Question

as a homework assignment, we’re writing a software rasterizer. I’ve noticed my z buffering is not working as well as it should, so I’m trying to debug it by outputting it to the screen. (Black is near, white is far away).

However, I’m getting peculiar values for the z per vertex. This is what I use to transform the points:

float Camera::GetZToPoint(Vec3 a_Point)
{
    Vec3 camera_new = (m_MatRotation * a_Point) - m_Position;

    return (HALFSCREEN / tanf(_RadToDeg(60.f * 0.5f)) / camera_new.z);
}

m_MatRotation is a 3×3 matrix. Multiplying it by a vector returns a transformed vector.

I get maximum and minimum values between 0 and x, where x is a seemingly random number.

Am I doing this transformation right? If so, how can I normalize my Z values so it lies between two set points?

Thanks in advance.

Answer 1

To normalize the Z values you have to define a near clipping plane and a far clipping plane. Then you normalize Z such that its 0 at the near plane and 1 at the far plane.

However, you would usually do that after projection. It looks like your last line is where projection occurs.

A number of other things:

• You compute the full matrix-vector multiplication but keep only the Z, this is wasteful. You should consider transforming the points and keeping all their X, Y, Z coordinates ;
• You recompute tanf() at every vertex, but its constant ;
• I would suggest you use a projection matrix rather than the tanf computation ;
• Start with a simple orthogonal projection, it will be easier to debug.