Image Chromaticity on the GPU

Images often represent color using three channels: (R, G, B) - red, green, and blue. For this document, we assume the range of each channel is [0.0, 1.0].

Normalized Chromaticity

Formulas

Output (r, g, b)

(1.0, 0.0, 0.0):

100% red

(0.0, 1.0, 0.0):

100% green

(0.0, 0.0, 1.0):

100% blue

Output (r, g)

(1.0, 0.0):

100% red

(0.0, 1.0):

100% green

(0.0, 0.0):

100% blue (implied)

Normalized RG/RGB

\[ \begin{align}\begin{aligned}r = \frac{R}{R+G+B} g = \frac{G}{R+G+B} b = \frac{B}{R+G+B}\\r+g+b = 1\end{aligned}\end{align} \]

Normalized RG/RGB White-Point

\[ \begin{align}\begin{aligned}R = 1 G = 1 B = 1\\r = \frac{R}{R+G+B} g = \frac{G}{R+G+B} b = \frac{B}{R+G+B}\\r+g+b = 1\end{aligned}\end{align} \]

float3 RGBtoChromaticityRGB(float3 Color)
{
   // Optimizes 2 ADD instructions into 1 DP3 instruction.
   float SumRGB = dot(Color, 1.0);
   float3 Chromaticity = saturate(Color / SumRGB);

   // Output the chromaticity's white point if the divisor is 0.0.
   // Prevents undefined behavior when dividing by 0.
   Chromaticity = (SumRGB == 0.0) ? float3(1.0 / 3.0) : Chromaticity;

   return Chromaticity;
}

Spherical Chromaticity

This section introduces a color space that represents chromaticity using angles.

Precision Loss in RG Chromaticity

A significant drawback of RG chromaticity is precision loss. All possible values map to a right triangle, effectively halving the precision in integer buffers.

Spherical chromaticity encodes data that utilizes the entire RG8 range by calculating angles between the color channels.

/*
   This code is based on the algorithm described in the following paper:
   Author(s): Joost van de Weijer, T. Gevers
   Title: "Robust optical flow from photometric invariants"
   Year: 2004
   DOI: 10.1109/ICIP.2004.1421433

   https://www.researchgate.net/publication/4138051_Robust_optical_flow_from_photometric_invariants
*/

float3 RGBtoSphericalRGB(float3 RGB)
{
   const float InvPi = 1.0 / acos(-1.0);

   // Precalculate (x*x + y*y)^0.5 and (x*x + y*y + z*z)^0.5
   float L1 = length(RGB.xyz);
   float L2 = length(RGB.xy);

   // .x = radius; .y = inclination; .z = azimuth
   float3 RIA;
   RIA.x = L1 / sqrt(3.0);
   RIA.y = (L1 == 0.0) ? 1.0 / sqrt(3.0) : saturate(RGB.z / L1);
   RIA.z = (L2 == 0.0) ? 1.0 / sqrt(2.0) : saturate(RGB.x / L2);

   // Scale the angles to [-1.0, 1.0) range
   RIA.yz = (RIA.yz * 2.0) - 1.0;

   // Calculate inclination and azimuth and normalize to [0.0, 1.0)
   RIA.yz = acos(RIA.yz) * InvPi;

   return RIA;
}