I've read several articles about intra prediction in HEVC and I still have some questions. 

For a PU of NxN pixels, we use 4xN + 1 reference samples (the row above the PU, the column at the left of the PU and the sample at the top left). Then, based on the MPM, a mode is selected to work with.

I now have a row of reference samples, a column of reference samples and a mode. Based on this, how are the samples inside the PU calculated ?

In this article <http://codepaint.kaist.ac.kr/wp-content/uploads/2013/10/Intra-Coding-of-the-HEVC-Standard.pdf> there are ready-to-use formulae which take coordinate and selected mode as parameters. Is it really that simple ?

Now, imagine we have a picture of a checkerboard. How intra prediction can be used ? In some cases, we might not want to use reference samples of previously decoded PU. How to deal with that ?

Thanks