:ImageUniqueID" : string#

A unique identifier for the image, as 16 ASCII hexadecimal digits representing a 128-bit number.

GPS Exif metadata#

The following GPS-related Exif metadata tags correspond to items in the “standard” set of metadata.

"GPS:LatitudeRef" : string#

Whether the GPS:Latitude tag refers to north or south: N or S.

"GPS:Latitude" : float[3]#

The degrees, minutes, and seconds of latitude (see also GPS:LatitudeRef).

"GPS:LongitudeRef" : string#

Whether the GPS:Longitude tag refers to east or west: E or a W.

"GPS:Longitude" : float[3]#

The degrees, minutes, and seconds of longitude (see also GPS:LongitudeRef).

"GPS:AltitudeRef" : string#

A value of 0 indicates that the altitude is above sea level, 1 indicates below sea level.

"GPS:Altitude" : float#

Absolute value of the altitude, in meters, relative to sea level (see GPS:AltitudeRef for whether it’s above or below sea level).

"GPS:TimeStamp" : float[3]#

Gives the hours, minutes, and seconds, in UTC.

"GPS:Satellites" : string#

Information about what satellites were visible.

"GPS:Status" : string#

A indicates a measurement in progress, V indicates measurement interoperability.

"GPS:MeasureMode" : string#

2 indicates a 2D measurement, 3 indicates a 3D measurement.

"GPS:DOP" : float#

Data degree of precision.

"GPS:SpeedRef" : string#

Indicates the units of the related GPS:Speed tag: K for km/h, M for miles/h, N for knots.

"GPS:Speed" : float#

Speed of the GPS receiver (see GPS:SpeedRef for the units).

"GPS:TrackRef" : string#

Describes the meaning of the GPS:Track field: T for true direction, M for magnetic direction.

"GPS:Track" : float#

Direction of the GPS receiver movement (from 0–359.99). The related GPS:TrackRef indicate whether it’s true or magnetic.

"GPS:ImgDirectionRef" : string#

Describes the meaning of the GPS:ImgDirection field: T for true direction, M for magnetic direction.

"GPS:ImgDirection" : float#

Direction of the image when captured (from 0–359.99). The related GPS:ImgDirectionRef indicate whether it’s true or magnetic.

"GPS:MapDatum" : string#

The geodetic survey data used by the GPS receiver.

"GPS:DestLatitudeRef" : string#

Whether the GPS:DestLatitude tag refers to north or south: N or S.

"GPS:DestLatitude" : float[3]#

The degrees, minutes, and seconds of latitude of the destination (see also GPS:DestLatitudeRef).

"GPS:DestLongitudeRef" : string#

Whether the GPS:DestLongitude tag refers to east or west: E or W.

"GPS:DestLongitude" : float[3]#

The degrees, minutes, and seconds of longitude of the destination (see also GPS:DestLongitudeRef).

"GPS:DestBearingRef" : string#

Describes the meaning of the GPS:DestBearing field: T for true direction, M for magnetic direction.

"GPS:DestBearing" : float#

Bearing to the destination point (from 0–359.99). The related GPS:DestBearingRef indicate whether it’s true or magnetic.

"GPS:DestDistanceRef" : string#

Indicates the units of the related GPS:DestDistance tag: K for km, M for miles, N for knots.

"GPS:DestDistance" : float#

Distance to the destination (see GPS:DestDistanceRef for the units).

"GPS:ProcessingMethod" : string#

Processing method information.

"GPS:AreaInformation" : string#

Name of the GPS area.

"GPS:DateStamp" : string#

Date according to the GPS device, in format YYYY:MM:DD.

"GPS:Differential" : int#

If 1, indicates that differential correction was applied.

"GPS:HPositioningError" : float#

Positioning error.

IPTC metadata#

The IPTC (International Press Telecommunications Council) publishes conventions for storing image metadata, and this standard is growing in popularity and is commonly used in photo-browsing programs to record captions and keywords.

The following IPTC metadata items correspond exactly to metadata in the OpenImageIO conventions, so it is recommended that you use the standards and that plugins supporting IPTC metadata respond likewise:

IPTC tag

OpenImageIO metadata convention




IPTC keywords should be concatenated, separated by semicolons (;), and stored as the Keywords attribute.







The remainder of IPTC metadata fields should use the following names, prefixed with IPTC: to avoid conflicts with other plugins or standards.

"IPTC:ObjecTypeReference" : string#

Object type reference.

"IPTC:ObjectAttributeReference" : string#

Object attribute reference.

"IPTC:ObjectName" : string#

The name of the object in the picture.

"IPTC:EditStatus" : string#

Edit status.

"IPTC:SubjectReference" : string#

Subject reference.

"IPTC:Category" : string#


"IPTC:ContentLocationCode" : string#

Code for content location.

"IPTC:ContentLocationName" : string#

Name of content location.

"IPTC:ReleaseDate" : string#
"IPTC:ReleaseTime" : string#

Release date and time.

"IPTC:ExpirationDate" : string#
"IPTC:ExpirationTime" : string#

Expiration date and time.

"IPTC:Instructions" : string#

Special instructions for handling the image.

"IPTC:ReferenceService" : string#
"IPTC:ReferenceDate" : string#
"IPTC:ReferenceNumber" : string#

Reference date, service, and number.

"IPTC:DateCreated" : string#
"IPTC:TimeCreated" : string#

Date and time that the image was created.

"IPTC:DigitalCreationDate" : string#
"IPTC:DigitalCreationTime" : string#

Date and time that the image was digitized.

"IPTC:ProgramVersion" : string#

The version number of the creation software.

"IPTC:AuthorsPosition" : string#

The job title or position of the creator of the image.

"IPTC:City" : string#
"IPTC:Sublocation" : string#
"IPTC:State" : string#
"IPTC:Country" : string#
"IPTC:CountryCode" : string#

The city, sublocation within the city, state, country, and country code of the location of the image.

"IPTC:Headline" : string#

Any headline that is meant to accompany the image.

"IPTC:Provider" : string#

The provider of the image, or credit line.

"IPTC:Source" : string#

The source of the image.

"IPTC:Contact" : string#

The contact information for the image (possibly including name, address, email, etc.).

"IPTC:CaptionWriter" : string#

The name of the person who wrote the caption or description of the image.

"IPTC:JobID" : string#
"IPTC:MasterDocumentID" : string#
"IPTC:ShortDocumentID" : string#
"IPTC:UniqueDocumentID" : string#
"IPTC:OwnerID" : string#

Various identification tags.

"IPTC:Prefs" : string#
"IPTC:ClassifyState" : string#
"IPTC:SimilarityIndex" : string#

Who knows what the heck these are?

"IPTC:DocumentNotes" : string#

Notes about the image or document.

"IPTC:DocumentHistory" : string#

The history of the image or document.

SMPTE metadata#

"smpte:TimeCode" : int[2]#

SMPTE time code, encoded as an array of 2 32-bit integers (as a TypeDesc, it will be tagged with vecsemantics TIMECODE).

"smpte:KeyCode" : int[7]#

SMPTE key code, encoded as an array of 7 32-bit integers (as a TypeDesc, it will be tagged with vecsemantics KEYCODE).

Extension conventions#

To avoid conflicts with other plugins, or with any additional standard metadata names that may be added in future versions of OpenImageIO, it is strongly advised that writers of new plugins should prefix their metadata with the name of the format, much like the "Exif:" and "IPTC:" metadata.