CompositeVideoClip with CompositeVideoClip ignores alpha channel

[samikhailov]

Expected Behavior

When using a CompositeVideoClip that contains another CompositeVideoClip containing an image with an alpha channel, transparency works correctly.

Actual Behavior

When using a CompositeVideoClip that contains another CompositeVideoClip containing an image with an alpha channel, the transparent part is filled with black.

Steps to Reproduce the Problem

bg_cl = VideoFileClip("path.mp4")
image_cl = ImageClip("image.png")  # an image with alpha channel 
union_cl = CompositeVideoClip([bg_cl, image_cl]).with_duration(3)
union_cl.write_videofile("ok.mp4", fps=3, codec="libx264", threads=4)

result_cl = CompositeVideoClip([union_cl])
result_cl.write_videofile("not_ok.mp4", fps=3, codec="libx264", threads=4)

ok.mp4 not_ok.mp4

Specifications

• Python Version: 3.9.16
• MoviePy Version: 2.0.0.dev0 @69ac313
• Platform Name: macOS
• Platform Version: Ventura 13.2.1

Extra

There is no problem in the MoviePy v1 in this part.

[samikhailov]

If I add the use_bgclip=True flag, then the background of the image becomes transparent.

union_cl = CompositeVideoClip([bg, image_cl], use_bgclip=True).with_duration(3)