How to add a new syntax element in HM (HEVC test Model)

Question

I've been working on the HM reference software for a while, to improve something in the intra prediction part. Now a new intra prediction algorithm is added to the code and I let the encoder choose between my algorithm and the default algorithm of HM (according to the RDCost of course).

What I need now, is to signal a flag for each PU, so that the decoder will be able to perform the same algorithm as the encoder decides in the rate distortion loop.

I want to know what exactly should I do to properly add this one bit flag to the stream, without breaking anything in the code.

Assuming that I want to use a CABAC context model to keep the track of my flag's statistics, what else should I do:

1. adding a new context model like ContextModel3DBuffer m_cCUIntraAlgorithmSCModel to the TEncSbac.h file.
2. properly initializing the model (both at encoder and decoder side) by looking at how the HM initialezes other context models.
3. calling the function m_pcBinIf->encodeBin(myFlag, cCUIntraAlgorithmSCModel) and m_pcTDecBinIfdecodeBin(myFlag, cCUIntraAlgorithmSCModel) at the encoder side and decoder side, respectively.

I take these three steps but apparently it breaks something.

PS: Even an equiprobable signaling (i.e. without using CABAC contexts) will be useful. I just want to send this flag peacefully!

Thanks in advance.

Answer 1

I could solve this problem finally. It was a bug in the CABAC context initialization.

But I want to share this experience as many people may want to do the same thing.

The three steps that I explained are essentially necessary to add a new syntax element, but one might be very careful with the followings:

1. In the beginning, you need to decide either you want to use a separate context model for your syntax element? Or you want to use an existing one? In case of CABAC separation, you should define a ContextModel3DBuffer and the best way to do that is: finding a similar syntax element in the code; then duplicating its ``ContextModel3DBuffer'' definition and ALL of its occurences in the code. This way assures that you are considering everything.
2. Encoding of each syntax elements happens in two different places: first, in the RDO loop to make a "decision", and second, during the actual encoding phase and when the decisions are being encoded (e.g. encodeCtu function).
3. The order of encoding/decoding syntaxt elements should be the same at the encoder/decoder sides. For example if your new syntax element is encoded after splitFlag and before predMode at the encoder side, you should decode it exactly between splitFlag and predMode at the decoder side.
4. The context model is implemented as a 3D matrix in order to let track the statistics of syntaxt elements separately for different block sizes, componenets etc. This means that when you want to call the function encodeBin, you may make sure that a correct index is being used. I've made stupid mistakes in this part!

Apart from the above remarks, I found a the function getState very useful for debugging. This function returns the state of your CABAC context model in an arbitrary place of the code when you have access to it. It is very useful to compare the state at the same place of the encoder and the decoder when you have a mismatch. For example, it happens a lot that you encode a 1 but you decode a 0. In this case, you need to check the state of your CABAC context before encoding and decoding. They should be the same. If they are not the same, track back the error to find the first place of mismatch.

I hope it was helpful.