View Full Version : relief mapping: artifacts at polygon boundaries

Neelix8472

08-10-2008, 03:51 AM

Hi,

I've been trying to implement the Relief Mapping algorithm found in GPUGems3, which works quite well. However, I've encountered some strange artifacts. When I enable mipmapping, I get wrong pixels at the boundaries between the two triangles as seen in the picture.

I've also tried a simple linear search which yields the same results.

With disabled mip-mapping the result is correct and without relief mapping and enabled mipmapping the result is also correct.

The resulting texture coords are correct, it's just that sampling these coords in a mipmapped texture leads to wrong results.

Has anyone encountered such a problem before?

Thanks,

http://img225.imageshack.us/img225/433/reliefbugqy5.jpg

arekkusu

08-10-2008, 10:51 AM

Is your sample inside of a conditional?

Derivatives are undefined inside conditionals. That's why we have texture*Grad().

Neelix8472

08-10-2008, 11:40 AM

The texture is not sampled inside a conditional.

I don't quite understand what you mean by texture*Grad(). As far as I understand it, mipmapping needs the derivative to work, but I don't see how I can influence this process at all.

Maybe I'm doing something else wrong in the shader?

vertex shader:

varying vec2 vTexCoord;

varying vec3 lightDir, vEyeVec, vTangent, vBinormal, vNormal, vPos;

uniform vec3 vPlayerPosition;

void main()

{

vTangent = gl_MultiTexCoord0.rgb;

vBinormal = gl_MultiTexCoord1.rgb;

vNormal = gl_Normal.rgb;

mat3 TBNMatrix = mat3(vTangent, vBinormal, vNormal);

vEyeVec = vPlayerPosition - gl_Vertex.rgb;

vEyeVec *= TBNMatrix;

vTexCoord = gl_MultiTexCoord2.rg;

gl_Position = ftransform();

}

fragment shader:

varying vec2 vTexCoord;

varying vec3 lightDir, vEyeVec, vTangent, vBinormal, vNormal, vPos;

uniform vec3 vPlayerPosition;

uniform sampler2D oDiffuse;

uniform sampler2D oHeight;

void main (void)

{

const float fDepthScale = 0.1;

const int iNumSteps = 50;

int iStep = 0;

float fStep = (1.0 / float(iNumSteps));

vec2 vDelta = (vEyeVec.xy * fDepthScale / (float(iNumSteps) * vEyeVec.z));

vec2 vCoord = vTexCoord.xy;

float fHeight = 1.0;

vec4 vNormal = vec4(0.0);

//linear search

while (iStep < iNumSteps)

{

vNormal = (1.0 - texture2D (oHeight, vCoord));

iStep++;

fHeight -= fStep;

vCoord -= vDelta;

if(vNormal.r > fHeight)

{

break;

}

}

gl_FragColor.rgb = texture2D (oDiffuse, vCoord.xy).rgb;

}

Neelix8472

08-10-2008, 12:55 PM

Now I understand you^^

I actually didn't know there was a texture2DGrad function.

By using

vec2 dPdx = dFdx(vTexCoord);

vec2 dPdy = dFdy(vTexCoord);

and

gl_FragColor.rgb = texture2DGrad(oDiffuse, vCoord, dPdx, dPdy).rgb;

to sample the texture I did solve the problem.

Thank you very much for your help!

NULL_PTR

08-10-2008, 01:25 PM

On ATI, you have to use texture2DdP_ARB or texture2DGradARB

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