Image Serving color management

Last update: November 3, 2023

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Image Serving supports color space conversions based on color space profiles conforming to the ICC (International Color Consortium) specification.

Default color spaces

Each image catalog (and the default catalog) can define a set of ICC profiles which constitute the default color spaces for this catalog - one input and one output profile each for gray-scale, RGB, and CMYK data. See
attribute::IccProfileRgb
attribute::IccProfileGray
attribute::IccProfileCmyk
attribute::IccProfileSrcRgb
attribute::IccProfileSrcGray
attribute::IccProfileSrcCmyk.

Input color space

Source images may embed ICC profiles to define the input color space. If no profile is embedded in a source image, attribute::IccProfileSrc* of the applicable image catalog corresponding to the pixel type of the source image is used. If this attribute is not defined in the image catalog, attribute::IccProfile* is used. If that catalog attribute is not defined either, the image is not color-managed and only naïve transforms are applied.

Output color space

The color space of the final image result of a request is defined with the icc= command. If icc= is not specified, the default output color space (from the request’s main catalog) which corresponds to the pixel type of the output image is used as the output color space. If no output profile is defined in the main or default catalog, and if the base layer is an image with an embedded profile matching the output pixel type, then that profile is used for the output color space. Otherwise, the output color space remains undefined – only naïve color conversions are applied when converting between pixel types and no color profile can be embedded in the output image.

The output color space of a nested/embedded Image Serving request is always the same as the output color space of the outer, embedding request.

Solid colors

Color values specified with color=, bgcolor=, or the RTF command \iscolortbl are associated with the input color space if the color value includes the suffix ‘S’, otherwise they are associated with the output color space. Color values specified with bgc= or the RTF commands \colortbl and \cmykcolortbl are always associated with the corresponding default or actual output color space.

NOTE
At this time, bgc= does not fully participate in color management-the ‘S’ suffix is ignored when specified with bgc=, and naïve conversion is applied when the pixel type of the color value specified with bgc= differs from the pixel type of the output image. Otherwise, bgc= is associated with the actual output color space.

Nested and embedded requests

The output color space for nested IS requests and embedded IR requests is automatically set to the output color space of the outermost request, unless the nested request specifies an explicit output color space with icc=. In addition, nested/embedded requests also inherit the default output color spaces from the main catalog of the outermost request, to ensure consistent handling of solid color values.

Color space conversion

Image Serving generally attempts to delay color conversions during processing. If all layers of an image have the same layer color space, conversion to the output color space is done after merging and final scaling. If multiple layer color spaces are involved, each layer is transformed to the output color space prior to merging.

NOTE
The commands op_brightness=, op_colorbalance=, op_colorize=, op_contrast=, op_hue=, and op_saturation= are RGB operations. These operations maintain color fidelity only if the layer color space has RGB pixel type. If other than RGB, the data is converted to RGB using naïve color conversion, and the result has limited color fidelity. The layer color space for such layers should be considered indeterminate.

Color conversion options are provided with icc= or, if icc= is not specified, with attribute::IccRenderIntent, attribute::IccBlackPointCompensation, and attribute::IccDither.