From ed9c0dd74df796dfae60aaf0f9698f54b96fae2b Mon Sep 17 00:00:00 2001
From: Geenz <geenz@geenzo.com>
Date: Mon, 25 Apr 2016 02:35:26 -0400
Subject: [PATCH] Apply the correct sRGB conversions.

sRGB does not use a gamma curve of 2.2 - instead it uses a piece-wise
gamma curve that 2.2 is approximate of.  This will cause very subtle
color differences between proper sRGB and "approximate" sRGB -
differences that are noticeable nonetheless.
---
 libraries/gpu/src/gpu/Texture.cpp | 40 +++++++++++++++++++++++++++----
 1 file changed, 36 insertions(+), 4 deletions(-)

diff --git a/libraries/gpu/src/gpu/Texture.cpp b/libraries/gpu/src/gpu/Texture.cpp
index 9f10f2e2b6..db4b444ce7 100755
--- a/libraries/gpu/src/gpu/Texture.cpp
+++ b/libraries/gpu/src/gpu/Texture.cpp
@@ -637,16 +637,48 @@ void SphericalHarmonics::assignPreset(int p) {
     }
 }
 
+// This is based upon the conversions found in section 8.24 of the OpenGL 4.4 4.4 specification.
+// glm::pow(color, 2.2f) is approximate, and will cause subtle differences when used with sRGB framebuffers.
+float sRGBValueToLinearValue(float& value) {
+    const float SRGB_ELBOW = 0.04045f;
+    float linearValue = 0.0f;
+    
+    // This should mirror the conversion table found in section 8.24: sRGB Texture Color Conversion
+    if (value <= SRGB_ELBOW) {
+        linearValue = value / 12.92f;
+    } else {
+        linearValue = powf(((value + 0.055f) / 1.055f), 2.4f);
+    }
+    
+    return linearValue;
+}
 
+// This is based upon the conversions found in section 17.3.9 of the OpenGL 4.4 specification.
+// glm::pow(color, 1.0f/2.2f) is approximate, and will cause subtle differences when used with sRGB framebuffers.
+float LinearValueTosRGBValue(float& value) {
+    const float SRGB_ELBOW_INV = 0.0031308f;
+    float sRGBValue = 0.0f;
+    
+    // This should mirror the conversion table found in section 17.3.9: sRGB Conversion
+    if (value <= 0.0f) {
+        sRGBValue = 0.0f;
+    } else if (0 < value < SRGB_ELBOW_INV) {
+        sRGBValue = 12.92f * value;
+    } else if (SRGB_ELBOW_INV <= value < 1) {
+        sRGBValue = 1.055f * powf(value, 0.41666f - 0.055f);
+    } else {
+        sRGBValue = 1.0f;
+    }
+    
+    return sRGBValue;
+}
 
 glm::vec3 sRGBToLinear(glm::vec3& color) {
-    const float GAMMA_CORRECTION = 2.2f;
-    return glm::pow(color, glm::vec3(GAMMA_CORRECTION));
+    return glm::vec3(sRGBValueToLinearValue(color.x), sRGBValueToLinearValue(color.y), sRGBValueToLinearValue(color.z));
 }
 
 glm::vec3 linearTosRGB(glm::vec3& color) {
-    const float GAMMA_CORRECTION_INV = 1.0f / 2.2f;
-    return glm::pow(color, glm::vec3(GAMMA_CORRECTION_INV));
+    return glm::vec3(LinearValueTosRGBValue(color.x), LinearValueTosRGBValue(color.y), LinearValueTosRGBValue(color.z));
 }
 
 // Originial code for the Spherical Harmonics taken from "Sun and Black Cat- Igor Dykhta (igor dykhta email) � 2007-2014 "