Use ffmpeg to flip mp4 video horizontally without losing quality, and inceasing file size (too much)

Question

As the title says, I want to flip an mp4 video horizontally, without losing quality, and without increasing the file size (if possible). Apparently due to having to re-encode, the file size will increase no matter what.

My current command is something like:

ffmpeg -i input.mp4 -vf hflip -qscale 0 -c:a copy output.mp4

Answer 1

Technically, not possible. Filtering requires encoding. When using lossy encoders you lose quality and have generation loss . Best you can do is provide a quality level or bitrate that provides an acceptable tradeoff between quality loss and file size. Assuming you are encoding with libx264 see FFmpeg Wiki: H.264 for detailed instructions.

One method to keep quality and file size is flipping during playback, but I doubt that is an acceptable solution:

ffplay -vf hflip input.mp4