Colour Gamuts

Important Features

Tone Reproduction Curves (TRCs)

What is a TRC

1. Neutral
3. Exaggerate highlights
4. Exaggerate mid-tones
5. Minimize mid-tones

Colour Spaces

HSV

• h/60 = i [integer part] + f [fractional part]
• R = V ( 1 - s * { 1 [i=2,3], 0 [i=0,5], f [i=1], (1-f) [i=4] } )
• G = V ( 1 - s * { 1 [i=4,5], 0 [i=1,2], f [i=3], (1-f) [i=0] } )
• B = V ( 1 - s * { 1 [i=2,3], 0 [i=0,5], f [i=1], (1-f) [i=4] } )

RGYB

• R = rL
• G = gL
• B = 1 - max(r,g)

YIQ

NTSC colour encoding

• Y = .30 R + .59 G + .11 B
• I = .60 R - .25 G - .32 B
• Q = .21 R - .52 G + .31 B

Comparing Images

Here's three ways that it is done

1. Sum ( (R - R")^2 + (G-G')^2 + (B - B')^2 ) ^(1/2) over pixels
2. Sum ( (X - X")^2 + (Y-Y')^2 + (Z - Z')^2 ) ^(1/2) over pixels
3. Sum ( (L - L")^2 + (u-u')^2 + (v - v')^2 ) ^(1/2) over pixels

Why are neither of these satisfactory?

Suggestions for improvement.

Colour Reproduction (aka Gamut Mapping)

Talk about fire engines: NTSC phosphor chromaticities

• R: 0.67, 0.33
• G: 0.21, 0.71
• B: 0.14, 0.08

Here are the heuristics used by printers

1. Get the neutral colours neutral