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Normals For Distorted Sphere

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Is there a conceptual flaw in this GLSL vertex shader method for calculating a normal for a sphere?
varying vec3 norm;

struct spherical { float r, phi, theta; };

spherical cartesianToSpherical(vec3 cPoint) {
	spherical sCoords;
	float xyLen = length(cPoint.xy);
	sCoords.r = length(cPoint);  
	sCoords.phi = acos(cPoint.z / sCoords.r);
	sCoords.theta = atan(cPoint.x, cPoint.y);
	return sCoords;
}

vec3 sphericalToCartesian(spherical sPoint) {
	vec3 cCoords;
	cCoords.x = sPoint.r * sin(sPoint.phi) * cos(sPoint.theta);
	cCoords.y = sPoint.r * sin(sPoint.phi) * sin(sPoint.theta);
	cCoords.z = sPoint.r * cos(sPoint.phi);
	return cCoords;
}

void main()
{
	vec4 vert = gl_Vertex;
	
	spherical sVert = cartesianToSpherical(vert.xyz);
	
	float offset = 0.01;
	
	spherical tanPos = spherical(sVert.r,sVert.phi + offset,sVert.theta);
	vec3 tangent = sphericalToCartesian(tanPos);
	tangent -= vert.xyz;

	spherical bitanPos = spherical(sVert.r,sVert.phi,sVert.theta + offset);
	vec3 bitangent = sphericalToCartesian(bitanPos);
	bitangent -=vert.xyz;
	
	norm = gl_NormalMatrix * normalize(cross(tangent,bitangent));
	
	//Transform vertex by modelview and projection matrices
	gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
	
	//Forward current color and texture coordinates after applying texture matrix
	gl_FrontColor = gl_Color;
	gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;
}
I know how to work out the normal for a sphere the easy way, incidentally. This is just a test. The idea would be to generate 2D/2D perlin noise values based on the cartesian coords of the vertex, tangent and bitangent and use this to displace the points along a line from the point to the centre of the mesh). Hope this makes sense, a|x http://machinesdontcare.wordpress.com

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