ImageCombineType.Add ignores source alpha when blending EditableImages

[reeeesaur (reeee)]

When I use DrawImage with ImageCombineType.Add, the source image does not respect transparency. If the source image is partially transparent, it still adds full color to the destination as if it had no transparency at all.

Both produce the same color (#ff00ff), despite the second source being partially transparent.
addition blending.rbxl (61.9 KB)

Expected behavior

Because the red image is 50% transparent, only half of the red color should be added to the blue image. The result should look like dark purple.

[varialbrickflip]

We’re happy to share that this bug has been fixed! Thanks for the report. If you encounter any further issues, please don’t hesitate to reach out.

[Mewious]

I was using this as a way to make images fully white

with this logic here

local editableImage = AssetService:CreateEditableImageAsync(Content.fromUri("rbxassetid://" .. tostring(props.Image)))
editableImage:DrawRectangle(
	Vector2.zero,
	editableImage.Size,
	Color3.new(1, 1, 1),
	1,
	Enum.ImageCombineType.Add
)

it was really great for performance rather then making every single pixel white
this is now doesn’t work

[varialbrickflip]

The fixed behavior is how the feature is supposed to work.
If you need to clear the image repeatedly, one workaround would be to precompute the whitened image once and then draw it using Enum.ImageCombineType.Overwrite.

[Mewious]

I made a bug report regarding AlphaBlend