After a few hours in the world of Balder, I managed to fight the evil wizard of the Z-buffer and projection mapping for rendering sprites correctly. Or, at least I hope so!

I used the distance vector from origo( using the cameras local coordinate system ) to the sprite P in order to scale it correctly using a pseudodepth function: 2*NearPlane/P.z.

 

Also, I finished implementing the basic lights: directional light and omni lights. Both lights got support for ambient, diffuse and specular lights, including self-shadowing.