The fragment shader expects a normal, but the vertex shader was not providing one. Fix: added a new vertex shader that has normals + smooth color interpolation. I also split gpu_shader_3D_vert in two: - one with just position - one with position + normal For each of the builtin shaders, we should be able to look at the GLSL and tell exactly what it's doing. Using #defines and #ifdefs for rendering options makes the shaders hard to read and easy to break.