Accessing _GrabTexture from within Cg programs

I am having trouble generating texture coordinates for the grabtexture for use in my Cg fragment code.

There seems to have been a way that worked on 1.x, but doesn't anymore. I took some code from Aras' refraction shader on the wiki, and it seems to work as badly as my own code on 2.0:

Code (csharp):

1.  
2. // Irrelevant details clipped out
3. sampler2D _MainTex : register(s1);
4. samplerRECT _GrabTexture : register(s0);
5.  
6. struct v2f {
7.     V2F_POS_FOG;
8.     float4 uvrefr    : TEXCOORD0;
9.     float2 uv         : TEXCOORD1;
10.     float3 normal    : TEXCOORD2;
11. };
12.  
13. v2f vert (appdata_base v)
14. {
15.     ...
16.     o.uvrefr = mul( glstate.matrix.texture[0], v.vertex );
17.     ...
18.     return o;
19. }
20.  
21. half4 frag( v2f i ) : COLOR
22. {
23.     ...
24.     half4 refr = texRECTproj( _GrabTexture, i.uvrefr );
25.     ...
26. }

I assume, that what is needed is doing an objectspace to viewspace to pixelspace transformation.

I have attempted doing:

Code (csharp):

1. struct v2f {
2.     float4 pos : POSITION;
3.     float2 tex;
4.     float4 uvgrab;
5.     float3 viewDirT;   
6. };
7.  
8. v2f_trace binary_trace_vert (appdata_tan v)
9. {
10.     v2f_trace o;
11.     o.pos = mul( glstate.matrix.mvp, v.vertex );
12.  
13.         // Pretending to know the width and height
14.         // of the viewport
15.     const float viewportwidth = 300;
16.     const float viewportheight = 200;
17.     float4x4 viewtopixels =  
18.   float4x4(viewportwidth/2, 0, 0, 0,
19.            0,  viewportheight/2, 0, 0,
20.            0, 0, 1, 0,
21.            viewportwidth/2, viewportheight/2, 0, 1 );
22.     o.uvgrab = mul(viewtopixels, o.pos);
23.         ...
24. };
25.  
26. float4 binary_trace_frag( v2f_trace i) : COLOR
27. {
28.    ...
29.    // This doesn't really work:
30.    float4 grabdata = texRECTproj(_GrabTexture, i.uvgrab );
31.    // What seems to get close is:
32.    float4 grabdata = texRECT( _GrabTexture, (i.uvgrab.xy + float2(1,1)) * viewportwidth);
33.  
34.    ...
35. }
36.  

So there are two problems here:
- The projective transform doesn't seem to work as I would expect
- I don't really know the size of the viewport from within a shader. Or do I?

 

No worries. I know you are a busy guy

And not for me. I will try to make a new clean attempt to figure out if it was just me, or if it is an issue with my particular card. (Radeon X1900)

Well, this explanation fits pretty much what I expected, and I did try assigning registers and dummy SetTexture, but that shader got really messed up, so I am going to try from scratch with something simple.

Thanks for confirming how it should work

 

I figured it out: USER ERROR!

Yep, I had somehow overlooked the Unity Pro standard refraction shader and was trying to use the 1.x one.

At least this shows how to combine GrabTexture with a custom fragment shader. I will be wanting to write my own vertex shader too, which is going to be interesting, but my confidence in Unity and the Cg compiler has been restored for now.