GrahamH

07-01-2008, 09:23 AM

I have written a fragment shader that takes a texture as an input does a little math and outputs values into an FBO. Everything works fine on an nVidia G80, but goes wildly wrong on an ATI 2400 XT. Here is the code:

/************************************************** ***********************/

uniform sampler2DRect knotPoints;

#define kp(index) (texture2DRect(knotPoints, vec2(index)).r)

uniform sampler2DRect verts;

//Uniform Inputs

uniform ivec4 numParams; // { degree, cp, kp, foo }

int FindSpan(float u) {

//Check the special case

if (u >= kp(numParams.y)) return numParams.y-1;

//Set up search

ivec3 lmh = ivec3(numParams.x, 0, numParams.y);

lmh.y = (lmh.x + lmh.z) / 2;

//Return the span value

return lmh.y;

}

void main(void) {

int i, j, span, index;

//Get basic vertex info

vec4 inVert = texture2DRect(verts, floor(gl_FragCoord.xy));

//Find the span for the vertex

span = FindSpan(inVert.x);

//Return the values

gl_FragColor = vec4(inVert.x, span, numParams.x, numParams.y);

}

/************************************************** ***********************/

As you can see, it is pretty simple. This issue is that when I run it on the ATI card (on XP) the divide by 2 in FindSpan returns a very wrong answer (like 1e-10). If I remove the divide it correctly adds the two values.

Am I doing something wrong or are there known issues with this card?

Thanks,

Graham

/************************************************** ***********************/

uniform sampler2DRect knotPoints;

#define kp(index) (texture2DRect(knotPoints, vec2(index)).r)

uniform sampler2DRect verts;

//Uniform Inputs

uniform ivec4 numParams; // { degree, cp, kp, foo }

int FindSpan(float u) {

//Check the special case

if (u >= kp(numParams.y)) return numParams.y-1;

//Set up search

ivec3 lmh = ivec3(numParams.x, 0, numParams.y);

lmh.y = (lmh.x + lmh.z) / 2;

//Return the span value

return lmh.y;

}

void main(void) {

int i, j, span, index;

//Get basic vertex info

vec4 inVert = texture2DRect(verts, floor(gl_FragCoord.xy));

//Find the span for the vertex

span = FindSpan(inVert.x);

//Return the values

gl_FragColor = vec4(inVert.x, span, numParams.x, numParams.y);

}

/************************************************** ***********************/

As you can see, it is pretty simple. This issue is that when I run it on the ATI card (on XP) the divide by 2 in FindSpan returns a very wrong answer (like 1e-10). If I remove the divide it correctly adds the two values.

Am I doing something wrong or are there known issues with this card?

Thanks,

Graham