ffmpeg alphamerge two videos into a gif with transparent background

Question

I had this question answered that helped me merge two mp4 videos with alphamerge with the following command to an .mov file

ffmpeg -i video.mp4 -i mask.mp4 -filter_complex "[1][0]scale2ref[mask][main];[main][mask]alphamerge" -c:v qtrle output.mov

Now, I was wondering how I would change this for the output of a gif. When I tried

ffmpeg -i word.mp4 -i word.matte.mp4 -filter_complex "[1][0]scale2ref[mask][main];[main][mask]alphamerge" -c:v qtrle output.gif

I got this error

[swscaler @ 0x5601daaa4d40] No accelerated colorspace conversion found from yuv420p to argb.
[gif @ 0x5601d9ad2d00] GIF muxer supports only a single video GIF stream.
Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument

I'm using ffmpeg 4.2.4

Answer 1

Combining your command with the command from How do I convert a video to GIF using ffmpeg, with reasonable quality?:

ffmpeg -i word.mp4 -i word.matte.mp4 -filter_complex "[1][0]scale2ref[mask][main];[main][mask]alphamerge,fps=10,scale=320:-1,split[s0][s1];[s0]palettegen[p];[s1][p]paletteuse" output.gif

Note the removal of -c:v qtrle. You can't put QuickTime Animation RLE video into GIF output.

If you want a background photo combine command from How to overlay with ffmpeg?

ffmpeg -i word.mp4 -i word.matte.mp4 -i background.jpg -filter_complex "[1][0]scale2ref[mask][main];[main][mask]alphamerge[fg];[2][fg]overlay=(main_w-overlay_w)/2:(main_h-overlay_h)/2:format=auto,fps=10,scale=320:-1,split[s0][s1];[s0]palettegen[p];[s1][p]paletteuse" output.gif