Merge pull request #338 from KhronosGroup/directional-light

Test for KHR_lights_punctual and BRDF normal distribution function

2.0/DirectionalLight/README.md

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+# Directional Light
+
+## Screenshot
+
+## Purpose
+
+The purpose of this test model is to verify that the BRDF and microfacet model used by a renderer preserves energy.  
+It is intended as a test to measure the amount of light factored in by the distribution functions (normal, shadowing and occlusion) - in this case especially the normal distribution function.  
+At least for realtime rasterizers it is common that the factor returned by this function is applied to the incoming light to calculate specular highlights.  
+In order to preserve energy it is important that this factor does not add light energy.  
+
+The directional lightsource is defined as:
+
+Intensity = 1.0 lumen / m2
+Color = [0.9, 0.8, 0.1]  - in linear RGB space.  
+
+Material roughess:
+Left = 0, Middle = 0.16, Right = 0.33
+
+This means that the color of the specular highlight shall retain the same hue as the lightsource.  
+Where hue is the color without reference to how dull or satururated the color is.  
+This value can be calculated by dividing each RGB component by the total:  
+redHue = R / R + G + B  
+greenHue = G / R + G + B  
+blueHue = B / R + G + B  
+
+Lightsource color = RGB[0.9, 0.8, 0.1]  
+Hue of the lightsource = [0.9 / 1.8, 0.8 / 1.8, 0.1 / 1.8] = [0.5, 0.444, 0.0555]  
+
+The displayed (concentrated) specuar highlight shall retain this hue.  
+Note that the lightvalues are in linear RGB space.    
+If display uses sRGB then the sRGB transfer (gamma) function will be applied to resulting scene linear light values.  
+In this case the values will be roughly [0.96, 0.91, 0.38] depending on exactly what transfer function is applied (this is not specified in the core glTF spec).  
+
+This will of course not be an exact measure since values are read and written with different precision.  
+There will also be a mix with basecolor as the material changes roughness.
+
+## How to check for correct output
+
+The main function tested is the specular highlights, as created by the included directional light.  
+As the lightsource has an intensity of 1 (lumen / m2) and display output can be considered to be candela / m2 it should never be the case that light contribution go above:
+(Color * Intensity) = [0.9, 0.8, 0.1] - in linear space.  
+
+Check the specular hightlight with a color picker and verify that the original hue is retained.  
+For an sRGB display this should be around, depending on the exact gamma/transfer function that is used.  
+HEX : F3E759
+
+Important to note is that the RG values should not be so high that they are clipped, ie if they are FFFF then that may be a sign that energy has been added and further investigation is needed.  
+
+
+## License Information
+
+[![CC0](http://i.creativecommons.org/p/zero/1.0/88x31.png)](http://creativecommons.org/publicdomain/zero/1.0/)  

2.0/DirectionalLight/gltf/DirectionalLight.gltf

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+{
+  "asset": {
+    "generator": "Khronos glTF Blender I/O v1.7.33 + manual edit",
+    "version": "2.0"
+  },
+  "extensionsUsed": [
+    "KHR_lights_punctual"
+  ],
+  "extensionsRequired": [
+    "KHR_lights_punctual"
+  ],
+  "extensions": {
+    "KHR_lights_punctual": {
+      "lights": [
+        {
+          "color": [
+            0.9,
+            0.8,
+            0.1
+          ],
+          "intensity": 1,
+          "type": "directional",
+          "name": "Sun"
+        }
+      ]
+    }
+  },
+  "scene": 0,
+  "scenes": [
+    {
+      "name": "Scene",
+      "nodes": [
+        0,
+        1,
+        2,
+        3,
+        4
+      ]
+    }
+  ],
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+        0.0
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+      "scale": [3, 3, 3]
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+    {
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+      "name": "m0%_r16%",
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+    },
+    {
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+                "light" : 0
+            }
+        },
+        "name" : "Sun_Orientation",
+        "rotation" : [
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+        ]
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+            ],
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+            ]
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+      },
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+    }
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+        ],
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+        "roughnessFactor": 0
+      }
+    },
+    {
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+          0.6,
+          0.6,
+          1
+        ],
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+      }
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