sudo apt-get install webp
dwebp file.webp -o file.png

#check:
convert file.webp ppm:- | sha1sum
convert file.png ppm:- | sha1sum
#or
if [ "$(convert file.webp ppm:- | sha1sum)" == "$(convert file.png ppm:- | sha1sum)" ]; then echo "equal"; else echo "not equal"; fi

#recursively converting:
find . -name '*.webp' -type f -exec bash -c 'dwebp "$0" -o "${0%.webp}.png"' {} \;
#find . -type f -name '*.webp' -delete

cwebp -z 9 "file.png" -o "file.webp"

#recursively converting:
find . -name '*.png' -type f -exec bash -c 'cwebp -z 9 "$0" -o "${0%.png}.webp"' {} \;
#find . -type f -name '*.png' -delete

Webm is a container similar to mkv and can contain various encoded video or audio streams. To change the container from webm to mkv, the following command can be used:

ffmpeg -i file.webm -c:a copy -c:v copy file.mkv

#recursively changing containers:
find . -name '*.webm' -type f -exec bash -c 'ffmpeg -i "$0" -c:a copy -c:v copy "${0%.webm}.mkv"' {} \;
#find . -type f -name '*.webm' -delete

To only change the container use the following example command - provided the codecs are compatible with mp4. The quality and size stays the same:

ffmpeg -i input.mkv -codec copy output.mp4

The following command scales a video to 720p x264 codec with 30fps target framerate, copying the audio as is and any subtitles while reducing the overall quality with CRF of 28.

ffmpeg -i input.mp4 -vf scale=-1:720 -c:v libx264 -r 30 -crf 28 -c:a copy -scodec copy output.720p.mkv

CRF option explained:

1. The range of the quantizer scale is 0-51: where 0 is lossless, 23 is default, and 51 is worst possible. A lower value is a higher quality and a subjectively sane range is 18-28. Consider 18 to be visually lossless or nearly so: it should look the same or nearly the same as the input but it isn't technically lossless.
2. The range is exponential, so increasing the CRF value +6 is roughly half the bitrate while -6 is roughly twice the bitrate. General usage is to choose the highest CRF value that still provides an acceptable quality. If the output looks good, then try a higher value and if it looks bad then choose a lower value.

Scale option -1 means the output has to be divisible by 1 with same aspect ratio. Scale option -2 means the output has to be divisible by 2, etc.

To limit bitrate to 2Mbit, add

-b:v  2M -maxrate 2M -bufsize 1M

Useful bash script, put in ~/.local/bin and chmod +x it after:

video_downscale.sh

#!/bin/bash
maxwidth=1280
maxheight=720
bitrate="2M"
bufsize="1M"
# mp4 is better for streaming, mkv supports all sorts of mixed codecs and subtitles
#container="mkv"
container="mp4"
#encoder="libx264" #libx264 = CPU, better quality and much smaller filesize; h264_amf = AMD GPU; h265_nvenv = Nvidia GPU; h264_qsv = Intel GPU
 
# ffpb is a wrapper for ffmpeg to show a progress bar and the remaining time.
# install it using:
# pip install ffpb
 
# Check if ffpb is installed, then use it, otherwise use normal ffmpeg
builtin type -P "ffpb" &> /dev/null && binary="ffpb" || binary="ffmpeg"
 
# Check if a parameter is provided
if [ -z "$1" ]; then
    echo "Usage: "$(basename $0)" <file or files>"
    exit 1
fi
 
for input in "$@";
do
    if [ -f "$input" ]; then
        echo "Processing file: $input"
        output="${input%.*}.convertednew.$container"
        # Check if mp4 container is desired and if subtitle is ASS format, then convert to srt, otherwise just copy subs
        subtitles_present=$(ffprobe -v error -select_streams s -show_entries stream=codec_name -of csv=p=0:s=x "$input")
        if [ "$subtitles_present" == "ass" ] && [ "$container" == "mp4" ];
        then
            subtitle_option="-c:s mov_text -metadata:s:s:0 language=en"
        else
            subtitle_option="-scodec copy"
        fi
        #-c:a copy   #copies audio as is, but mp4 works best with aac and wma cannot be in mp4 files
        #$binary -i "$input" -vf scale=-2:$resolution -c:v libx264 -r 30 -crf 28 -c:a aac -scodec copy -b:v $bitrate -maxrate $bitrate -bufsize $bufsize "$output"
        # using complex filter to prevent upscaling and only ever downscale
        cmd=$binary' -i "'"$input"'" -filter_complex "scale=ceil(iw*min(1\,min('$maxwidth'/iw\,'$maxheight'/ih))/2)*2:-2" -c:v libx264 -r 30 -crf 28 -c:a aac '$subtitle_option' -b:v '$bitrate' -maxrate '$bitrate' -bufsize '$bufsize' "'"$output"'"'
        eval $cmd
        if [ $? -eq 0 ];
        then
            actualheight=$(ffprobe -v error -select_streams v -show_entries stream=height -of csv=p=0:s=x "$output")
            #output2="${output%.*}."$actualheight"p.$container"
            output2="${output//.convertednew./.$actualheight""p.}"
            mv "$output" "$output2"
            echo "Output file: $output2"
        else
            echo "An error occured. File not converted properly!"
            echo "Full ffmpeg command:"
            echo "${cmd//ffpb/ffmpeg}"
        fi
    fi
done

To add this to Nemo filemanager as right-click option for video files, create a nemo_action file in ~/.local/share/nemo/actions

video_downscale.nemo_action

[Nemo Action]
Name=Video Downscale to max 720p
Comment=Video Downscale to max 720p
Exec=video_downscale.sh %F;echo;read -rsn1 -p "Press any key to continue . . .";echo
Icon-Name=stock_down
Selection=notnone
Extensions=mp4;wmv;avi;mkv;mov
#Quote=double
EscapeSpaces=true
Terminal=true