Address some SkLightingShader TODOs

src/effects/SkLightingShader.h

 
 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 
 #ifndef SkLightingShader_DEFINED
 #define SkLightingShader_DEFINED
 
 #include "SkPoint3.h"
 #include "SkShader.h"
 
 class SK_API SkLightingShader {
 public:
     struct Light {
         SkVector3   fDirection;       // direction towards the light (+Z is out of the screen).
                                       // If degenerate, it will be replaced with (0, 0, 1).
-        SkColor     fColor;           // linear (unpremul) color. Note: alpha assumed to be 255.
+        SkColor3f   fColor;           // linear (unpremul) color. Range is 0..1 in each channel.
     };
 
     /** Returns a shader that lights the diffuse and normal maps with a single light.
 
         It returns a shader with a reference count of 1.
         The caller should decrement the shader's reference count when done with the shader.
         It is an error for count to be < 2.
         @param  diffuse     the diffuse bitmap
         @param  normal      the normal map
         @param  light       the light applied to the normal map
-        @param  ambient     the linear (unpremul) ambient light color. Note: alpha assumed to be 255.
+        @param  ambient     the linear (unpremul) ambient light color. Range is 0..1/channel.
         @param  localMatrix the matrix mapping the textures to the dest rect 
 
         NULL will be returned if:
             either 'diffuse' or 'normal' are empty
             either 'diffuse' or 'normal' are too big (> 65535 on a side)
             'diffuse' and 'normal' aren't the same size
+
+        The lighting equation is currently:
+            result = LightColor * DiffuseColor * (Normal * LightDir) + AmbientColor
+
+        The normal map is currently assumed to be an 8888 image where the normal at a texel
+        is retrieved by:
+            N.x = R-127;
+            N.y = G-127;
+            N.z = B-127;
+            N.normalize();
+        The +Z axis is thus encoded in RGB as (127, 127, 255) while the -Z axis is
+        (127, 127, 0).
     */
     static SkShader* Create(const SkBitmap& diffuse, const SkBitmap& normal,
-                            const SkLightingShader::Light& light, const SkColor ambient,
+                            const SkLightingShader::Light& light, const SkColor3f& ambient,
                             const SkMatrix* localMatrix);
 
     SK_DECLARE_FLATTENABLE_REGISTRAR_GROUP()
 };
 
 #endif