The docs for color.RGBA mention: "An alpha-premultiplied color component C
has been scaled by alpha (A), so has valid values 0 <= C <= A.", so I
believe "color.RGBA{R: 100, G: 150, B: 200, A: 0}" is an invalid value.
Perhaps you have non-alpha-premultiplied colors? Using RGBA.Set with an
NRGBA value seems to work as I expect (https://go.dev/play/p/OjzdswX-lk_L),
same with just using an image.NRGBA in the first place
(https://go.dev/play/p/VEvsKjh8lNU)
On Wednesday, August 20, 2025 at 10:25:39 PM UTC-6 SugarMGP wrote:
> func main(){
> src := image.NewRGBA(image.Rect(0, 0, 2, 2))
> src.SetRGBA(0, 0, color.RGBA{R: 100, G: 150, B: 200, A: 0})
>
> dst := image.NewRGBA(src.Bounds())
> draw.Draw(dst, dst.Bounds(), &image.Uniform{color.White}, image.Point{},
> draw.Src)
> draw.Draw(dst, dst.Bounds(), src, image.Point{}, draw.Over)
>
> 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)
> }
>
> result: Pixel (0,0) RGBA: 100 150 200 255
>
> However, when I tried to blend a transparent pixel onto a white
> background, the result was not white, which is what really confused me.
> 在2025年8月21日星期四 UTC+8 07:10:20<Rob Pike> 写道：
>
>> 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 <a2350...@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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>>> .
>>>
>>
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