Merge two videos with transparency in ffmpeg

With the overlay filter you don't need the movie= format. It's easier, so use this format instead:

ffmpeg -y -i inputmovie -i inputpicture.png -filter_complex 'overlay=0:0' -s 1280x720 ...other parameters... outputfile.mp4

The size I put in is as an example only- yours will be different. Note that the size is the same for the picture as well as the movie frame, and no scaling is actually being done here. The 0:0 is for alignment of the picture to the top left of the movie frame- this is logical since they are both the same size.

H.264, which is what libx264 encodes to, does not support transparency. Neither does mpeg4, the default codec for AVI. MOV output defaults to libx264 as well.

You can do this in one command:

ffmpeg -i original.mp4 -framerate 60 -pattern_type glob -i images/*.png \
-filter_complex "[0:v]scale=1920x960,setdar=16:9[base];[1:v]scale=1920x960,setdar=16:9[ovr];\
 [ovr][base]blend=all_mode='overlay':all_opacity=0.7[v]"
-map [v] result.mp4

You don't need to scale both videos.If you just want the PNGs to scale the same as the base video, you can use scale2ref:

ffmpeg -i original.mp4 -framerate 60 -pattern_type glob -i images/*.png \
-filter_complex "[1:v][0:v]scale2ref=iw:ih[ovr][base]; \
 [ovr][base]blend=all_mode='overlay':all_opacity=0.7[v]"
-map [v] result.mp4

Now, you have specified a custom DAR (1920x960 is not 16:9). If that needs to be set, use

[1:v][0:v]scale2ref=iw:ih,setdar=16:9[ovr][base];

This uses a different method to apply the overlay.

ffmpeg -i original.mp4 -framerate 60 -pattern_type glob -i "images/*.png" \
-filter_complex "[1:v][0:v]scale2ref=iw:ih[ovr][base]; \
 [ovr]colorchannelmixer=aa=0.7[ovrl]; [base][ovrl]overlay[v]"
-map [v] result.mp4