Hi all,
I recently encountered an issue when using Go’s standard library
image/draw. Specifically, I found that fully transparent pixels (alpha = 0)
sometimes retain non-zero RGB values. This can cause unexpected colors when
compositing the image onto a white background.
*Here is a minimal reproducible example:*
func main() {
// Create a source image with a fully transparent pixel
src := image.NewRGBA(image.Rect(0, 0, 2, 2))
src.SetRGBA(0, 0, color.RGBA{R: 100, G: 150, B: 200, A: 0}) // fully
transparent
dst := image.NewRGBA(src.Bounds())
draw.Draw(dst, dst.Bounds(), src, image.Point{}, draw.Src)
r, g, b, a := dst.At(0, 0).RGBA()
fmt.Printf("Pixel (0,0) RGBA: %d %d %d %d\n", r>>8, g>>8, b>>8, a>>8)
}
*Output:*
Pixel (0,0) RGBA: 100 150 200 0
As you can see, even though alpha is 0, the RGB channels retain their
original values. This causes issues when you try to composite the image
onto a white background, because these “dirty” RGB values can show up in
certain blending scenarios.
In my code, I wrote a manual loop to premultiply or blend alpha into white,
which works correctly. But using draw.Draw, these transparent pixels keep
their original RGB values.
My questions are:
-
Is this the intended behavior of image/draw?
-
Or is this a bug (or at least a usability issue) in the standard library
that should be fixed?
Thank you very much for your opinions and insights!
*Note:* My English is not very good, so I used an LLM to translate this
post.
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