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Geometrian

Peculiar Banding Normal Mapping

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Hi, I've got a strange problem normal mapping. For the most part, the mapping looks fine, but there is a strange bright band that sometimes appears--more with some maps than others. Especially with low intensity normal maps, the band becomes a serious issue. I think the problem lies in the shader code, which is a Frankenstein of shaders I found, assembled naively by me. Here's a screenshot of a particularly nasty banding pattern (normally, it's not this bad): Normal Map Failure Here's the vertex shader (glsl):
varying vec3 lightVec; 
varying vec3 eyeVec;
varying vec2 texCoord;
attribute vec3 vTangent; 
					 

void main(void)
{
    gl_Position = ftransform();
    texCoord = gl_MultiTexCoord0.xy;


    vec3 c1 = cross(gl_Normal, vec3(0.0, 0.0, 1.0)); 
    vec3 c2 = cross(gl_Normal, vec3(0.0, 1.0, 0.0)); 
    if(length(c1)>length(c2))
    {
        vTangent = c1;
    }
    else
    {
        vTangent = c2;
    }
    vTangent = normalize(vTangent);


    vec3 n = normalize(gl_NormalMatrix * gl_Normal);
    vec3 t = normalize(gl_NormalMatrix * vTangent);
    vec3 b = cross(n, t);

    vec3 vVertex = vec3(gl_ModelViewMatrix * gl_Vertex);
    vec3 tmpVec = gl_LightSource[0].position.xyz - vVertex;

    lightVec.x = dot(tmpVec, t);
    lightVec.y = dot(tmpVec, b);
    lightVec.z = dot(tmpVec, n);

    tmpVec = -vVertex;
    eyeVec.x = dot(tmpVec, t);
    eyeVec.y = dot(tmpVec, b);
    eyeVec.z = dot(tmpVec, n);
}

and here is the fragment shader:
varying vec3 lightVec;
varying vec3 eyeVec;
varying vec2 texCoord;
uniform sampler2D normalMap;
uniform float invRadius;

void main (void)
{
	float distSqr = dot(lightVec, lightVec);
	float att = clamp(1.0 - invRadius * sqrt(distSqr), 0.0, 1.0);
        att = 1.0;
	vec3 lVec = lightVec * inversesqrt(distSqr);

	vec3 vVec = normalize(eyeVec);
	
	vec3 bump = normalize( texture2D(normalMap, texCoord).xyz * 2.0 - 1.0);

	vec4 vAmbient = gl_LightSource[0].ambient * gl_FrontMaterial.ambient;

	float diffuse = max( dot(lVec, bump), 0.0 );
	
	vec4 vDiffuse = gl_LightSource[0].diffuse * gl_FrontMaterial.diffuse * diffuse;	

	float specular = pow(clamp(dot(reflect(-lVec, bump), vVec), 0.0, 1.0), gl_FrontMaterial.shininess );
	
	vec4 vSpecular = gl_LightSource[0].specular * gl_FrontMaterial.specular * specular;	
	
	gl_FragColor = ( vAmbient + vDiffuse + vSpecular) * att;
}

I really need this problem fixed. Can anyone give me any pointers or help on the matter? Thanks, Geometrian

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