Downscaling VP9 from 4K to 1080p without re-compression

Question

I have a video file that is a 4K upscale of a 1080p video in VP9 (upscaled to 4K to force Youtube to use higher bitrate for the video).

I want to downscale the 4K video file to 1080p while (effectively) losing no data, ie not forcing re-compression of the finished product.

I'm not familiar with VP9 so I'm not sure if this even makes sense in the context of how the codec works, but any advice is helpful.

A note, if it would result in a file size reduction even if slight then that would be ideal.

Answer 1

Downscaling requires using the scale filter, and filtering requires encoding.

Options:

• accept some quality loss (but you may not notice)
• use a lossless encoder (giant files, probably not what you actually want)
• use your player to downscale as it plays

Accept some quality loss

Just provide it enough bits and you may not notice. Assuming you want VP9 output, an example using a somewhat arbitrary -crf value:

ffmpeg -i input.webm -vf "scale=-2:1080" -c:v libvpx-vp9 -crf 20 -b:v 0 -c:a copy output.webm

Adjust -crf as needed. Lower is better quality.

See FFmpeg Wiki: VP9.

Using a lossless encoder

Assuming you want VP9 output:

ffmpeg -i input.mp4 -vf "scale=-2:1080" -c:v libvpx-vp9 -lossless 1 -c:a copy output.webm

This will make a huge sized file.

See FFmpeg Wiki: VP9.

Use a player

This will use a player named ffplay to downscale while playing. No output file is created.

ffplay -vf scale=-2:1080 input.webm