Disappearing transparent pixels when using .resize() feature

EliTheDev commented 1 year ago

Below is simplified code for a script I'm programming to overlay PNG files with subtitles over a video:

import moviepy.editor as moviepy
from moviepy import *

input_vid = moviepy.VideoFileClip("input.mp4").set_duration(10)
input_width = moviepy.VideoFileClip("input.mp4").w
input_height = moviepy.VideoFileClip("input.mp4").h

clips = [input_vid];
img = moviepy.ImageClip('./subtitles/1.png')
clips.append(   img.set_start(0).set_duration(5).set_pos(lambda t: (    input_width / 2 - img.w*(1+t/5)/ 2, input_height / 2 - img.h*(1+t/5)/2  )).resize(lambda t: 1+t/5)     )   #  #.set_pos(((input_width / 2) - (img_w / 2), (input_height / 2) - (img_h/2)))   )

print('Compositing...')
video = moviepy.CompositeVideoClip(clips).set_duration(10)
print('Done.')
print('Exporting...')
video.write_videofile("moviepy_test.mp4",codec='libx264')

-->

Expected Behavior

The photo contains a transparent shadow generated with PIL. It's imported as a PNG.

It should overlay over the video like so and slowly grow in size:

Actual Behavior

Instead, the overlay does increase is size however the transparent pixels disappear:

Steps to Reproduce the Problem

Import video and photo with transparent pixels
Increase the photo over time with lambda and .resize()
Composite the two together
Export the result

import moviepy.editor as moviepy

#import base video and image with transparent pixels
input_vid = moviepy.VideoFileClip("input.mp4").set_duration(5)
opaque_img = moviepy.ImageClip('./subtitles/1.png').set_duration(3)

#overlay the two together and resize the overlay with lambda animation
output_vid = moviepy.CompositeVideoClip([ input_vid, opaque_img.resize(lambda t: 1+t).set_position(('center', 'center')) ])

#export
output_vid.write_videofile("output.mp4",codec='libx264')