How to paste an image on top of another, the empty pixels of the background remain empty Python PIL

Question

I want to paste the first image (which is a simple vertical gradient, from transparent to black), at the bottom of the background. The empty pixels in the background need to remain empty, and the rounded corners must be preserved. I know I need to use some combination of alpha_composite, paste and masking but really could not figure it out. How can I achieve this using python pillow library?

Paste image

Background

Expected result

Answer 1

Here's a way to do that:

#!/usr/bin/env python3

from PIL import Image

# Open background and ensure RGBA
bg = Image.open('bg.png').convert('RGBA')

# Open foreground, ensure RGBA and match to size of background
fg = Image.open('fg.png').convert('RGBA').resize(bg.size)

# Put aside background alpha channel
bgA = bg.getchannel('A')

# Paste foreground over background with alpha
bg.paste(fg, mask=fg)

# Reapply transparent cut-outs from original background
bg.putalpha(bgA)

bg.save('result.png')