Away3D（九）：Lighting & shading
Away3D中的光照可以分为三个基本要素：
- the light source(s) defining the properties of the light(s)
- light source(s) 定义了光照的属性
- shaders providing algorithms that determine how a light source interacts with a material
- shaders 提供一些决定光源如何同材质交互的算法
- a material type combining different shaders along with base colours or bitmaps
- 一种组合 了不同 shaders 和 colors 或者 bitmaps 的 material type
第一个同PV3D不同之处是我们可以指定超过一个光源 。这样会加大CPU的负担，但同时也可以制造出更加真实的场景渲染。
三种主要的 light sources：
- AmbientLight3D which has no position and provides a uniform ambient light to all objects
- AmbientLight3D （环境光源 ）没有空间坐标，他为所有的 object 提供了一种统一的环境光照
- DirectionalLight3D which provides light from a specified position but with a strength that does not diminish with distance
- DirectionalLight3D （定向光源 ），有一个空间坐标，有一个不会减弱的光照强度
- PointLight3D providing light from a specified position that weakens in intensity with distance
- PointLight3D （点光源 ），有一个空间坐标，光照强度会随着距离的增加而减弱
以下几种 shaders：
- AmbientShader which provides ambient shading to a triangle face
- AmbientShader 为一个三角面提供环境阴影
- DiffusePhongShader which, depending on the angle between the light source and a triangle face, produces a diffuse shading pattern
- DiffusePhongShader 根据光源和三角面之间所成的角度，产生一个散射阴影
- SpecularPhongShader which produces specular shading on triangles to imitate a surface that is reflecting light, depending on the observer’s position and the light position
- SpecularPhongShader 根据观察者和光源的位置，为三角面产生一个模拟的反射光源的效果
- EnviroShader allowing for environment mapping, producing a reflection from a surrounding environment on an object, independent of a light source. Requires bitmap data for the environment
- EnviroShader 根据光源位置和环境贴图，给 object 添加一个环境贴图反射的映射
- DiffuseDot3Shader to increase scene reality by adding more small scale structure through the use of normal mapping to an object. Requires bitmap data for the normal map
- DiffuseDot3Shader 通过增加更多的 structure 来增加场景的真实感
以上shaders，除了 EnviroShader ，其余的 light source 都是DirectionalLight3D
我们可以选择使用CompositeMaterials ，这种材质包含了一些不同的 shaders 和基于这些 shaders 的materials。
material列表如下：
PhongBitmapMaterial used to create ambient, diffuse and specular lighting on a texture-mapped material
Base material : BitmapMaterial
Shaders : AmbientShader , DiffusePhongShader and SpecularPhongShader
PhongMovieMaterial used to create ambient, diffuse and specular lighting on a material based on another Flash movie
Base material : MovieMaterial
Shaders : AmbientShader , DiffusePhongShader and SpecularPhongShader
PhongColorMaterial used to create ambient, diffuse and specular lighting on a simple coloured material
Base material : ColorMaterial
Shaders : AmbientShader , DiffusePhongShader and SpecularPhongShader
EnviroBitmapMaterial used to create environmental lighting on a texture-mapped material
Base material : BitmapMaterial
Shaders : EnviroShader
Dot3BitmapMaterial used to create ambient and normal-mapped lighting on a texture-mapped material
Base material : BitmapMaterial
Shaders : AmbientShader and DiffuseDot3Shader ,
Dot3MovieMaterial used to create ambient and normal-mapped lighting on a material based on another Flash movie
Base material : MovieMaterial
Shaders : AmbientShader and DiffuseDot3Shader
以上这些 materials 都是很耗性能的 ，如果你想使用简单点的 shade ，你可以选择CenterLightingMaterial 这种材质的效果就相同于PV3D里的 flat-shaded materials ，Away3D里有两种选择 ：
- ShadingColorMaterial for simple flat shading of a ColorMaterial
- WhiteShadingBitmapMaterial for simple flat shading of a BitmapMaterial , but assuming that the light source is always white (independent of colour given to light source).
// create a new directional white light source with specific ambient, diffuse and specular parameters
var light:DirectionalLight3D = new DirectionalLight3D({color:0xFFFFFF, ambient:0.25, diffuse:0.75, specular:0.9});
light.x = 100;
light.z = 500;
light.y = 500;
scene.addChild(light);
// Create an object container to group the objects on the scene
group = new ObjectContainer3D();
scene.addChild(group);
// Create a new sphere object using a very shiny phong-shaded bitmap material representing the earth
var earthMaterial:PhongBitmapMaterial = new PhongBitmapMaterial(Cast.bitmap(earthBitmap));
earthMaterial.shininess = 100;
sphere = new Sphere({material:earthMaterial, radius:50, segmentsW:10, segmentsH:10});
sphere.x = ORBITAL_RADIUS;
sphere.ownCanvas = true;
group.addChild(sphere);
// Create a new cube object using a tiled, phong-shaded bitmap material
var tiledAway3DMaterial:PhongBitmapMaterial = new PhongBitmapMaterial(Cast.bitmap(away3DBitmap), {repeat:true, scaleX:.5, scaleY:.5});
cube = new Cube({material:tiledAway3DMaterial, width:75, height:75, depth:75});
cube.z = -ORBITAL_RADIUS;
cube.ownCanvas = true;
group.addChild(cube);
// Create a cylinder mapping the earth data again
cylinder = new Cylinder({material:earthMaterial, radius:25, height:100, segmentsW:16});
cylinder.x = -ORBITAL_RADIUS;
cylinder.ownCanvas = true;
group.addChild(cylinder);
// Create a torus object and use a checkered, flat-shaded (from white light) bitmap material
var checkerBitmapMaterial:WhiteShadingBitmapMaterial = new WhiteShadingBitmapMaterial(Cast.bitmap(checkerBitmap));
torus = new Torus({material:checkerBitmapMaterial, radius:40, tube:10, segmentsT:8, segmentsR:16});
torus.z = ORBITAL_RADIUS;
torus.ownCanvas = true;
group.addChild(torus);
// Create a new cube object using a smoothed, precise, phong-shaded, mat (not shiny) bitmap material
var away3DMaterial:PhongBitmapMaterial = new PhongBitmapMaterial(Cast.bitmap(away3DBitmap), {smooth:true, precision:2});
away3DMaterial.shininess = 0;
centerCube = new Cube({material:away3DMaterial, width:75, height:75, depth:75});
centerCube.ownCanvas = true;
group.addChild(centerCube);
// add mouse listeners to all the 3D objects
sphere.addOnMouseDown(onMouseDownOnObject);
cube.addOnMouseDown(onMouseDownOnObject);
cylinder.addOnMouseDown(onMouseDownOnObject);
torus.addOnMouseDown(onMouseDownOnObject);
centerCube.addOnMouseDown(onMouseDownOnObject);
sphere.ownCanvas = true;
这句话很重要，没有这句话，当 objects 被别的 objects 遮挡了 ，他们可能不会被正确渲染