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HEVC-deep-learning-pipeline

Integrating neural network models in HEVC encoder, to test the complexity reduction using deep-learning-based method in HEVC intra-prediction.

Introduction

Using neural networks, we can directly predict the Coding Unit (CU) depths for each frame. The intention is to speed up the encoding process of HEVC encoder. Thus, after we have a trained model, another thing that needs to be done is to integrate the deep learning prediction process into the HEVC encoder. Not only is this a pratical thing but also it makes the evaluation of the performance of our neural network model easier.

Usage

This pipeline is used for evaluating the performance of a neural network model in HEVC intra-prediction process. Compare the difference in encoding time, BDBR, BD-PSNR with the original HEVC encoder.

Requirements:

  • FFmpeg
  • Python3
  • PyTorch

First, prepare at least four YUV test sequence, but you can only encode one YUV at a time. Modify the bitstream.cfg according to the properties of the YUV file. Then, build the deep-learning-based HEVC encoder from source in folder HM_dl, move the TAppEncoder.exe or executables for other platforms to the repository directory, and use

.\TAppEncoder.exe -c .\\encoder_intra_main.cfg -c .\\bitstream.cfg

Then the encoding of the YUV file using deep learning model with begin. When it's done, it will output some information on the command line, like the encoding time, YUV-PSNR and so on.

To test the performance, you need to also use the original HEVC encoder on the YUV file and get the information it outputs on the command line. When you have done testing at least four YUV files with both encoders. You can use the information and calculate the BDBR and BD-PSNR. Refer to ./calc_BDBR for details.

Pipeline description

First, before the HEVC encoder actually begins the encoding process, it triggers gen_frames.py. This python script will read the bitstream.cfg, get the YUV file information, and use ffmpeg command to extract all the frames of the YUV file that is about to be encoded to the ./rec/frames folder for the prediction of the neural network. The script also creates an empty ./pred folder to store the prediction result (the CU depths information) of the neural network.

After the execution of gen_frames.py is done. A thread is called to execute use_model.py. Then , the thread is detached and HEVC starts encoding.

use_model.py uses the neural network model in wolverinn/HEVC-CU-depths-prediction-CNN. It reads frames from ./rec/frames and processs them. And then use the neural network trained model in ./rec to preedict CU depths from the input. For each 64x64 CTU, the script ouputs a .txt file to store the depth information in an 1x16 vector (also refer to the previous repository to see why 16 labels are enough to represent CU depths information). The file is stored like ./pred/FrameNumber/CtuNumber.txt.

Back to the HEVC encoder side, when the encoder starts to encode a 64x64 CTU in the xth frame, it first checks whether the file ./pred/x/CtuNumber.txt exists. This check is necessary because the prediction and the encoding process are running paralelly. If not, it waits. If the check passes, it reads the depth information, a 16 vector from the txt file.

Then, when the encoder searches recursively in each CU for the best partition. In each depth, it compares the current depth with the predicted depth from the neural network. If current_depth < predicted_depth, then it skips the calculation for current depth and continues to search for next depth. If current_depth = predicted_depth, then it only does calculation for current depth and stops searching further depths. If current_depth < predicted_depth, then it not only skips the current depth calculation, but also stops searching further depths.

This makes sure that calculation will happen only if the current depth is the predicted depth from the neural network model, which achieves the purpose of saving time by skipping unnecessary calculation for RD-cost.

Modification of HEVC source code

We modified the HEVC trunk source code to make our pipeline. Only the encoder part was modified. Search modified2019 in the project source code for detailed modification.

  • /source/App/TAppEncoder/encmain.cpp
  • /TLibCommon/TComPicYuv.h: add member variable m_iFrameRcvd and member function setFrame and getFrame to store and get FrameNumber information in class TComPicYuv.
  • /source/App/TAppEncoder/TAppEncTop.cpp: store the current frame number m_iFrameRcvd in class TComPicYuv pcPicYuvOrg.
  • /TLibEncoder/TEncGop.h: add a parameter Int m_iFrame for member function compressGOP.
  • /TLibEncoder/TEncTop.cpp: extract current frame number from pcPicYuvOrg to variable m_iFrame, pass the variable to the function m_cGOPEncoder.compressGOP. Note: m_cGOPEncoder.compressGOP exists in two places.
  • /TLibEncoder/TEncSlice.h: add a parameter Int m_iFrame for member function compressSlice and precompressSlice.
  • /TLibEncoder/TEncGop.cpp: add a parameter Int m_iFrame for function compressGOP (consistant with the declaration in TEncGop.h). Pass the variable m_iFrame to function m_pcSliceEncoder->precompressSlice and m_pcSliceEncoder->compressSlice.
  • /TLibEncoder/TEncCu.h: add a parameter Int m_iFrame for member function compressCtu.
  • /TLibEncoder/TEncSlice.cpp: add a parameter Int m_iFrame for function precompressSlice and compressSlice (consistant with the declaration in TEncSlice.h). Note: compressSlice exists in two places. Pass the variable m_iFrame to function compressCtu.
  • /TLibEncoder/TEncCu.cpp: add a parameter Int m_iFrame for function compressCtu (consistant with the declaration in TEncCu.h).
  • /TLibCommon/TComDataCu.h: add member variable pred_depth and member function set_pred and get_pred to store and get depth information in class TComDataCU.

TEncCu.cpp -- function compressCtu()

use m_iFrame to get the frame number, use pCtu->getCtuRsAddr() to get the current ctu number, and read depth prediction from file ./pred/FrameNumber/CtuNumber.txt (availability check first).

Store the depth information in m_ppcBestCU[0]->pred_depth. Use member function m_ppcBestCU[0]->set_pred(UInt *label).

TEncCu.cpp -- function xCompressCU()

use member function m_ppcBestCU[0]->get_pred() to get the predicted CU depth. Use variable uiLPelX and uiTPelY to locate current CU location and get the corresponding depth. Compare the predicted depth to uiDepth. Determine two things: if the current depth needs calculation/ if the next depth needs calculation.

If current_calculation is False, then skip xCheckRDCostIntra. Instead, directly assign:

rpcBestCU->getTotalCost() = MAX_DOUBLE / 16;
rpcBestCU->getTotalDistortion() = MAX_UINT >> 3;
rpcBestCU->getTotalBits() = MAX_UINT >> 3;
rpcBestCU->setPartitionSize(0, SIZE_2Nx2N);
rpcBestCU->setPredictionMode(0, MODE_INTRA);

If next_calculation is False, then stop calling xCompressCU recursively. But directly assign:

pcSubBestPartCU->getTotalCost() = MAX_DOUBLE / 16;
pcSubBestPartCU->getTotalDistortion() = MAX_UINT >> 3;
pcSubBestPartCU->getTotalBits() = MAX_UINT >> 3;
pcSubBestPartCU->setPartitionSize(0, SIZE_2Nx2N);
pcSubBestPartCU->setPredictionMode(0, MODE_INTRA);

Reference