在2D实拍视频中合成3D动画
发表时间：2005-10-26 6. Multi-pass rendering.
6. 多次渲染
To allow me to control the attributes of each shadow individually in post production I'm going to render each shadow out separately. In my example there are three shadows, a downward shadow beneath objects, and then a long shadow cast behind. I've split the render of the long shadow into two passes, one for the vertical objects such as the cup and the CPU, and one for the horizontal objects such as the desk. This is because the horizontal shadows are much lighter than those cast on vertical objects and I'd like the ability to control this in post production.
为了在后期处理中能分别控制每一个阴影的属性，我将会分别渲染出每个阴影。在例子中有三个阴影：在物体下指向下的阴影，然后是物体后的长阴影。我把长阴影的渲染分成两步，一是对于垂直的物体例如、杯子和主机，另一个是为水平物体例如桌面。这是因为水平方向的阴影比在垂直物体上投射的阴影要轻的多，我想要在后期处理中能控制这些特性。
So now we want to render out out shadows for the whole sequence. MAX comes with a material called 'shadow/matte' that will only render shadows cast on the object this is ideal for our purposes. We can apply this material to the objects in the scene such as my proxy CPU, desk, wall and cup.
现在我要为整个场景渲染出阴影。MAX有一种材质叫做'shadow/matte'（阴影/遮罩），它将只渲染被投射在其物体上的阴影，这对我们达到目的是理想的材质。我们可对场景中的虚拟物体例如主机、桌子、墙壁和杯子赋予这种材质。
Now if we set the environment background to white and render out a shot you'll see you only get the object and the shadows it casts.
现在如果我们将场景的环境背景设置为白色并渲染出，你发现你只得到了小车物体和它投下的阴影。
The problem is we still have the 3D object in the shot, which we really don't want when we come to post production. When using ray traced shadows MAX won't allow you to see shadows without the object being visible, so we'll use a plugin texture called which is available from the nice people at : http://www.blur.com/BlurHome/tech/
问题是在镜头中仍有我们在后期不想看到的3D物体。当时用光线跟踪类型的阴影时MAX不允许在物体不可见是投射阴影，所以我们将使用一个叫做'cast shadows only'（只投射阴影）的纹理插件。插件可从http://www.blur.com/BlurHome/tech/下载。
This material can now be applied to the object (in my example the car) so that when we render we only see shadows. Now you'll have to play with the shadow cast/receive options for your proxy objects as obviously we don't want them to cast shadows on each other.
这个材质现在可以被赋予物体（在这个例子中是小车），结果是当渲染时我们将只看见阴影。现在我们必须调整虚拟物体的阴影产生、接收选项，以避免它们之间产生阴影。
Now we can render our shadows out one by one. In my case this involved rendering out 300 frames for each shadow, each time turning lights on or off and altering the cast/receive shadows options for various objects.
现在我们能一个个渲染出阴影了。这个例子中我为每阴影渲染300帧图像，每次为不同的物体设置阴影产生、接收选项和调整光源的开闭．
As you can see above you now have three sets of shadows. Take these frames and store them for later, once again keeping a proxy of each for compositing.
从上图可看出现在有三种类型的阴影。储存这些帧，再次为每段设定一个代理。
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