$ muxmovie -transparency-mode help
Error getting transparency mode. Correct syntax is one of:
No transparency.
Use the alpha channel that's in the movie.
The specified color is transparent; all others are opaque. red, green, and blue should be between 0 and 255.
The specified color is transparent; all others are opaque. red, green, and blue should be between 0 and 65535.
Blend the two images using the specified proportion for each channel. red-prop, green-prop, and blue-prop should be real numbers between 0 and 1.