[Libav-user] Building an index for arbitrary-frame-accurate VBR seeking use byte offset

[stelefx at gmail.com]

Replying to myself here-

Looks like the looping was caused because I didn't 
avcodec_flush_buffers() between frame seeks. Also the 
AVSEEK_FLAG_BACKWARD is unnecessary when using AVSEEK_FLAG_BYTES.

I've still got one AVR file that has an odd jump near the beginning, 
even though the index seems to be doing the right thing.

On 12/17/21 12:18 PM, rin tec wrote:
>
>
> <stelefx at gmail.com>
>
>     I have various videos that are VBR and don't have an accurate
>     duration
>     or frame rate.
>
>     I wrote some code to index the files by reading through the whole
>     thing
>     by packet and recording the pts, byte offset, and duration for each
>     frame, and recording which ones are I-frames. This also allows
>     getting
>     an accurate duration for the files that didn't have one too.
>
>     Now I'm attempting to use the index to jump to any arbitrary
>     frame, by
>     first seeking to the byte offset of the I-frame before the desired
>     frame, and then decoding packets until the desired number of
>     non-I-frames are processed.
>
>     I've tried this both using the byte offset and with the pts of that
>     reference I-frame, but in many cases I'm getting repeating "loops" in
>     the video.
>
>     This is probably my misunderstanding of what the fields in the
>     AVFrame
>     are. While building the index, I'm using a loop of av_read_frame()
>     and
>     avcodec_decode_video2(). When I get a picture, and if key_frame is
>     set,
>     I record the frame's pkt_pos, pts, and pkt_dts values in a dictionary
>     keyed by the frame number.
>
>     When I seek to an arbitrary frame, I look up the first entry in the
>     dictionary with the frame # I'm looking for, and then to:
>
>     av_seek_frame(context, video_stream_index,
>     pkt_pos_for_nearest_iframe,
>     AVSEEK_FLAG_BACKWARD | AVSEEK_FLAG_BYTES);
>
>     Then if the frame I'm looking for is not an I-frame, I repeat the
>     av_frame_read/avcodec_decode_video2 loop until I get to that frame
>     number.
>
>     Does anyone have any experience with this kind of thing who could
>     tell
>     me what I'm doing wrong?
>
>
>     ______________________________
>
>
> Check the video stream first? Sometimes the video stream were already 
> broken so the codec cannot decode the stream.
>
> Or you also can use other codec(such as libx264 or libx265). And check 
> the others codec output.
>
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