[FFmpeg-user] Question about handling / unwanted changes of SAR in ffmpeg
uwe.freese at gmx.de
Tue Dec 25 21:54:57 EET 2018
I'm confused about how ffmpeg handles the SAR (sample aspect ratio) and
changes it. Maybe someone can explain why this happens:
I've a video from TV with original size 720x576 pixels (PAL), which
shall be shown as 16:9 aspect ratio.
The SAR of the input video is correctly shown as 64:45 (resulting in
720*64 / 576*45 = 46080 / 25920 = 16 / 9 display aspect ratio).
When I crop black borders and encode this video using x264 codec and mkv
format, ffprobe shows about the result:
Stream #0:0: Video: h264 (High), yuv420p(progressive), 688x560 [SAR
64:45 DAR 2752:1575], SAR 172:121 DAR 7396:4235, 50 fps, 50 tbr, 1k tbn,
100 tbc (default)
What does SAR (and DAR) mean in the brackets compared to the second SAR
172:121, which is slightly different?
Why isn't the second one also telling the same SAR 64:45 (which would be
the correct one)?
It gets stranger when I convert the video a second time using "ffmpeg -i
video_encode_x264.mkv test.mkv". ffmpeg seems to use the second (wrong)
SAR and shows:
Stream #0:0: Video: h264 (libx264) (H264 / 0x34363248), yuv420p,
688x560 [SAR 172:121 DAR 7396:4235], q=-1--1, 50 fps, 1k tbn, 50 tbc
Why is ffmpeg now using the second (wrong) SAR?
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