From d756e185730f46fd78e88215e0b4b9fd282fd1d7 Mon Sep 17 00:00:00 2001
From: Runitai Linden <davep@lindenlab.com>
Date: Thu, 26 Mar 2020 16:48:33 -0500
Subject: SL-12902 Fix for doing the technically correct but compatibility
 wrong thing WRT light color values.

---
 indra/llmath/llmath.h  | 17 +++++++++++++----
 indra/llmath/v3color.h |  9 +++++++++
 indra/llmath/v4color.h | 11 +++++++++++
 3 files changed, 33 insertions(+), 4 deletions(-)

(limited to 'indra/llmath')

diff --git a/indra/llmath/llmath.h b/indra/llmath/llmath.h
index 57f2489a2d..8f01ad6c1c 100644
--- a/indra/llmath/llmath.h
+++ b/indra/llmath/llmath.h
@@ -537,9 +537,12 @@ inline void ll_remove_outliers(std::vector<VEC_TYPE>& data, F32 k)
 	}
 }
 
-// This converts from a non-linear sRGB floating point value (0..1) to a linear value.
-// Useful for gamma correction and such.  Note: any values passed through this should not be serialized.  You should also ideally cache the output of this.
-inline float sRGBtoLinear(const float val) {
+// Converts given value from a linear RGB floating point value (0..1) to a gamma corrected (sRGB) value.
+// Some shaders require color values in linear space, while others require color values in gamma corrected (sRGB) space.
+// Note: in our code, values labeled as sRGB are ALWAYS gamma corrected linear values, NOT linear values with monitor gamma applied
+// Note: stored color values should always be gamma corrected linear (i.e. the values returned from an on-screen color swatch)
+// Note: DO NOT cache the conversion.  This leads to error prone synchronization and is actually slower in the typical case due to cache misses
+inline float linearTosRGB(const float val) {
     if (val < 0.0031308f) {
         return val * 12.92f;
     }
@@ -548,7 +551,13 @@ inline float sRGBtoLinear(const float val) {
     }
 }
 
-inline float linearTosRGB(const float val) {
+// Converts given value from a gamma corrected (sRGB) floating point value (0..1) to a linear color value.
+// Some shaders require color values in linear space, while others require color values in gamma corrected (sRGB) space.
+// Note: In our code, values labeled as sRGB are gamma corrected linear values, NOT linear values with monitor gamma applied
+// Note: Stored color values should generally be gamma corrected sRGB.  
+//       If you're serializing the return value of this function, you're probably doing it wrong.
+// Note: DO NOT cache the conversion.  This leads to error prone synchronization and is actually slower in the typical case due to cache misses.
+inline float sRGBtoLinear(const float val) {
     if (val < 0.04045f) {
         return val / 12.92f;
     }
diff --git a/indra/llmath/v3color.h b/indra/llmath/v3color.h
index ac78197510..43a632408c 100644
--- a/indra/llmath/v3color.h
+++ b/indra/llmath/v3color.h
@@ -484,4 +484,13 @@ inline const LLColor3 srgbColor3(const LLColor3 &a) {
 	return srgbColor;
 }
 
+inline const LLColor3 linearColor3(const LLColor3 &a) {
+    LLColor3 linearColor;
+    linearColor.mV[0] = sRGBtoLinear(a.mV[0]);
+    linearColor.mV[1] = sRGBtoLinear(a.mV[1]);
+    linearColor.mV[2] = sRGBtoLinear(a.mV[2]);
+
+    return linearColor;
+}
+
 #endif
diff --git a/indra/llmath/v4color.h b/indra/llmath/v4color.h
index 6b91b4f191..00fc955384 100644
--- a/indra/llmath/v4color.h
+++ b/indra/llmath/v4color.h
@@ -656,6 +656,7 @@ void LLColor4::clamp()
 	}
 }
 
+// Return the given linear space color value in gamma corrected (sRGB) space
 inline const LLColor4 srgbColor4(const LLColor4 &a) {
     LLColor4 srgbColor;
 
@@ -667,5 +668,15 @@ inline const LLColor4 srgbColor4(const LLColor4 &a) {
     return srgbColor;
 }
 
+// Return the given gamma corrected (sRGB) color in linear space
+inline const LLColor4 linearColor4(const LLColor4 &a)
+{
+    LLColor4 linearColor;
+    linearColor.mV[0] = sRGBtoLinear(a.mV[0]);
+    linearColor.mV[1] = sRGBtoLinear(a.mV[1]);
+    linearColor.mV[2] = sRGBtoLinear(a.mV[2]);
+    linearColor.mV[3] = a.mV[3];
+}
+
 #endif
 
-- 
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