Section: Re-encoding & Transcoding
The built-in video transcoding function in the LBRY desktop application starts the FFMPEG utility process with the following parameters:
ffmpeg -i "$(path_to_original_file).ext" -y -c:s copy -c:d copy \
-c:v libx264 -crf 24 -preset faster -pix_fmt yuv420p \
-vf "scale=if(gte(iw\,ih)\,min(1920\,iw)\,-2):if(lt(iw\,ih)\,min(1920\,ih)\,-2)" \
-maxrate 5500K -bufsize 5000K -movflags +faststart \
-c:a aac -b:a 160k "$(path_to_original_file)_fixed.mp4"
The meanings of the passed parameters:
-c:s copy
and
-c:d copy
tell FFMPEG to copy subtitles and data, respectively.
-c:v libx264 -crf 24 -preset faster -pix_fmt yuv420p
tells FFMPEG to transcode the video using the H.264 codec with constant rate factor equal to 24, with the faster preset and using the YUV color space with the 4:2:0 scheme for chroma subsampling.
-vf "scale=if(gte(iw\,ih)\,min(1920\,iw)\,-2):if(lt(iw\,ih)\,min(1920\,ih)\,-2)"
specifies that the height and width of the video cannot be greater than 1920 pixels, and both values must be a multiple of two.
-maxrate 5500K -bufsize 5000K
specifies the maximum bitrate to be 5500 Kb/s with a buffer equal to 5000 Kb/s.
-movflags +faststart
tells FFMPEG to move the «moov atom» (the metadata) from the end of the file to its beginning to improve playback in browsers.
-c:a aac -b:a 160k
tells FFMPEG to transcode the audio with bitrate equal to 160 kb/s.
An alternative option for transcoding a horizontal video with increased bitrate and quality and better compression:
ffmpeg -i "$(path_to_original_file).ext" -y -c:s copy -c:d copy \
-c:v libx264 -crf 17 -preset slower -pix_fmt yuv420p \
-maxrate 8M -bufsize 8M -movflags +faststart \
-c:a aac -b:a 160k "$(path_to_original_file) (Transcoded).mp4"
One may select optimal CRF values by performing tests on a sample of the target file with a faster preset. The average bitrate of the transcoded test file should be close (on the left side) to the target bitrate. The CRF value of 17 is sufficient to produce a video with indistinguishable quality (using a slow preset). Note that with a fixed CRF value, the target bitrate may not always be reached, and the encoder will use more bits if necessary.
A sample can be cut from the source file like this:
ffmpeg -i input.ext -ss 00:01:00 -to 00:02:00 -c copy sample.ext
References: