FFmpeg v4l2m2m的capture和output
发现FFmpeg v4l2m2m中encoder和decoder中的output和capture的细节，理解起来有点奇怪，记录如下。
encoder v4l2_encode_init
external/ffmpeg/ffmpeg/libavcodec/v4l2_m2m_enc.c
/* output context */
output->av_codec_id = AV_CODEC_ID_RAWVIDEO;
output->av_pix_fmt = avctx->pix_fmt;
/* capture context */
capture->av_codec_id = avctx->codec_id;
capture->av_pix_fmt = AV_PIX_FMT_NONE;
decoder v4l2_decode_init
external/ffmpeg/ffmpeg/libavcodec/v4l2_m2m_dec.c
/* output context */
output->av_codec_id = avctx->codec_id;
output->av_pix_fmt = AV_PIX_FMT_NONE;
/* capture context */
capture->av_codec_id = AV_CODEC_ID_RAWVIDEO;
capture->av_pix_fmt = avctx->pix_fmt;
在编码器encoder中，capture对应输出编码后的视频数据，而output对应的是输入原始视频数据。编码器的主要作用是将原始视频数据压缩成指定codec格式的数据流，以便于传输和存储。
在解码器decoder中，capture对应的是解码后的视频数据，而output对应的是输入的原始视频数据。解码器的主要作用是将编码后的视频数据解压缩成原始视频数据，以便于播放和处理。
这里，不管是encoder还是decoder，这个output实际上就相当于encoder或者decoder的input，不理解为什么不定义为input呢？
- 从下面的log看，使用
h264_v4l2m2m解码，输入是h264（output），输出是NM21(capture)，确实在v4l2的实现里面把输入叫做output，输出叫capture
$ ffmpeg -vcodec h264_v4l2m2m -i /data/h264.mp4 -pix_fmt nv21 /data/yuv420p-1080x720.yuv -loglevel trace
[h264_v4l2m2m @ 0x1cdbaa0] probing device /dev/video26
[h264_v4l2m2m @ 0x1cdbaa0] driver 'aml-vcodec-dec' on card 'platform:amlogic' in mplane mode
[h264_v4l2m2m @ 0x1cdbaa0] Using device /dev/video26
[h264_v4l2m2m @ 0x1cdbaa0] driver 'aml-vcodec-dec' on card 'platform:amlogic' in mplane mode
[h264_v4l2m2m @ 0x1cdbaa0] requesting formats: output=H264 capture=NM21
[h264_v4l2m2m @ 0x1cdbaa0] output: H264 16 buffers initialized: 0064x0064, sizeimage 01048576, bytesperline 00000000
/dev/video的设备列表：
sh-5.1# ls /dev/video*
/dev/video10 /dev/video14 /dev/video18 /dev/video23 /dev/video32 /dev/video36 /dev/video_composer.1
/dev/video11 /dev/video15 /dev/video19 /dev/video26 /dev/video33 /dev/video37 /dev/videotunnel
/dev/video12 /dev/video16 /dev/video20 /dev/video30 /dev/video34 /dev/video38
/dev/video13 /dev/video17 /dev/video21 /dev/video31 /dev/video35 /dev/video_composer.0
对比下encoder的log输出，在当前的设备上，枚举了video设备，没有合适的设备，最后configure失败：
ffmpeg -pix_fmt nv21 -video_size 1080x720 -i /data/yuv420p-1080x720.yuv -vcodec h264_v4l2m2m /data/output.mp4 -loglevel trace
[graph 0 input from stream 0:0 @ 0xd105d0] Setting 'video_size' to value '1080x720'
[graph 0 input from stream 0:0 @ 0xd105d0] Setting 'pix_fmt' to value '24'
[graph 0 input from stream 0:0 @ 0xd105d0] Setting 'time_base' to value '1/25'
[graph 0 input from stream 0:0 @ 0xd105d0] Setting 'pixel_aspect' to value '0/1'
[graph 0 input from stream 0:0 @ 0xd105d0] Setting 'frame_rate' to value '25/1'
[graph 0 input from stream 0:0 @ 0xd105d0] w:1080 h:720 pixfmt:nv21 tb:1/25 fr:25/1 sar:0/1
[AVFilterGraph @ 0xd0fde0] query_formats: 3 queried, 2 merged, 0 already done, 0 delayed
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video26
[h264_v4l2m2m @ 0xd0dd10] driver 'aml-vcodec-dec' on card 'platform:amlogic' in mplane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 capture format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video12
[h264_v4l2m2m @ 0xd0dd10] driver 'amlvideo2' on card 'amlvideo2' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video11
[h264_v4l2m2m @ 0xd0dd10] driver 'amlvideo2' on card 'amlvideo2' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video23
[h264_v4l2m2m @ 0xd0dd10] driver 'amlvideo' on card 'amlvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video10
[h264_v4l2m2m @ 0xd0dd10] driver 'amlvideo' on card 'amlvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video38
[h264_v4l2m2m @ 0xd0dd10] driver 'v4lvideo' on card 'v4lvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video37
[h264_v4l2m2m @ 0xd0dd10] driver 'v4lvideo' on card 'v4lvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video36
[h264_v4l2m2m @ 0xd0dd10] driver 'v4lvideo' on card 'v4lvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video35
[h264_v4l2m2m @ 0xd0dd10] driver 'v4lvideo' on card 'v4lvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video34
[h264_v4l2m2m @ 0xd0dd10] driver 'v4lvideo' on card 'v4lvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video33
[h264_v4l2m2m @ 0xd0dd10] driver 'v4lvideo' on card 'v4lvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video32
[h264_v4l2m2m @ 0xd0dd10] driver 'v4lvideo' on card 'v4lvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video31
[h264_v4l2m2m @ 0xd0dd10] driver 'v4lvideo' on card 'v4lvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video30
[h264_v4l2m2m @ 0xd0dd10] driver 'v4lvideo' on card 'v4lvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video21
[h264_v4l2m2m @ 0xd0dd10] driver 'ionvideo' on card 'ionvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video20
[h264_v4l2m2m @ 0xd0dd10] driver 'ionvideo' on card 'ionvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video19
[h264_v4l2m2m @ 0xd0dd10] driver 'ionvideo' on card 'ionvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video18
[h264_v4l2m2m @ 0xd0dd10] driver 'ionvideo' on card 'ionvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video17
[h264_v4l2m2m @ 0xd0dd10] driver 'ionvideo' on card 'ionvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video16
[h264_v4l2m2m @ 0xd0dd10] driver 'ionvideo' on card 'ionvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video15
[h264_v4l2m2m @ 0xd0dd10] driver 'ionvideo' on card 'ionvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video14
[h264_v4l2m2m @ 0xd0dd10] driver 'ionvideo' on card 'ionvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video13
[h264_v4l2m2m @ 0xd0dd10] driver 'ionvideo' on card 'ionvideo' in splane mode
[h264_v4l2m2m @ 0xd0dd10] v4l2 output format not supported
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/videotunnel
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video_composer.1
[h264_v4l2m2m @ 0xd0dd10] probing device /dev/video_composer.0
[h264_v4l2m2m @ 0xd0dd10] Could not find a valid device
[h264_v4l2m2m @ 0xd0dd10] can't configure encoder