This is correct behavior. The original pixel has value (100, 150, 200, 0)
and you use the Src operator, which copies the source to the destination
unmodified.
-rob
On Thu, Aug 21, 2025 at 4:55 AM SugarMGP <a2350745...@gmail.com> wrote:
> 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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