ffmpeg intro

Always make sure that you have the latest version installed
ffmpeg.zeranoe.com

STANDARD VIDEO ENCODE
ffmpeg -i input.file -ss 00:00:10.000 -to 00:00:20.000 -c:v libvpx -crf 4 -vf scale=640:-1 output.file

Merging audio
video.file -i audio.file  -c:v copy/libvpx -c:a copy/libvorbis -strict experimental output.file
Extracting/converting audio
input.file output.file
Extracting and/or converting into multiple different formats
input.file output.file1 output.file2 output.file3 etc
Using image instead of video
-loop 1 -i image.file -i audio.file output.file

OVERLAY
One overlay:
-filter_complex "[0:v][1:v] overlay=x:y:enable='between(t,0,20)'"
[0:v] refers to the first video stream (or the first input file) [1:v] refers to the second video stream (or the second input file) that is used as the overlay.
overlay=x:y means that the overlay will be placed y pixels down and x pixels to the right from the top left corner that is 0:0.
enable='between(t,0,20)' means that the overlay will be placed between 0 and 20 seconds, remove to make the overlay present from start to finish
Multiple overlays:
-filter_complex -"[0:v][1:v] overlay=x:y:enable='between(t,0,20)' [tmp]; [tmp][2:v] overlay=x:y:enable='between(t,0,20)' [tmp]; [tmp][3:v] overlay=x:y:enable='between(t,0,20)'"
[tmp] is a temporary output file that you then need to specify as the source (instead of [0:v]) or else ffmpeg will ignore the subsequent commands leading to only one overlay present in output file

HARDCODE SUBTITLES INTO VIDEO
Image based:
input.file -filter_complex "[0:v][0:s(:#)]overlay[v]" -map "[v]" -map 0:a output.file
change # according to which subtitle you want. 0 is first subtitle stream, 1 is the second. etc.
Ass sub external:
input.file -vf subtitles=sub.file output.file
Ass sub internal:
input.file -vf subtitles=input.file output.file (add :si=1 for second stream)
Can only hardcode .ass subtitles
Converting to ass
sub.srt sub.ass
Change subtitle color and font
input.file subtitles=sub.srt:force_style='FontName=DejaVu Serif,PrimaryColour=&HAA00FF00' output.file
Extracting subtitle from video
input.file -an -vn -c:s:0 subfile sub.subfile
remember to hardcode video before changing resolution

MERGE VIDEO
Vertically merge:
-i input1.file (top) -i input2.file (bottom) -filter_complex vstack output.file
Horizontally merge:
-i input1.file (left) -i input2.file (right) -filter_complex hstack output.file
note:when stacking vertically/horizontally quality of video might degrade
Splice video:
-i input1.file -i input2.file -filter_complex "[0:v:0] [1:v:0] concat=n=2:v=1 [v]" -map "[v]" output.file
Splice video with audio:
-i input1.file -i input2.file "[0:v:0] [0:a:0] [1:v:0] [1:a:0] concat=n=2:v=1:a=1 [v] [a]" -map "[v]" -map "[a]" output.file
Splice multiple videos
-f concat -i textfile.file -fflags +genpts output.file
note: textfile should have multiple lines looking like this:
file 1.mp4
file 2.mp4
file 3.mp4
file 4.mp4
(mp4 can be changed to other video formats)

FILTERS
-ss (start of video)
-to (end of video)
-c:v (video codec)
-c:a (audio codec)
-b:v [enter number followed by K (Kilobit) or M (Megabit)] (video bitrate)
-b:a [cannot be set lower than 50k] (audio bitrate)
-vf scale=height:width [-1 on either side to scale resolution to corresponding height/width] (change resolution)
-auto-alt-ref [0/1] (disables/enables alternate reference frames)

OPTIONAL
-y (automatically replace duplicate file)
-threads # (number of threads used)
-metadata title="title" (embedding title to video)
-an (disable audio)
-sn (disable subtitles)
-vn (disable video)
-r (framerate (not recommended))
-aq 5 [higher is better] (constant audio quality)
-crf [4-63 lower is better (0 for lossless)] (constant video quality)
-bufsize (for more accurate average bitrate)
-minrate (sets minimum video bitrate (useless))
-maxrate (sets maximum video bitrate, used in conjunction with -bufsize)
-acodec [can be used instead of -c:a] (audio codec)
-vcodec [can be used instead of -c:v] (video codec)
-movflags +faststart (will make the video play before completely dl'd by browser (only for browser viewing))
-stream_loop -1 (loops video)
-loop 1 (loops image (not gif))

OTHER FILTERS
-strict experimental
-filter_complex
-vf=video filter
-af=audio filter

AUDIO CODECS
libvorbis (.ogg, low quality)
libfaac (.aac)
libmp3lame (.mp3)
libopus (.ogg, high quality, does not support quality based commands)

VIDEO CODECS
libvpx
libx264

COMMANDS
-formats (lists all supported formats)
-codecs (lists all supported codecs)
-filters (lists all supported filters)
-help (lists commands)