lavfi

# FFMPEG
# lavfi (Libavfilter input virtual device.) youtube video link https://youtu.be/ysMyZggmQfQ


# to make a 4 minute plain black video:
ffmpeg -f lavfi -i color=c=black:duration=240:size=1920x1080 blank4min.mkv


# Here's a link to lavfi in the online documentation: https://ffmpeg.org/ffmpeg-all.html#lavfi


# In the first example you create a color video stream and play it back with ffplay:


ffplay -f lavfi -graph "color=c=pink [out0]" dummy

# the same command but removed the graph and "out0" labels:

ffplay -f lavfi color=c=pink


# three different video test filtered sources and played them:

ffplay -f lavfi -graph "testsrc [out0]; testsrc,hflip [out1];testsrc,negate [out2]" test3



# an example to read an audio stream from a file using the amovie source and play it back with ffplay:

ffplay -f lavfi "amovie=Doom666.mp3"

#(If you look hard you might be able to see the hidden image?)
# I chose the Doom soundtrack because I knew that they had somehow put image into the sound, 
# I don't understand it and I don't know how they did it.
# I found a better visualisation in this link: https://www.youtube.com/watch?v=yzFit0nldf4

# I also found an ffmpeg example in the following link: https://www.bogotobogo.com/FFMpeg/ffmpeg_video_test_patterns_src.php

# This example will create a 10 second output, 30 fps (300 frames total):

ffmpeg -f lavfi -i testsrc=duration=10:size=1920x1080:rate=30 testsrc.mp4

# There are some other examples on the links that I've provided, 
# and the ffmpeg manual goes into more detail about what lavfi is.