glTF 2.0: New KHR_lights extension

[McNopper]

As the lights have to be extracted from the KHR_materials_common extension, here is a first try by example:

{
  "lights": [
    {
      "ambient": {
        "color": [
          1,
          1,
          1
        ]
      }
    },
    {
      "directional": {
        "color": [
          1,
          1,
          1
        ]
      }
    },
    {
      "point": {
        "color": [
          1,
          1,
          1
        ],
        "constantAttenuation": 1,
        "linearAttenuation": 0.5,
        "quadraticAttenuation": 0.25
      }
    },
    {
      "spot": {
        "color": [
          1,
          1,
          1
        ],
        "constantAttenuation": 1,
        "fallOffAngle": 40,
        "fallOffExponent": 0,
        "linearAttenuation": 0.5,
        "quadraticAttenuation": 0.25
      }
    }
  ]
}

Directional, point and spot are obvious for PBR and non-PBR materials.

Regarding ambient and for PBR, this needs to be further discussed and explained. But I suggest to treat the value as if an environment map is used, which does have just this color.

[McNopper]

In general, a node will have no or one entry of a light with its index.
Exception is, that no ambient light is allowed to be assigned to a node.

An ambient light can only be assigned to a scene.
In opposite, for a scene, only the ambient light can be assigend.

Is this approach accepted?

[lexaknyazev]

An ambient light can only be assigned to a scene.

Lgtm. Ambient light has no transform to apply.
Also, add required light.type string property (as camera.type).

[McNopper]

I will implement this extension. If it works out fine, I will create a schema and bring it back to the glTF working group.

[McNopper]

"extensions" : {
        "KHR_lights" : {
            "lights" : [
                {
                    "color" : [
                        1.0, 
                        1.0, 
                        1.0
                    ], 
                    "name" : "Directional", 
                    "type" : "directional"
                }, 
                {
                    "color" : [
                        1.0, 
                        1.0, 
                        1.0
                    ], 
                    "name" : "Point", 
                    "point" : {
                        "quadraticAttenuation" : 1.0
                    }, 
                    "type" : "point"
                }, 
                {
                    "color" : [
                        1.0, 
                        1.0, 
                        1.0
                    ], 
                    "name" : "Spot", 
                    "spot" : {
                        "fallOffAngle" : 0.7853981852531433, 
                        "fallOffExponent" : 19.200000762939453, 
                        "quadraticAttenuation" : 1.0
                    }, 
                    "type" : "spot"
                }, 
                {
                    "color" : [
                        0.0, 
                        0.0, 
                        0.0
                    ], 
                    "name" : "Ambient_Scene", 
                    "type" : "ambient"
                }
            ]
        }
    },

This is the current export. Moved the color 'up', as it does appear in all light types.

[stevenvergenz]

Should this extension also address the use case of pre-computed lighting? Perhaps an additional optional texture map in materials?

[McNopper]

Yes, pre-computed lighting should also go into materials like e.g. occlusion. I suggest to create another extension for this, where all light maps come in.

[UX3D-nopper]

As promised in yesterdays call, this is the current syntax of the lights export by example:

{
    "accessors" : [
        {
            "bufferView" : 0, 
            "componentType" : 5121, 
            "count" : 6, 
            "max" : [
                3
            ], 
            "min" : [
                0
            ], 
            "type" : "SCALAR"
        }, 
        {
            "bufferView" : 1, 
            "componentType" : 5126, 
            "count" : 4, 
            "max" : [
                1.0, 
                0.0, 
                1.0
            ], 
            "min" : [
                -1.0, 
                0.0, 
                -1.0
            ], 
            "type" : "VEC3"
        }, 
        {
            "bufferView" : 2, 
            "componentType" : 5126, 
            "count" : 4, 
            "max" : [
                0.0, 
                1.0, 
                -0.0
            ], 
            "min" : [
                0.0, 
                1.0, 
                -0.0
            ], 
            "type" : "VEC3"
        }
    ], 
    "asset" : {
        "generator" : "Khronos Blender glTF 2.0 exporter", 
        "version" : "2.0"
    }, 
    "bufferViews" : [
        {
            "buffer" : 0, 
            "byteLength" : 6, 
            "byteOffset" : 0, 
            "target" : 34963
        }, 
        {
            "buffer" : 0, 
            "byteLength" : 48, 
            "byteOffset" : 6, 
            "target" : 34962
        }, 
        {
            "buffer" : 0, 
            "byteLength" : 48, 
            "byteOffset" : 54, 
            "target" : 34962
        }
    ], 
    "buffers" : [
        {
            "byteLength" : 102, 
            "uri" : "04_lights.bin"
        }
    ], 
    "cameras" : [
        {
            "name" : "Camera", 
            "perspective" : {
                "aspectRatio" : 1.703595982340029, 
                "yfov" : 0.5033799409866333, 
                "zfar" : 100.0, 
                "znear" : 0.10000000149011612
            }, 
            "type" : "perspective"
        }
    ], 
    "extensions" : {
        "KHR_lights" : {
            "lights" : [
                {
                    "color" : [
                        1.0, 
                        1.0, 
                        1.0
                    ], 
                    "name" : "Directional", 
                    "type" : "directional"
                }, 
                {
                    "color" : [
                        1.0, 
                        0.0, 
                        0.0
                    ], 
                    "constantAttenuation" : 1.0, 
                    "name" : "Point_Constant", 
                    "type" : "point"
                }, 
                {
                    "color" : [
                        0.0, 
                        1.0, 
                        0.0
                    ], 
                    "linearAttenuation" : 0.1, 
                    "name" : "Point_Linear", 
                    "type" : "point"
                }, 
                {
                    "color" : [
                        0.0, 
                        0.0, 
                        1.0
                    ], 
                    "name" : "Point_Quadratic", 
                    "quadraticAttenuation" : 0.1, 
                    "type" : "point"
                }, 
                {
                    "color" : [
                        1.0, 
                        0.0, 
                        0.0
                    ], 
                    "constantAttenuation" : 1.0, 
                    "fallOffAngle" : 0.1745329201221466, 
                    "fallOffExponent" : 0.0, 
                    "name" : "Spot_Constant", 
                    "type" : "spot"
                }, 
                {
                    "color" : [
                        0.0, 
                        1.0, 
                        0.0
                    ], 
                    "fallOffAngle" : 0.3490658402442932, 
                    "fallOffExponent" : 64.0, 
                    "linearAttenuation" : 0.1, 
                    "name" : "Spot_Linear", 
                    "type" : "spot"
                }, 
                {
                    "color" : [
                        0.0, 
                        0.0, 
                        1.0
                    ], 
                    "fallOffAngle" : 0.5235987901687622, 
                    "fallOffExponent" : 128.0, 
                    "name" : "Spot_Quadratic", 
                    "quadraticAttenuation" : 0.1, 
                    "type" : "spot"
                }, 
                {
                    "color" : [
                        0.0, 
                        0.0, 
                        0.0
                    ], 
                    "name" : "Ambient_Scene", 
                    "type" : "ambient"
                }
            ]
        }
    }, 
    "extensionsRequired" : [
        "KHR_materials_common", 
        "KHR_lights"
    ], 
    "extensionsUsed" : [
        "KHR_materials_common", 
        "KHR_lights"
    ], 
    "materials" : [
        {
            "emissiveFactor" : [
                0.0, 
                0.0, 
                0.0
            ], 
            "extensions" : {
                "KHR_materials_common" : {
                    "ambientFactor" : [
                        1.0, 
                        1.0, 
                        1.0
                    ], 
                    "diffuseFactor" : [
                        0.800000011920929, 
                        0.800000011920929, 
                        0.800000011920929, 
                        1.0
                    ], 
                    "shininessFactor" : 12.298039215686275, 
                    "specularFactor" : [
                        0.5, 
                        0.5, 
                        0.5
                    ], 
                    "type" : "commonPhong"
                }
            }, 
            "name" : "Plane"
        }
    ], 
    "meshes" : [
        {
            "name" : "Plane", 
            "primitives" : [
                {
                    "attributes" : {
                        "NORMAL" : 2, 
                        "POSITION" : 1
                    }, 
                    "indices" : 0, 
                    "material" : 0
                }
            ]
        }
    ], 
    "nodes" : [
        {
            "camera" : 0, 
            "name" : "Correction_Camera", 
            "rotation" : [
                -0.7071067690849304, 
                -0.0, 
                0.0, 
                0.7071067690849304
            ]
        }, 
        {
            "children" : [
                0
            ], 
            "name" : "Camera", 
            "rotation" : [
                0.5730898976325989, 
                0.0, 
                -0.0, 
                0.8194926381111145
            ], 
            "translation" : [
                0.0, 
                12.0, 
                30.0
            ]
        }, 
        {
            "extensions" : {
                "KHR_lights" : {
                    "light" : 0
                }
            }, 
            "name" : "Correction_Directional", 
            "rotation" : [
                -0.7071067690849304, 
                -0.0, 
                0.0, 
                0.7071067690849304
            ]
        }, 
        {
            "children" : [
                2
            ], 
            "name" : "Directional", 
            "rotation" : [
                0.2588190734386444, 
                0.0, 
                -0.0, 
                0.965925931930542
            ], 
            "translation" : [
                -8.0, 
                4.0, 
                8.0
            ]
        }, 
        {
            "mesh" : 0, 
            "name" : "Plane", 
            "scale" : [
                10.0, 
                10.0, 
                10.0
            ]
        }, 
        {
            "extensions" : {
                "KHR_lights" : {
                    "light" : 1
                }
            }, 
            "name" : "Correction_Point_Constant", 
            "rotation" : [
                -0.7071067690849304, 
                -0.0, 
                0.0, 
                0.7071067690849304
            ]
        }, 
        {
            "children" : [
                5
            ], 
            "name" : "Point_Constant", 
            "translation" : [
                -8.0, 
                4.0, 
                -0.0
            ]
        }, 
        {
            "extensions" : {
                "KHR_lights" : {
                    "light" : 2
                }
            }, 
            "name" : "Correction_Point_Linear", 
            "rotation" : [
                -0.7071067690849304, 
                -0.0, 
                0.0, 
                0.7071067690849304
            ]
        }, 
        {
            "children" : [
                7
            ], 
            "name" : "Point_Linear", 
            "translation" : [
                0.0, 
                4.0, 
                -0.0
            ]
        }, 
        {
            "extensions" : {
                "KHR_lights" : {
                    "light" : 3
                }
            }, 
            "name" : "Correction_Point_Quadratic", 
            "rotation" : [
                -0.7071067690849304, 
                -0.0, 
                0.0, 
                0.7071067690849304
            ]
        }, 
        {
            "children" : [
                9
            ], 
            "name" : "Point_Quadratic", 
            "translation" : [
                8.0, 
                4.0, 
                -0.0
            ]
        }, 
        {
            "extensions" : {
                "KHR_lights" : {
                    "light" : 4
                }
            }, 
            "name" : "Correction_Spot_Constant", 
            "rotation" : [
                -0.7071067690849304, 
                -0.0, 
                0.0, 
                0.7071067690849304
            ]
        }, 
        {
            "children" : [
                11
            ], 
            "name" : "Spot_Constant", 
            "rotation" : [
                -0.2588190734386444, 
                0.0, 
                -0.0, 
                0.965925931930542
            ], 
            "translation" : [
                -8.0, 
                4.0, 
                -8.0
            ]
        }, 
        {
            "extensions" : {
                "KHR_lights" : {
                    "light" : 5
                }
            }, 
            "name" : "Correction_Spot_Linear", 
            "rotation" : [
                -0.7071067690849304, 
                -0.0, 
                0.0, 
                0.7071067690849304
            ]
        }, 
        {
            "children" : [
                13
            ], 
            "name" : "Spot_Linear", 
            "rotation" : [
                -0.2588190734386444, 
                0.0, 
                -0.0, 
                0.965925931930542
            ], 
            "translation" : [
                0.0, 
                4.0, 
                -8.0
            ]
        }, 
        {
            "extensions" : {
                "KHR_lights" : {
                    "light" : 6
                }
            }, 
            "name" : "Correction_Spot_Quadratic", 
            "rotation" : [
                -0.7071067690849304, 
                -0.0, 
                0.0, 
                0.7071067690849304
            ]
        }, 
        {
            "children" : [
                15
            ], 
            "name" : "Spot_Quadratic", 
            "rotation" : [
                -0.2588190734386444, 
                0.0, 
                -0.0, 
                0.965925931930542
            ], 
            "translation" : [
                8.0, 
                4.0, 
                -8.0
            ]
        }
    ], 
    "scene" : 0, 
    "scenes" : [
        {
            "extensions" : {
                "KHR_lights" : {
                    "light" : 7
                }
            }, 
            "name" : "Scene", 
            "nodes" : [
                16, 
                14, 
                12, 
                10, 
                8, 
                6, 
                4, 
                3, 
                1
            ]
        }
    ]
}

[UX3D-nopper]

Also, regarding the ambient light and PBR, here is a quick render from Blender:
From my side, we can keep ambient light with PBR, but we should not use it as a factor for the later environment light.

[donmccurdy]

We are implementing this extension in three.js with mrdoob/three.js#11551. Would this article be an appropriate introduction to the lighting model described here? I am making the following assumptions:

• constantAttenuation applies a factor multiplier to the intensity of the light uniformly, at all distances.
• linearAttenuation applies a factor multiplier to intensity per unit distance, such that intensity will reach 0 at distance 1 / linearAttenuation.

I am not sure I understand the difference between quadraticAttenuation and fallOffExponent. I assume we want to modify intensity by a factor 1/d^power, where physically realistic lighting would have power=2. Is that power defined by quadraticAttenuation or fallOffExponent, and what is the other value?