Pixel Coords in Pixel Shader

OpenGL OpenGL: Advanced Coding
1

hi.

i’ve tried to read the pixel value(into a texture),
when I perform my pixel shader.I would want read this value in the same coords where the pixel shader is executed.

I make it in this way:

Vertex-Frogram ------

struct Out{
float4 HPOS : POSITION;
float2 LookUp : TEXCOORD0;
};

Temp=mul(ModelViewProj,IN.position_main)
Temp.x=Temp.x/Temp.z;
Temp.y=Temp.y/Temp.z;

Out.LookUp.x=(Temp.x*0.5+0.5)640;
Out.LookUp.y=(Temp.y0.5+0.5)*480;

Fragment Program -----

struct Inputs{
float4 HPOS : POSITION;
float2 LookUp : TEXCOORD0;
};

struct Outputs{
float4 Color : COLOR;
};

Out main(uniform sampler2D Texure)
{
Out.Color=tex2D(Texure,LookUp.xy);
};

is this the correct way to perform this task?

thanks in advance Sandro

2

Hello,

I have the same problem : I want to do a multipass rendering (front to back).

Thus I would like to read a texture (pbuffer) that represents the former pass while I am drawing some polygons.

And so I need to convert the fragment position to texture/screen coordinates.

I want to have the maximal performance so I do not want to perform this at the fragment level but at the vertex level:

  • Vertex program converts position (after transformation) to texture coordinates for my screen aligned pbuffer.
  • In the fragment program a “simple” TXP instruction performs the texture sampling.

I do not achieve this because I am mixing lots of matrices and I cannot figure out how to perform the convenient rescaling at the vertex level and to combine this with projective mapping.

Paul

PS I am still using fragment and vertex programs.

3

Hello,
You can get the current fragment coordinates in screen pixels, declaring a WPOS parameter directly in your fragment shader (however, this requires to run shader 2.0 or more, or this won’t be available).

4

i tryed to declare wpos,but doens’t work.this is my shader

struct Inputs{
float4 HPOS : WPOS;
float3 Normal : TEXCOORD1;
};

Outputs main(Inputs IN,
uniform samplerRECT Buffer)
{
Outputs OUT;

OUT.Color=0.1f+texRECT(Buffer,IN.HPOS.xy);

}

is correct it?

thanks
sandro

5

Originally posted by CoinCoin:
Hello,
You can get the current fragment coordinates in screen pixels, declaring a WPOS parameter directly in your fragment shader (however, this requires to run shader 2.0 or more, or this won’t be available).
dont know if this is any better in glsl (somehow i doubt it), but accessing the fragment position in a fragment program turned out to be slow as hell for me (so in fact storing the transformation result in a second attribute and calculating it myself was a ton faster)… might of course be completely different on nvidia hardware ,-)

6

Hello,
Accessing WPOS did not cause any perf issue, but I only did test that on NVidia hardware…
Sandruzzo, your texture sampling at the current pixel seems correct to me.