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/*
[configuration]
[OptionRangeFloat]
GUIName = Amplificiation
OptionName = AMPLIFICATION
MinValue = 1.0
MaxValue = 6.0
StepAmount = 0.25
DefaultValue = 2.5
[/configuration]
*/
// ICtCP Colorspace as defined by Dolby here:
// https://professional.dolby.com/siteassets/pdfs/ictcp_dolbywhitepaper_v071.pdf
/***** Transfer Function *****/
const float a = 0.17883277;
const float b = 1.0 - 4.0 * a;
const float c = 0.5 - a * log(4.0 * a);
float HLG_f(float x)
{
if (x < 0.0) {
return 0.0;
}
else if (x < 1.0 / 12.0) {
return sqrt(3.0 * x);
}
return a * log(12.0 * x - b) + c;
}
float HLG_inv_f(float x)
{
if (x < 0.0) {
return 0.0;
}
else if (x < 1.0 / 2.0) {
return x * x / 3.0;
}
return (exp((x - c) / a) + b) / 12.0;
}
float4 HLG(float4 lms)
{
return float4(HLG_f(lms.x), HLG_f(lms.y), HLG_f(lms.z), lms.w);
}
float4 HLG_inv(float4 lms)
{
return float4(HLG_inv_f(lms.x), HLG_inv_f(lms.y), HLG_inv_f(lms.z), lms.w);
}
/***** Linear <--> ICtCp *****/
const mat4 RGBtoLMS = mat4(
1688.0, 683.0, 99.0, 0.0,
2146.0, 2951.0, 309.0, 0.0,
262.0, 462.0, 3688.0, 0.0,
0.0, 0.0, 0.0, 4096.0)
/ 4096.0;
const mat4 LMStoICtCp = mat4(
+2048.0, +3625.0, +9500.0, 0.0,
+2048.0, -7465.0, -9212.0, 0.0,
+0.0, +3840.0, -288.0, 0.0,
+0.0, +0.0, +0.0, 4096.0)
/ 4096.0;
float4 LinearRGBToICtCP(float4 c)
{
return LMStoICtCp * HLG(RGBtoLMS * c);
}
/***** ICtCp <--> Linear *****/
mat4 ICtCptoLMS = inverse(LMStoICtCp);
mat4 LMStoRGB = inverse(RGBtoLMS);
float4 ICtCpToLinearRGB(float4 c)
{
return LMStoRGB * HLG_inv(ICtCptoLMS * c);
}
void main()
{
float4 color = Sample();
// Nothing to do here, we are in SDR
if (!OptionEnabled(hdr_output) || !OptionEnabled(linear_space_output)) {
SetOutput(color);
return;
}
// Renormalize Color to be in [0.0 - 1.0] SDR Space. We will revert this later.
const float hdr_paper_white = hdr_paper_white_nits / hdr_sdr_white_nits;
color.rgb /= hdr_paper_white;
// Convert Color to Perceptual Color Space. This will allow us to do perceptual
// scaling while also being able to use the luminance channel.
float4 ictcp_color = LinearRGBToICtCP(color);
// Scale the color in perceptual space depending on the perceived luminance.
//
// At low luminances, ~0.0, pow(AMPLIFICATION, ~0.0) ~= 1.0, so the
// color will appear to be unchanged. This is important as we don't want to
// over expose dark colors which would not have otherwise been seen.
//
// At high luminances, ~1.0, pow(AMPLIFICATION, ~1.0) ~= AMPLIFICATION,
// which is equivalent to scaling the color by AMPLIFICATION. This is
// important as we want to get the most out of the display, and we want to
// get bright colors to hit their target brightness.
//
// For more information, see this desmos demonstrating this scaling process:
// https://www.desmos.com/calculator/syjyrjsj5c
float exposure = length(ictcp_color.xyz);
ictcp_color *= pow(HLG_f(AMPLIFICATION), exposure);
// Convert back to Linear RGB and output the color to the display.
// We use hdr_paper_white to renormalize the color to the comfortable
// SDR viewing range.
SetOutput(hdr_paper_white * ICtCpToLinearRGB(ictcp_color));
}
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