"illegal image mode" when trying to subtract a png from another by using putalpha

[Nicolas-Gth]

This is my code

reflexionBackground = Image.new('RGBA', image.size, (255, 0, 0, 0))
reflexion = Image.open("./assets/ProgressBar/Reflexion.png")
reflexionBackground.alpha_composite(reflexion, (50, 55))
reflexionBackground.save("reflexionBackground.png")

substractShapeBackGround = Image.new('RGBA', image.size, (255, 0, 0, 0))
substractShape = Image.open("./assets/ProgressBar/SubstractShape.png")
substractShapeBackGround.alpha_composite(substractShape, (fillXCoord, 0))
substractShapeBackGround.save("substractShapeBackGround.png")

substractShapeBackGround.convert('L').resize(reflexionBackground.size)
reflexionBackground.putalpha(substractShapeBackGround)

reflexionBackground.png :

substractShapeBackground :

I'd like to substract the black shape from reflexionBackground.png

[radarhere]

I think you've correctly understood that the alpha argument to putalpha() "can either be an “L” or “1” image"

However, what I think you've missed is that convert() "Returns a converted copy of this image". A copy. It does not modify the original. The same goes for resize().

So I would suggest just changing

substractShapeBackGround.convert('L').resize(reflexionBackground.size)

to

substractShapeBackGround = substractShapeBackGround.convert('L').resize(reflexionBackground.size)

[Nicolas-Gth]

I added a zip file with the assets

My goal is to get this :

[radarhere]

If I do reflexionBackground = Image.new('RGBA', image.size, (0, 0, 0, 0)) and substractShapeBackground = Image.new('RGBA', image.size, (0, 0, 0, 0)) I get

But if I do reflexionBackground = Image.new('RGBA', image.size, (0, 0, 0, 0)) and substractShapeBackground = Image.new('RGBA', image.size, (255, 255, 255, 0)) I get

I don't understand why changing the RGB values makes a difference if they're set to transparent, and I would like to have my reflection with transparency on a fully transparent background.

substractShapeBackGround.convert('L') is converting an RGBA image to L, a greyscale mode. To do that, Pillow simply ignores the transparency at that point, and only uses the RGB values. So that is why different RGB values produces different results.

If your intention was to get the alpha channel, I would suggest substractShapeBackGround.getchannel("A") instead. Using that, both of your scenarios here produce the same reflexionBackground in the end.

[Nicolas-Gth]

Mmh, maybe I wasn't clear when explaining what I am trying to obtain.

So after executing this code :

reflexionBackground = Image.new('RGBA', image.size, (0, 0, 0, 0))
reflexion = Image.open("./assets/ProgressBar/Reflexion.png")
reflexionBackground.alpha_composite(reflexion, (50, 55))
reflexionBackground.save("reflexionBackground.png")

substractShapeBackground = Image.new('RGBA', image.size, (255, 255, 255, 0))
substractShape = Image.open("./assets/ProgressBar/SubstractShape.png")
substractShapeBackground.alpha_composite(substractShape, (fillXCoord, 0))

substractShapeBackground = substractShapeBackground.convert('L')
substractShapeBackground.save("substractShapeBackground.png")

I have these two images :

What I would like to obtain :

What

reflexionBackground.putalpha(substractShapeBackground)
reflexionBackground.save("reflexionBackgroundFinal.png")

gives me :

[radarhere]

Ok, then, how about this? The following code

output = Image.new("RGBA", reflexionBackground.size)
output.paste(reflexionBackground, mask=substractShapeBackground)
output.save("current.png")

creates

[Nicolas-Gth]

Oh, yes it's working ! Thank you so much