cliping a webm file using moviepy and ffmpeg parameters

Question

Using moviepy, I am trying to trim a section of a webm file like this:

my_file.write_videofile(name, codec = 'libvpx')

Of course, I have already defined the beginning and end of the clip, etc. The code is returning the segment I want, however, I am noticed a decrease in the quality of the file. I am not resizing or constraiing the file size anywhere, so I don't understand why the clip has an inferior quality compared to the original.

There are some parameters that I could play with, which I suspect are set as defaults in moviepy to speed of video manipulation, but the documentation of moviepy does not say anything about them:

ffmpeg_params :

Any additional ffmpeg parameters you would like to pass, as a list of terms, like [‘-option1’, ‘value1’, ‘-option2’, ‘value2’]

Anybody outhere is familiar with the right parameters to keep the quality of the original file? As an alternative, is anybody is familiar with any other library to trim webm files?

Below are two pics showing the difference in quality. The first one is a frame of the trimmed file, the second one is approximately the same frame, for the original file.

Thank you

Answer 1

The parameter you are looking for is "bitrate" (for some reason I omitted it in the docs, it will be fixed for the next versions). If you don't provide it, ffmpeg has a default value which is indeed very low.

myclip.write_videofile("test_1.webm", bitrate="50k") # low quality.
myclip.write_videofile("test_2.webm", bitrate="50000k") # high quality.

You can also tune the bitrate of the audio with `audio_bitrate='50k' by the way. The bitrate gives ffmpeg an upper bound on what the bitrate can be, but most of the time when you provide "50000k" the actual bitrate will be below "50000k". 50000k provides nice-quality videos, but keep in mind that webm is still a lossy format.