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map the video of video.mp4 to the video of mix.mp4
map the audio of audio.mp3 to the audio of mix.mp4
make sure that video.mp4 and audio.mp3 have the same duration
fade [type] fade-in-length [stop-time [fade-out-length]]
Apply a fade effect to the beginning, end, or both of the audio.
An optional type can be specified to select the shape of the fade curve: q for quarter of a sine wave, h for
half a sine wave, t for linear (`triangular') slope, l for logarithmic, and p for inverted parabola. The
default is logarithmic.
A fade-in starts from the first sample and ramps the signal level from 0 to full volume over fade-in-length sec?
onds. Specify 0 seconds if no fade-in is wanted.
For fade-outs, the audio will be truncated at stop-time and the signal level will be ramped from full volume
down to 0 starting at fade-out-length seconds before the stop-time. If fade-out-length is not specified, it
defaults to the same value as fade-in-length. No fade-out is performed if stop-time is not specified. If the
file length can be determined from the input file header and length-changing effects are not in effect, then 0
may be specified for stop-time to indicate the usual case of a fade-out that ends at the end of the input audio
All times can be specified in either periods of time or sample counts. To specify time periods use the format
hh:mm:ss.frac format. To specify using sample counts, specify the number of samples and append the letter `s'
to the sample count (for example `8000s').
With the -vn switch we make our intentions clear and ask FFmpeg not to bother itself with the video. Next we specify the audio codec copy, which tells FFmpeg to use the same codec to encode the audio, which it uses to decode it. To keep things simple, we'll just keep the sampling and bitrate values the same.
convert -crop 51x34 +repage http://www.google.com/images/flags/sprite_flag-50.png flag.png