July 30, 2010 8 Comments
This is going to be a very brief tutorial; the idea for it came about from a comment on my very first tutorial about using multiple viewports. I assumed that using multiple viewports would be a simple matter of just calling a SetViewport method just like in DX9, but it isn’t. I tried finding some info online but there is absolutely nothing available so I had to figure it out on my own. There are two methods to get multiple viewports working. The first requires a state change when selecting the viewports but I don’t think that the cost of that is too prohibitive since you would probably only swap viewports once per viewport during the scene rendering. The second method involves using a geometry shader to specify which viewport to use during the clipping/screen mapping stage in the pipeline.
What is a viewport
Well lets first discuss what a viewport actually is, if you Google a bit you’ll find almost no information regarding viewports or what they actually are (and there is absolutely no info in the DX documentation). A viewport is a rectangle that defines the area of the frame buffer that you are rendering to. Viewports do have depth values which affect the projected z range of any primitives in the viewport but this is only used in very advanced cases so you should always set the near depth to 0 and the far depth to 1. If we imagine a car game in which we have a rear view mirror, a simple method to draw the rear view mirror contents is to set the viewport to the mirror area, rotate the camera to face backwards and render the scene. Another common use in games is when you see another player’s viewpoint within your HUD (Ghost recon does this quite often), once again to render this all that is require is to set the viewport to the area of your final image you want to render to, then you render the scene from the other players viewpoint. Read more of this post