Support for (pixel-level) transparency when composing images

[sebnozzi]

Given this icon, which has a transparent background:
(it only seems white on top of github's white background)

And this image:

When I compose the icon over the image, I would expect to see this:

But instead I see this:
(note, again, that the rectangle surrounding the icon is transparent; you are seeing through the resulting image into the page's background)

This is the code I used:

val pic = Image(new File("picture.jpg"))
val icon = Image(new File("icon.png"))

def combineImages(composite: Composite, targetPath: String) = {
  val result = pic.composite(composite, icon)
  result.writer(Format.PNG).withCompression(9).write(new File(targetPath))
}

combineImages(new NormalComposite(1.0), "result_normal_1.0.png")

It seems that the pixels from the incoming (second / icon) image are being taken "absolutely", replacing the pixels on the target (first / picture) image.

Each pixel should be combined not only taking into account the alpha parameter given to the composite object, but the transparency of each individual pixel.

For a reference implementation, compare with gimp (which was used to generate the "expected" result)

[sksamuel]

This is in master now. You need to use the AlphaComposite, so

val background = ...
val icon = ...
val composed = background.composite(AlphaComposite(0.5f), icon)

Or whatever alpha you like. See AlphaCompositeTest for details.

[sksamuel]

[sebnozzi]

Works fine!

I am of the opinion that this should be the behaviour for the NormalComposite. But I'll respect your design decisions and close this issue, since the functionality is there after all. Thanks!

[sksamuel]

Yeah I guess that's better. I'll support it in Normal as well.