Getting an arbitrary point position in Screen Space in a shader

Hi,

I'm trying to get the screen space position of a world space point i set through a property and i assumed this would work:

Code (CSharp):

1. Properties
2. {
3. ...
4. _RandomPointWorldPos("Random Point Position In World Space", Vector) = (0.0, 0.0, 0.0, 1.0)
5. }
6. SubShader
7. {
8. Pass
9. {
10. ...
11. uniform half4 _RandomPointWorldPos;
12.  
13. struct vertexOutput
14. {
15. float4 pos : SV_POSITION;
16. half4 screenPos : TEXCOORD0;
17. half4 randomPointScreenPos : TEXCOORD1;
18. };
19.  
20. vertexOutput vert(vertexInput input)
21. {
22. vertexOutput output;
23.  
24. output.pos = mul(UNITY_MATRIX_MVP, input.vertex);
25. output.screenPos = ComputeScreenPos(output.pos);
26. output.randomPointScreenPos = ComputeScreenPos( mul(UNITY_MATRIX_VP, _RandomPointWorldPos) );
27.  
28. return output;
29. }
30. ...
31. }
32. }

But it doesn't, as if _RandomPointWorldPos was already in screen pos and the whole ComputeScreenPos Stuff wasn't doing anything.
Maybe i'm doing it totally wrong here but i don't know how i could resolve this.

Thanks for your help!