Obtaining minimum and maximum rendered UV values (Direct3D)

Question

I need to calculate the minimum and maximum UV values assigned to the pixels produced when a given object is drawn onscreen from a certain perspective. For example, if I have a UV-mapped cube but only the front face is visible, min(UV) and max(UV) should be set to the minimum and maximum UV coordinates assigned to the pixels of the visible face.

I'd like to do this using Direct3D 9 shaders (and the lowest shader model possible) to speed up processing. The vertex shader could be as simple as taking each input vertex's UV coordinates and passing them on, unmodified, to the pixel shader. The pixel shader, on the other hand, would have to take the values produced by the vertex shader and use these to compute the minimum and maximum UV values.

What I'd like to know is:

1. How do I maintain state (current min and max values) between invocations of the pixel shader?
2. How do I get the final min and max values produced by the shader into my program?

Is there any way to do this in HLSL, or am I stuck doing it by hand? If HLSL won't work, what would be the most efficient way to do this without the use of shaders?

Answer 1

1) You don't. 2) You would have to do a read back at some point. This will be a fairly slow process and cause a pipeline stall.

In general I can't think of a good way to do this. What exactly are you trying to acheieve with this? There may be some other way to achieve the result you are after.

You "may" be able to get something going using multiple render targets and writing the UVs for each pixel to the render target. Then you'd need to pass the render target back to main memory and then parse it for your min and max values. This is a realy slow and very ugly solution.

If you can do it as a couple of seperate pass you may be able to render to a very small render target and use 1 pass with a Max and 1 pass with a Min alpha blend op. Again ... not a great solution.