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(Required except for image masks and images that use the JPXDecode filter) The number of bits used to represent each colour component.
Only a single value shall be specified; the number of bits shall be the same for all colour components. The value shall be 1, 2, 4, 8, or (in PDF 1.5) 16.
If ImageMask is , this entry is optional, but if specified, its value shall be 1.