Calculating lookat matrix using in vertex shader OpenGL 3.x

Question

I followed this post to build the GluLookAt type matrix.

            Method calculateLookAtMatrix(glm::vec3 eye, glm::vec3 center, glm::vec3 up)

    glm::vec3 zaxis = glm::normalize(center - eye);
    glm::vec3 xaxis = glm::normalize(glm::cross(up, zaxis));
    glm::vec3 yaxis = glm::cross(zaxis, xaxis);

    lookAtMatrix[0][0] = xaxis.x;
    lookAtMatrix[0][1] = yaxis.x;
    lookAtMatrix[0][2] = zaxis.x;

    lookAtMatrix[1][0] = xaxis.y;
    lookAtMatrix[1][1] = yaxis.y;
    lookAtMatrix[1][2] = zaxis.y;

    lookAtMatrix[2][0] = xaxis.z;
    lookAtMatrix[2][1] = yaxis.z;
    lookAtMatrix[2][2] = zaxis.z;

    lookAtMatrix[3][0] = glm::dot(xaxis, -eye);
    lookAtMatrix[3][1] = glm::dot(yaxis, -eye);
    lookAtMatrix[3][2] = glm::dot(zaxis, -eye);

    lookAtMatrix[3][3] = 1;

I calculate that CPU side and pass it in to the shader as a uniform in the same way as the perspective matrix.

The shader looks like this.

 gl_Position =   lookAtMatrix * position * perspectiveMatrix;

If pass in the lookAtMatrix as the identity matrix the scene is rendered correctly, however using the code above to set the matrix as ...

    eye(0.0f,0.0f,10.0f);
center(0.0f,0.0f,0.0f);
up(0.0f,0.0f,1.0f);

results in a blank scene. The scene is a grid 10x10 drawn on X/Y, I therefore would have been expecting a zoomed out view of the scene.

(I have played about with the rows and cols but I am 99.9% sure they are fine, as the perspective matrix is fine)

score 6 · Accepted Answer · answered Sep 19 '12 at 04:44

gl_Position =   lookAtMatrix * position * perspectiveMatrix;

This is not correct. You want to transform your position by the lookAtMatrix, and then transform the result of that by the perspectiveMatrix.

When using column vector/column major vectors and matrices, you want to multiply a matrix and vector by placing the vector on the right hand side of the matrix.

So your multiplication should ultimately look like this:

gl_Position =   perspectiveMatrix * lookAtMatrix * position;

Having said that, I'm not sure why you had the position on the left in the first place, so I wonder if you're doing some matrix transposition somewhere in your pipeline. If this solution doesn't work you may need to show more code.

score 3 · Answer 2 · answered Sep 18 '12 at 23:29

eye(0.0f,0.0f,10.0f);
center(0.0f,0.0f,0.0f);
up(0.0f,0.0f,1.0f);

The up vector direction is parallel to the look-at direction. This is not allowed.

Remember: the up direction defines what "up" means for the view. It defines the rotation around the look-at direction. If the up direction is the look-at direction, then the math no longer works.

Calculating lookat matrix using in vertex shader OpenGL 3.x

2 Answers2