Transparent BackgroundColor turns black

[tryashtar]

Hello :) I am trying to save an image with a white transparent background.

This is what I tried:

using var test = Image.Load<Rgba32>("in.png");
test.Mutate(x => { x.BackgroundColor(Color.FromRgba(255, 255, 255, 0)); });
var encoder = new PngEncoder() { TransparentColorMode = PngTransparentColorMode.Preserve };
test.SaveAsPng("out.png", encoder);

Using this image:

Unfortunately, the result turns out unchanged: it has a black transparent background. I tried messing around with different GraphicsOptions, but nothing seemed to do the trick. The only solution I've found is setting the alpha to 1 instead of 0, but that doesn't feel right :(

Is this possible? Did I miss something easy? Thanks for your work and help!

[JimBobSquarePants]

I cannot replicate this.

[JimBobSquarePants]

Ah... I read that at black, not black transparent (There's no such thing technically as black transparent btw).

That result is expected.

Replacing the background requires an operation called alpha composition. In order to do alpha composition, you need to premultiply the color RGB components by their A component.

(255, 255, 255) x 0 = (0, 0, 0)

[tryashtar]

Yeah, black transparency. Sorry if that wasn't clear in my post. It's a bit of a weird thing to want, so I was glad to see the PngTransparentColorMode.Preserve option. But I'm not sure why it's not doing what I expect here

[JimBobSquarePants]

Preserve is too late. The color has already been converted to (0, 0, 0, 0) due to the premultiplication.

[tryashtar]

Mm, I see. I guess I can always just ProcessPixelRows to set them how I want, if there's no easier way. Thanks for looking!

[JimBobSquarePants]

No worries.

Here's possibly the best article I've ever read on alpha compositing.
https://ciechanow.ski/alpha-compositing/

About half-way down there an interactive image which demonstrates exactly what happens when the source color alpha is 0.