FFMPEG: Frame parameter initializations in HEVC decoder

Question

I'm going through the HEVC decoder integrated in FFMPEG. I'm actually trying to understand its flow and working.

By flow, i mean the part in code where it is reading various parameters of the input .bin file. Like where is it reading the resolution, where is it deciding the fps that it needs to play, the output display format that is yuv420p etc.

Initially What i suspected is the demuxer of hevc situated at /libavformat/hevcdec.c In this file does the input file reading work. There is a probe function which is used to detect which decoder to select while decoding the input bin stream. Further we have a FF_DEF_RAWVIDEO_DEMUXER. Is it in this function that the resolution and other parameters read from the input file?

Secondly what i suspect is the hevc parser situated at: /libavcodec/hevc_parser.c but here i think it is just parsing the frame data, that is finding end of frame. So, is this assumption of mine right?

Any suggestions or any predictions will be really helpful to me. Please provide your valuable suggestions. Thanks in advance.

Answer 1

To understand more specifically what is going on in decoder, it's better to start your study with HEVC/H.265 standard (http://www.itu.int/rec/T-REC-H.265). It contains all the information you need to know to find the location of resolution, fps, etc.

If you want to get more details from FFMPEG, here are some hints:

• /libavcodec/hevc_parser.c contains H.265 Annex B parser, which converts byte stream into series of NAL units. Each NAL unit has its own format and should be parsed depending on its NAL unit type.
• If you are looking for basic properties of video sequence, you may be interested in SPS (Sequence Parameter Set) parsing. Its format is described in section 7.3.2.2.1 of the standard and there is a function ff_hevc_decode_nal_sps located in /libavcodec/hevc_ps.c which extracts SPS parameters from the bitstream.

Note: I was talking about FFMPEG version 2.5.3. Code structure can be different for other versions.