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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

damjeux
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Benjamin
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2 Answers2

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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 ?

As it is stated in this article first encoder should decide about the mode and the sizes of PUs and TUs during the RDO process. Among the list of modes lets say mode number 25 is chosen to predict the current block. Mode number 25 is one of angular modes so we will use the mentioned formula for angular modes and obtain the output. It worth mentioning that although formula is simple details of reference samples make it a little tricky.

Now, imagine we have a picture of a checkerboard. How intra prediction can be used ?

First the prediction modes should be found. Lets say we decided on mode X then we should refer to the related formula to mode X and form our prediction block simililar to what discussed in previous question.

In some cases, we might not want to use reference samples of previously decoded PU. How to deal with that ?

Intra prediction basically is formed based on these reference samples and if you are not using these pixels your not doing INTRA prediction. Maybe you should shift to INTER prediction where it uses other blocks in successive frames and MVs to predict the current block.

user3851234
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The question is interest for me. I can easy to say that the mode is selected by encode. In the HEVC encoder, it run all the mode(35, in the view of complexity,encoder uses fast algorithm to simplify the selection process, you can find some paper to read), finally encoder selects the best mode(RDO process). so,decoder can not select reference sample. decoder have to select the samples which are same with encoder.

In the SCC(screen content coding) coding which is a extension of the HEVC, using IBC(intra block copy) mode to select the reference sample in reconstructed area.

xiangjian Wu
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