Figure 2: The approximation result of PPF. is used to approximate . The top row is fully sampled. The bottom row is sparsely sampled by 15 projections. The left most column is the reconstruction result by FBP. The middle column is the PPF approximation results. The right most column is the difference between the reconstructed images and the approximation results, which is the PPFC transform. The grey scale window for the first and second columns is . For the third column, the window is .