Simplify linearTosRGB(sRGBToLinear(srgbIn) * k) (#592)

* Simplify linearTosRGB(sRGBToLinear(srgbIn) * k)

On Adreno, brings performance improvement (e.g. 5FPS in this case https://mapbox.atlassian.net/browse/MAPS3D-912?focusedCommentId=138071)

* simplify sRGBToLinear(vec3 srgbIn)

src/shaders/_prelude.glsl

@@ -20,7 +20,13 @@ vec3 linearTosRGB(vec3 color)
 // sRGB to linear approximation
 vec3 sRGBToLinear(vec3 srgbIn)
 {
- return vec3(pow(srgbIn.xyz, vec3(2.2)));
+ return pow(srgbIn, vec3(2.2));
+}
+
+// equivalent to linearTosRGB(sRGBToLinear(srgbIn) * k)
+vec3 linearProduct(vec3 srgbIn, vec3 k)
+{
+ return srgbIn * pow(k, vec3(1./2.2));
 }
 
 // Apply depth for wireframe overlay to reduce z-fighting

src/shaders/_prelude_lighting.glsl

@@ -44,7 +44,7 @@ vec3 apply_lighting(vec3 color, vec3 normal, float dir_factor) {
  float ambient_directional_factor = calculate_ambient_directional_factor(normal);
  vec3 ambient_contrib = ambient_directional_factor * u_lighting_ambient_color;
  vec3 directional_contrib = u_lighting_directional_color * dir_factor;
- return linearTosRGB(sRGBToLinear(color) * (ambient_contrib + directional_contrib));
+ return linearProduct(color, ambient_contrib + directional_contrib);
 }
 
 vec4 apply_lighting(vec4 color, vec3 normal, float dir_factor) {
@@ -69,7 +69,7 @@ vec3 apply_lighting_ground(vec3 color) {
  // Emulate sky being brighter close to the main light source
  float ambient_directional_factor = calculate_ambient_directional_factor(vec3(0.0, 0.0, 1.0));
  vec3 ambient_contrib = u_lighting_ambient_color * ambient_directional_factor;
- return linearTosRGB(sRGBToLinear(color) * (ambient_contrib + u_lighting_directional_color * NdotL));
+ return linearProduct(color, ambient_contrib + u_lighting_directional_color * NdotL);
 }
 
 vec4 apply_lighting_ground(vec4 color) {