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OpenGL GluLookAt clone - unexpected behaviour

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I've been trying replicate the behaviour of GluLookAt, using the information at http://www.opengl.or...l/gluLookAt.xml however my camera doesn't properly track the focus point.

This is the code I've written to calculate a matrix from the camera's attributes:


void MatrixFromLookatCam(camera_follow_t *camera, mat4x4_t matrix)
{
vec3_t lookDir;
vec3_t s, u;
vec3subtract(lookDir, camera->point, camera->origin);
vec3Normalise(lookDir, lookDir);
vec3Normalise(camera->up, camera->up);
vec3CrossProduct(s, lookDir, camera->up);
vec3Normalise(s, s);
vec3CrossProduct(u, s, lookDir);
vec3Normalise(u, u);

matrix[0] = s[0];
matrix[1] = u[0];
matrix[2] = -lookDir[0];
matrix[4] = s[1];
matrix[5] = u[1];
matrix[6] = -lookDir[1];
matrix[8] = s[2];
matrix[9] = u[2];
matrix[10] = -lookDir[2];
matrix[12] = -camera->origin[0];
matrix[13] = -camera->origin[1];
matrix[14] = -camera->origin[2];
}


At the rendering stage I'm multiplying the modelview matrix by this new matrix, then reverting it to the identity matrix at the end of the render.

To illustrate the problem:
When the focus point is moved from it's starting position ([0,0,0]) the camera doesn't follow it precisely. The focus point is indicated by the bottom-left corner of the red square.
[media]
[/media]

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From a glance, I can't spot anything obviously wrong in the code. If you need more reference, you can try e.g. cross-checking against float3x3::LookAt in MathGeoLib.

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For debugging, I would take a snapshot when you press a key, and draw the up,right,forward vectors and then move backwards and see where they are actually pointing. Of course scale those vectors so they go out far enough to see if it is hitting the red dot or anything else.

Are you calling
glLoadIdentity
glMultMatrixf()
?

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