This largely tutorial treatise presents a Fourier based model for 2D-projection, the latter being a most important ingredient in any iterative reconstruction method. For sampled images the model requires an assumed basis function, which implicitly defines the necessary window and interpolation functions. We unravel the basis and window functions for some projection techniques described as procedures. Circular symmetric basis functions make it simple to find interpolation coefficients but require well tuned interpolation functions to avoid aliasing. We find it unnecessary to distinguish between voxel and ray driven projection. These two techniques concern only the innermost loop and both can be applied to any interpolation function, and to projection and back-projection alike.