Implicit Normals?

I found an interesting snippet on the wiki of a voxel engine known as "Thermite3D". It speaks of a pair of fragment shader functions, dFdx and dFdy which give the partial derivative of a variable. They use it with vertices as a way to obtain normals.

If I have a texture which holds the world positions of the visible voxels, I could sample it and estimate the derivative. I can then combine this with another texture with the lighting information (generated by raycasting the landscape) and its associated material, to calculate the final colour value.

Unfortunately, the functions don't work with textures, but I can still do the estimation myself, as shown here.