Known errors

This is a list of errors that I know of / expect, but trust the user to avoid. Some of them can also be safely ignore.

Audio filters and stream copying:

The script will try to copy the input audio stream if three requirements are met:

• Has the same codec (Vorbis or Opus, based on the -n flag)
• The audio bitrate is less or equal the one being chosen by the script
• Trim mode is inactive If you want to apply audio filters via the -f flag, while the script tries to copy the input audio stream, then ffmpeg will throw an error (as you can't apply filters without encoding). To avoid this error use the trim mode for such files and simply hit enter, when asked for further input.

VP9 and -tune ssim:

The -tune ssim option leads to better results while using VP8. VP9 doesn't offer this option (despite ffmpeg's internal documentation saying the opposite), so you'll see an error message when using VP9 (Failed to set VP8E_SET_TUNING codec control: Invalid parameter. Additional information: Option --tune=ssim is not currently supported in VP9.). This message however can be safely ignored, as ffmpeg ignores the -tune option and continues as usual.

Pictures with transparency:

VP8 (and perhaps VP9, haven't tested it yet) is unable to handle input images with transparency (e.g. RGBA png files). It seems like there is a bug that forces the user to deactivate the alternate reference frame manually (should be off by default) in order to be able to use such pictures. Since I can't think of a way to detect those pictures with ffprobe, I'll leave it to the user to make sure, that no input picture has an alpha channel.

Gifs are the exception for this problem. The script gives them their own ffmpeg commands.