Colour Representation

A digital colour image is stored as a multi-channel structure, one image per attribute.

Channels

• RGB
  Maintains the intensity of red, green and blue in separate channels.
• Luminance and chrominance
  Separates brightness (luminance) from colour (chrominance) into separate channels.

• Discarding $C_r, C_b$ gives a monochrome image directly
• The human visual system is more sensitive to luminance than colour, so YCrCb formats often allocate bits in a 4:2:2 ratio
• Widely used for its inherent compression capability

Emission and absorption colours

• Emission based
  Determined by what light should be emitted to produce a colour. Absence of light is black, presence of all components is white.
• Absorption based
  Determined by what ink should be applied so that light reflected from the substrate produces a colour. Ink subtracts colours from white.

RGB is emission based. CMYK is absorption based.

True colour images

RGB uses 24 bits per pixel, or 3 bytes. CMYK needs 4 bytes per pixel.

Palette based representation

Storing a table of the colours actually used, then storing pointers into that table, is more storage-efficient than native RGB.

• Palette length determines how many distinct colours can be used at once, out of the full $2^{24}$
• Bits per pixel in image memory depend on the palette table length
• The palette table is usually stored with the image, allowing a custom colour set per image