使用ffmpeg生成视频缩略图而不会出现任何失真

Question

To get a 200x100 thumbnail from a video, I do ffmpeg -ss 100 -i /tmp/video.mp4 -frames:v 1 -s 200x100 image.jpg . But if the source video isn't in the same aspect ratio as 200x100 , the thumbnail gets distorted (either stretched or squished, horizontally or vertically) and it looks bad.

Is there a way that ffmpeg can figure out for example that a 500x200 video is 100px too wide, and remove 50px from the right and 50px from the left, making the video 400x200 ? And because 400x200 is the same aspect ratio as 200x100 , the thumbnail would have no distortion.

I know there are other tools that can do this to the thumbnails generated by ffmpeg , but I'd prefer doing it within ffmpeg and not having to process the thumbnails again.

Answer 1

If your thumbnail size is 200x100 fixed, then run

ffmpeg -ss 100 -i /tmp/video.mp4 -vf "scale='if(gt(dar,200/100),100*dar,200)':'if(gt(dar,200/100),100,200/dar)',setsar=1,crop=200:100"  -frames:v 1 image.jpg

The scale filter checks the aspect ratio of the source and scale so that one dimension fits the 200x100 canvas and the other overshoots, unless it's a perfect match. Then the crop filter crops it to 200x100 from the center thus taking care of the out of bounds region.

Answer 2

您可以在比例过滤器中使用force_original_aspect_ratio选项。

ffmpeg -ss 100 -i /tmp/video.mp4 -frames:v 1 -q:v 2 -vf "scale=200:100:force_original_aspect_ratio=increase,crop=200:100" image.jpg