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(Optional; shall not be present for image masks; PDF 1.3) An image XObject defining an image mask to be applied to this image (see 8.9.6.3,"Explicit masking"), or an array specifying a range of colours to be applied to it as a colour key mask (see 8.9.6.4, "Colour key masking").
If ImageMask is true, this entry shall not be present.