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• MediaStream to C++ data type conversion

  20 juillet 2016, par Mohammad Abu Musa

  I am sending a video and audio stream to C++ from Google Chrome, I am not sure how to cast the data.

  What is the data type of this and how do I convert it to C++ ?

  videoStream.getVideoTracks()[0]

  I want to build ffmpeg encoder using C++ but I can not figure out how to cast this type ?

  I was also wondering if there is method to build a test case similar to this data type ?

• AVFrame : How to get/replace plane data buffer(s) and size ?

  19 juillet 2018, par user10099431

  I’m working on gstreamer1.0-libav (1.6.3), trying to port custom FPGA based H264 video acceleration from gstreamer 0.10.

  The data planes (YUV) used to be allocated by a simple malloc back in gstreamer 0.10, so we simply replaced the AVFrame.data[i] pointers by pointers to memory in our video acceleration core. It seems to be MUCH more complicated in gstreamer 1.12.

  For starters, I tried copying the YUV planes from AVFrame.data[i] to a separate buffer - which worked fine ! Since I haven’t seen an immediate way to obtain the size of AVFrame.data[i] and I recognized that data[0], data[1], data[2] seem to be in a single continuous buffer, I simply used (data[1] - data [0]) for the size of the Y plane and (data[2] - data[1]) for the sizes of the U/V planes respectively. This works fine, expect for one scenario :

  • Input H264 stream with resolution of 800x600 or greater
  • The camera is covered (jacket, hand, ...)

  This causes a SEGFAULT in the memcpy of the V plane (data[2]) using the sizes determined as described above. Before covering the camera, the stream is displayed completely fine ... so for some reason the dark screen changes the plane sizes ?

  My ultimate goal is replacing the data[i] pointers allocated by gstreamer by my custom memory allocation (for futher processing) ... where exactly are these buffers assigned, can I change them and how can I obtain the size of each plane (data[0], data[1], data[2]) ?

• Extract luminance data using ffmpeg libavfilter, specifically PIX_FMT_YUV420P type

  12 mars 2014, par id128

  This pertains to ffmpeg 0.7 (yes I know it's old, but data access should be similar).

  I am writing a libavfilter to extract the luminance data from each frame. In draw_slice() function I have access to AVFilterLink structure which in turn gives me access to AVFilterBufferRef structure that have uint8_t *data[] pointers. With the PIX_FMT_YUV420P type, I think data[0], data[1], data[2] refers to Y U V channels respectively.

  My question is, with the pointer to data[0] (luminance plane), how do I interpret the data ? The pixfmt.h header file states :

  PIX_FMT_YUV420P, ///< planar YUV 4:2:0, 12bpp, (1 Cr & Cb sample per 2x2 Y samples)

  does that mean I have to interpret the luminance plane data every 2 bytes ? Also, what exactly is the datatype for the values pointed to by the pointer - int, float, etc ?

  Thanks in advance