From 50512366c3607dafa142f904e050687627e35378 Mon Sep 17 00:00:00 2001 From: Sam Belliveau Date: Tue, 26 Mar 2024 14:06:56 -0400 Subject: [PATCH] Convert to Hybrid Log Gamma --- Data/Sys/Shaders/PerceptualHDR.glsl | 78 ++++++++++++++++++----------- 1 file changed, 49 insertions(+), 29 deletions(-) diff --git a/Data/Sys/Shaders/PerceptualHDR.glsl b/Data/Sys/Shaders/PerceptualHDR.glsl index 6545992fc4..84cea65302 100644 --- a/Data/Sys/Shaders/PerceptualHDR.glsl +++ b/Data/Sys/Shaders/PerceptualHDR.glsl @@ -17,45 +17,65 @@ DefaultValue = 2.5 /***** Transfer Function *****/ -const float4 m_1 = float4(2610.0 / 16384.0); -const float4 m_2 = float4(128.0 * 2523.0 / 4096.0); -const float4 m_1_inv = float4(16384.0 / 2610.0); -const float4 m_2_inv = float4(4096.0 / (128.0 * 2523.0)); +const float a = 0.17883277; +const float b = 1.0 - 4.0 * a; +const float c = 0.5 - a * log(4.0 * a); -const float4 c_1 = float4(3424.0 / 4096.0); -const float4 c_2 = float4(2413.0 / 4096.0 * 32.0); -const float4 c_3 = float4(2392.0 / 4096.0 * 32.0); +float HLG_f(float x) +{ + if (x < 0.0) { + return 0.0; + } -float4 EOTF_inv(float4 lms) { - float4 y = pow(lms, m_1); - return pow((c_1 + c_2 * y) / (1.0 + c_3 * y), m_2); + else if (x < 1.0 / 12.0) { + return sqrt(3.0 * x); + } + + return a * log(12.0 * x - b) + c; } -float4 EOTF(float4 lms) { - float4 x = pow(lms, m_2_inv); - return pow(-(x - c_1) / (c_3 * x - c_2), m_1_inv); +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; } -// This is required as scaling in EOTF space is not linear. -float EOTF_AMPLIFICATION = EOTF_inv(float4(AMPLIFICATION)).x; +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; + 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, +6610.0, +17933.0, 0.0, - +2048.0, -13613.0, -17390.0, 0.0, - +0.0, +7003.0, -543.0, 0.0, - +0.0, +0.0, +0.0, 4096.0) / 4096.0; + +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 * EOTF_inv(RGBtoLMS * c); + return LMStoICtCp * HLG(RGBtoLMS * c); } /***** ICtCp <--> Linear *****/ @@ -65,7 +85,7 @@ mat4 LMStoRGB = inverse(RGBtoLMS); float4 ICtCpToLinearRGB(float4 c) { - return LMStoRGB * EOTF(ICtCptoLMS * c); + return LMStoRGB * HLG_inv(ICtCptoLMS * c); } void main() @@ -88,19 +108,19 @@ void main() // Scale the color in perceptual space depending on the percieved luminance. // - // At low luminances, ~0.0, pow(EOTF_AMPLIFICATION, ~0.0) ~= 1.0, so the + // 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(EOTF_AMPLIFICATION, ~1.0) ~= EOTF_AMPLIFICATION, - // which is equivilant to scaling the color by EOTF_AMPLIFICATION. This is + // At high luminances, ~1.0, pow(AMPLIFICATION, ~1.0) ~= AMPLIFICATION, + // which is equivilant 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 const float luminance = ictcp_color.x; - ictcp_color *= pow(EOTF_AMPLIFICATION, luminance); + ictcp_color *= HLG_f(pow(AMPLIFICATION, luminance)); // Convert back to Linear RGB and output the color to the display. // We use hdr_paper_white to renormalize the color to the comfortable