Screen Space Shader

Questions & Answers
1

I’m trying to generate screen space UV coordinates in a fragment shader and I can’t quite seem to get it to turn out right.

I’ve used a snipet that I found on the forums and it’s pretty close:

float4 clipSpace = mul(UNITY_MATRIX_MVP, v.vertex);
clipSpace.xy /= clipSpace.w;
clipSpace.xy = 0.5 * clipSpace.xy + 0.5;
o.uv2 = clipSpace.xy;

However objects with many faces it always seems to be a little bit off. I know this must be possible since this code in a surface shader works fine:

void surf(Input i, inout SurfaceOutput o)
{
    float2 screenUV = i.screenPos.xy / i.screenPos.w;
    ...

However I would rather not use a surface shader, I just want to know how the surface shader is calculating its screenPos variable and what I’m doing wrong.

The top image is the x coordinate of the screen space UV’s generated by a surface shader. Notice that the gradient is nice and smooth unlike the bottom image which is generated by my fragment shader.

Thanks for your help!

30603-screenspaceuvexample.jpg
30603-screenspaceuvexample.jpg639×1031 40.8 KB

2

In case anyone stumbles across this I figured it out:

	v2f vert(appdata_full v)
	{
		o.scrPos = ComputeScreenPos(o.pos);

…

	fixed4 frag(v2f i) : COLOR
	{
		float2 uv2 = (i.scrPos.xy/i.scrPos.w);

…

I guess it doesn’t work to do all the calculations per vert, some have to be done per pixel.