Where to alter reference code to extract motion vectors from HEVC encoded video

Question

So this question has been asked a few times, but I think my C++ skills are too deficient to really appreciate the answers. What I need is a way to start with an HEVC encoded video and end with CSV that has all the motion vectors. So far, I've compiled and run the reference decoder, everything seems to be working fine. I'm not sure if this matters, but I'm interested in the motion vectors as a convenient way to analyze motion in a video. My plan at first is to average the MVs in each frame to just get a value expressing something about the average amount of movement in that frame.

The discussion here tells me about the TComDataCU class methods I need to interact with to get the MVs and talks about how to iterate over CTUs. But I still don't really understand the following:

1) what information is returned by these MV methods and in what format? With my limited knowledge, I assume that there are going to be something like 7 values associated with the MV: the frame number, an index identifying a macroblock in that frame, the size of the macroblock, the x coordinate of the macroblock (probably the top left corner?), the y coordinate of the macroblock, the x coordinate of the vector, and the y coordinate of the vector.

2) where in the code do I need to put new statements that save the data? I thought there must be some spot in TComDataCU.cpp where I can put lines in that print the data I want to a file, but I'm confused when the values are actually determined and what they are. The variable declarations look like this:

  // create motion vector fields

  m_pCtuAboveLeft      = NULL;
  m_pCtuAboveRight     = NULL;
  m_pCtuAbove          = NULL;
  m_pCtuLeft           = NULL;

But I can't make much sense of those names. AboveLeft, AboveRight, Above, and Left seem like an asymmetric mix of directions?

Any help would be great! I think I would most benefit from seeing some example code. An explanation of the variables I need to pay attention to would also be very helpful.

Answer 1

At TEncSlice.cpp, you can access every CTU in loop

for( UInt ctuTsAddr = startCtuTsAddr; ctuTsAddr < boundingCtuTsAddr; ++ctuTsAddr )

then you can choose exact CTU by using address of CTU. pCtu(TComDataCU class)->getCtuRsAddr().

After that,

pCtu->getCUMvField()

will return CTU's motion vector field. You can extract MV of CTU in that object. For example,

TComMvField->getMv(g_auiRasterToZscan[y * 16 + x])->getHor() 

returns specific 4x4 block MV's Horizontal element.

You can save these data after m_pcCuEncoder->compressCtu( pCtu ) because compressCtu determines all data of CTU such as CU partition and motion estimation, etc.

I hope this information helps you and other people!