Computing pixel's screen position in a vertex shader: right or wrong?
- by cubrman
I am building a deferred rendering engine and I have a question. The article I took the sample code from suggested computing screen position of the pixel as follows:
VertexShaderFunction()
{
...
output.Position = mul(worldViewProj, input.Position);
output.ScreenPosition = output.Position;
}
PixelShaderFunction()
{
input.ScreenPosition.xy /= input.ScreenPosition.w;
float2 TexCoord = 0.5f * (float2(input.ScreenPosition.x,-input.ScreenPosition.y) + 1);
...
}
The question is what if I compute the position in the vertex shader (which should optimize the performance as VSF is launched significantly less number of times than PSF) would I get the per-vertex lighting insted. Here is how I want to do this:
VertexShaderFunction()
{
...
output.Position = mul(worldViewProj, input.Position);
output.ScreenPosition.xy = output.Position / output.Position.w;
}
PixelShaderFunction()
{
float2 TexCoord = 0.5f * (float2(input.ScreenPosition.x,-input.ScreenPosition.y) + 1);
...
}
What exactly happens with the data I pass from VS to PS? How exactly is it interpolated? Will it give me the right per-pixel result in this case? I tried launching the game both ways and saw no visual difference. Is my assumption right?
Thanks.
P.S. I am optimizing the point light shader, so I actually pass a sphere geometry into the VS.
