FFMPEG: crop a video without losing the quality

Question

I have mp4 video of 1920x1080. I would like to crop the video to 480x270 without quality loss.

I am using the following command:

ffmpeg -i input.mp4 -filter:v "crop=480:270:200:200" -crf 23 output.mp4 

I also tried:

ffmpeg -i input.mp4 -filter:v "crop=480:270:200:100" -c:a copy -qp 0 output.mp4 

I used -crf 23 and -qp 0 for loseless video cropping, but after cropping video has lost quality.

Does anyone know how I can crop the video without losing the quality?

Answer 1

You can't perform any filtering without losing quality when encoding to a lossy format, but you have some options.

Crop with your player

A possible solution would be to crop during playback, so you don't even need to re-encode. It is also useful to preview a crop.

This method will not create an output file. This will use your video player to crop while it is playing. See one of the other methods below if you want an output file.

With ffplay and crop filter:

ffplay -vf "crop=480:270:200:100" input.mp4 

With vlc (or cvlc):

vlc input.mp4 --crop=480x270+200+100 

Or you could crop with the VLC GUI: Tools → Effects & Filters → Video Effects → Crop.

Accept some quality loss (you may not even notice a difference)

Give it enough bits and you may not be able to tell there is a quality difference:

ffmpeg -i input -vf "crop=480:270:200:100" -c:v libx264 -crf 17 -c:a copy ouput.mp4 

See FFmpeg Wiki: H.264 Video Encoding Guide for more info.

Use a bitstream filter

The h264_metadata and hevc_metadata bitstream filters can set crop dimensions without modifying the video itself.

Note: Your player may not support this method.

Example for H.264 video:

ffmpeg -i input.mp4 -c copy -bsf:v h264_metadata=crop_left=100:crop_right=20:crop_top=10:crop_bottom=10 output.mp4 

• Sets the frame cropping offsets in the SPS. These values will replace the current ones if the stream is already cropped.

• These fields are set in pixels. Note that some sizes may not be representable if the chroma is subsampled or the stream is interlaced (see H.264 section 7.4.2.1.1).

Use a lossless format

ffmpeg can encode with several lossless encoders: ffv1, huffyuv, ffvhuff, utvideo, libx264 (using -crf 0 or -qp 0). The output will be lossless but the output file will be huge.

Note: Your player may not support this method.

ffmpeg -i input.mp4 -vf "crop=480:270:200:100" -c:v ffv1 -c:a copy output.mkv 

or

ffmpeg -i input.mp4 -vf "crop=480:270:200:100" -c:v libx264 -crf 0 -c:a copy output.mp4 

If your input is MJPEG

Stream copy the individual images with ffmpeg, crop them losslessly with jpegtran, then remux them with ffmpeg. This will result in no loss, but you will be restricted to the ancient MJPEG format.

Answer 2

At a basic level you cannot make use of a lossy encoding and then expect it to not lose quality when you decode and then encode again. The only way that works is to make use of a lossless codec, for example Quicktime with the Animation codec. This is just a basic truth of digital video production that you cannot work around by just passing command line options to ffmpeg.