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Ipito

Need help rotating about an arbitrary vector in OpenGL

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I've written the following function to rotate about an arbitrary vector (with non-zero components). Basically, I want to project the vector to the x-z plane, then to the z axis, then perform the rotation, and finally undo the rotations required to allign the vector to the z axis: void rotateArbitrary(float deg, float x, float y, float z) { float rad = degToRad(deg); // Rotation matrix for the rotation about the x, and z axis float rotateX[16]; float rotateY[16]; float rotateZ[16]; // Angles to rotate by float cosAlpha, sinAlpha, cosBeta, sinBeta; if(x > 0 && y > 0 && z > 0) { // The sine and cosine for the rotation about the x axis cosAlpha = z/sqrt(y*y + z*z); sinAlpha = y/sqrt(y*y + z*z); // The sine and cosine for the rotation about the y axis cosBeta = z/sqrt(x*x + z*z); sinBeta = x/sqrt(x*x + z*z); // Build up the rotation matrix // Rotate back about the x axis for 1 - alpha memset(rotateX, 0, sizeof(float) * 16); rotateX[0] = 1.0; rotateX[5] = cosAlpha; rotateX[9] = sinAlpha; rotateX[6] = -sinAlpha; rotateX[10] = cosAlpha; rotateX[15] = 1.0; glMultMatrixf(rotateX); // Rotate back about the y axis for 1 - beta memset(rotateY, 0, sizeof(float) * 16); rotateY[0] = cosBeta; rotateY[8] = -sinBeta; rotateY[5] = 1.0; rotateY[2] = sinBeta; rotateY[10] = cosBeta; rotateY[15] = 1.0; glMultMatrixf(rotateY); // Rotate the desired amount about the z axis memset(rotateZ, 0, sizeof(float) * 16); rotateZ[0] = cos(rad); rotateZ[4] = -sin(rad); rotateZ[1] = sin(rad); rotateZ[5] = cos(rad); rotateZ[10] = 1.0; rotateZ[15] = 1.0; glMultMatrixf(rotateZ); // Rotate through beta to the z axis memset(rotateY, 0, sizeof(float) * 16); rotateY[0] = cosBeta; rotateY[8] = sinBeta; rotateY[5] = 1.0; rotateY[2] = -sinBeta; rotateY[10] = cosBeta; rotateY[15] = 1.0; glMultMatrixf(rotateY); // Rotate through alpha to the x/z plane memset(rotateX, 0, sizeof(float) * 16); rotateX[0] = 1; rotateX[5] = cosAlpha; rotateX[9] = -sinAlpha; rotateX[6] = sinAlpha; rotateX[10] = cosAlpha; rotateX[15] = 1.0; glMultMatrixf(rotateX); } } However, when I call the function with: rotateArbitrary(30.0, 1.0, 0.0, 0.0); The result is different to if you call the standard: glRotatef(30.0, 1.0, 1.0, 1.0); It appears to be rotating the -30 degrees about the x axis, instead of 30 degrees. I've tried the rotation matrices on their own, and they seem to work fine. Any advice would be appreciated. Thanks

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