[FFmpeg-trac] #11565(avformat:new): dshow wrapper packet AV_PKT_FLAG_KEY flagging

[FFmpeg]

#11565: dshow wrapper packet AV_PKT_FLAG_KEY flagging
-------------------------------------+-------------------------------------
             Reporter:  Matt         |                     Type:
  Carpenter                          |  enhancement
               Status:  new          |                 Priority:  normal
            Component:  avformat     |                  Version:  6.1.1
             Keywords:  dshow        |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 Summary of the enhancement:
 Explanation:
 {{{
 % ffprobe -f dshow -video_pin_name 1 -i video="H264 USB Camera"
 -show_entries frame=key_frame

 ffmpeg version
 libavutil      56. 63.100 / 56. 63.100
 libavcodec     58.115.102 / 58.115.102
 libavformat    58. 65.101 / 58. 65.101
 libavdevice    58. 11.103 / 58. 11.103
 libavfilter     7. 95.100 /  7. 95.100
 libswscale      5.  8.100 /  5.  8.100
 libswresample   3.  8.100 /  3.  8.100
 libpostproc    55.  8.100 / 55.  8.100
 }}}

 The above command displays key_frames coming from my USB camera once-per-
 second. Camera is ELP-USBFHD06H-BL170 with the h264 stream on video pin 1.
 Inside my recorder program the following code is used to try and detect
 key_frames on the same camera.

 {{{
 input->input_ctx = avformat_alloc_context();
 input->input_ctx->video_codec_id = AV_CODEC_ID_H264;
 ...
 av_read_frame(input->input_ctx, in_packet);
 in_packet->flags & AV_PKT_FLAG_KEY) == AV_PKT_FLAG_KEY
 }}}

 The code above does not detect any packets having the key_frame flag set.
 Original question asked and answered:
 https://superuser.com/questions/1888999/trying-to-understand-how-key-
 frames-work-when-using-ffmpeg-on-windows-with-a-usb

 A workaround for my program was the following code to search for any IDR
 NAL units inside the packet, and consider any packets containing IDR NAL
 units as keyfames.

 {{{
 /*
 * Look inside the packet's frame buffer for NAL units of type IDR frame
 * @returns 0 if not 1 if found
 */
 int is_idr_frame(AVPacket *pkt) {
     if (pkt->size < 6) return 0;  // Ensure packet is large enough for
 parsing
     const uint8_t *data = pkt->data;

     // Look for NAL unit start codes (00 00 00 01 or 00 00 01)
     for (int i = 0; i < pkt->size - 4; i++) {
         if (data[i] == 0x00 && data[i + 1] == 0x00 &&
             ((data[i + 2] == 0x01) || (data[i + 2] == 0x00 && data[i + 3]
 == 0x01))) {
             uint8_t nal_type = data[i + 2] == 0x01 ? (data[i + 3] & 0x1F)
 : (data[i + 4] & 0x1F);
             if (nal_type == 5)  // NAL type 5 = IDR frame
                 return 1;
         }
     }
     return 0;
 }
 }}}

 Suggested enhancement for dshow wrapper - set the AV_PKT_FLAG_KEY on
 AVPacket.
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/11565>
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