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Calculating Tangent Space

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Hi, I'm trying to add bumpmapping to my deferred shading engine and im having trouble calculating the per-vertex tangents to pass into it. I have adapted the code from here but it doesnt always work, when ever the vertex has a normal with a negative value, the tangent isnt calculated. Could someone explain to me (in simple terms) why this is happening? Here is my code in case it helps:
void KMesh::CalculatePerVertexTangents()
	KPolygon* ptr = polygons;
	tangents = new KVec3[numVertices];

    KVec3 *tan1 = new KVec3[numVertices];
    memset(tan1, 0, sizeof(KVec3) * numVertices);

	for (int i = 0; i < numPolygons; i++)
		int numTris = ptr->GetNumTriangles();
		for (int j = 0; j < numTris; j++)
			int idx[3];
			ptr->GetTriangle(j, idx[0], idx[1], idx[2]);

			KVec3 v1 = vertices[idx[0]];
			KVec3 v2 = vertices[idx[1]];
			KVec3 v3 = vertices[idx[2]];

			KVec2 w1 = texCoords[idx[0]];
			KVec2 w2 = texCoords[idx[1]];
			KVec2 w3 = texCoords[idx[2]];

			float x1 = v2.X() - v1.X();
			float x2 = v3.X() - v1.X();
			float y1 = v2.Y() - v1.Y();
			float y2 = v3.Y() - v1.Y();
			float z1 = v2.Z() - v1.Z();
			float z2 = v3.Z() - v1.Z();
			float s1 = w2.X() - w1.X();
			float s2 = w3.X() - w1.X();
			float t1 = w2.Y() - w1.Y();
			float t2 = w3.Y() - w1.Y();

			float r = 1.0F / (s1 * t2 - s2 * t1);
			KVec3 sdir((t2 * x1 - t1 * x2) * r, (t2 * y1 - t1 * y2) * r,
					(t2 * z1 - t1 * z2) * r);

			tan1[idx[0]] += sdir;
			tan1[idx[1]] += sdir;
			tan1[idx[2]] += sdir;

	KVec3 n, t;

	for (int a = 0; a < numVertices; a++)
		n = normals[a];
        t = tan1[a];
        // Gram-Schmidt orthogonalize
		tangents[a] = (t - (n * n.Dot(t))).Normalise();

	delete [] tan1;

Thanks, Luke.

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Your code looks correct to me. It should work fine as long as your input data doesn't include triangles with degenerate texture mapping or left-handed texture spaces (since you removed the handedness calculation). I'm not sure what you mean by a tangent not being calculated when the normal has a negative value.

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