Hi Stanimir, Le lundi 20 avril 2020 à 17:38 +0300, Stanimir Varbanov a écrit : > Hi, > > I need to port a decoder v4l2 control in mainline Venus driver which > instructs the decoder to decode sync frames only (I frame/ IDR frame). > In practice the usage of such control is to generate thumbnails for > particular video. > > To do that I researched what we have currently in v4l2-controls.h and > found something similar but for encoders: > > V4L2_CID_MPEG_VIDEO_FORCE_KEY_FRAME > "Force a key frame for the next queued buffer. Applicable to encoders. > This is a general, codec-agnostic keyframe control." > > I think I have two options: > > 1. reuse V4L2_CID_MPEG_VIDEO_FORCE_KEY_FRAME for the decoder and document it > > 2. create a new v4l control V4L2_CID_MPEG_VIDC_VIDEO_SYNC_FRAME_DECODE Seems analogue to GStreamer flag GST_SEEK_FLAG_TRICKMODE_KEY_UNIT, if that helps your inspiration. It's used to speed up fast-forward (ore thumbnails generation). What the decoder will effectively do is to skip the P and the B frames. Some decoder offers fine grain control on that. That being said, in GStreamer (and probably most framework) frames are skipped in userspace. But decoder tend to wait for the next key frame to output the first one, so we endup having to do more work to force the frame out (like CMD_STOP/START cycle, or crafts an early AUD). Would be nice to document if that mode will help in this regard (or not). For protocols like DASH, when doing fast-forward we don't even download the non- key-frames from the server, so the skipping behaviour does not provide any gain any way. > > Any suggestions? > Could be as Hans suggested, or something less generic but that covers more trickmodes ? V4L2_CID_MPEG_VIDEO_DECODER_H264_TRICKMODE - No skip - Skip B Frames - I frame only Or maybe try and construct something base on what other do ? FFMPEG software decoder has the following: (0): Skip nothing - 0 (1): Skip B-frames - 1 (2): Skip IDCT/Dequantization - 2 (5): Skip everything - 5 CODA from Chips&Media is fancier, and propose some variants that have implication how muchs state is maintained, hence allowing or not the transition back to non-skipping outside IRAP boundary. NORMAL THUMBNAIL Skip nonIRAP Skip nonREF-PIC Skip Temporal Layer I didn't find such feature for Samsung MFC, but Amlogic is likely one candidate having this type of skipping feature (as it also has HW demuxers). Hope this helps getting the big picture, hence proposing uAPI that isn't too limited. regards, Nicolas
