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 <[email protected]> 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.
>>
>> --
>> You received this message because you are subscribed to the Google Groups
>> "golang-nuts" group.
>> To unsubscribe from this group and stop receiving emails from it, send an
>> email to [email protected].
>> To view this discussion visit
>> https://groups.google.com/d/msgid/golang-nuts/9b7cb846-622c-44c1-9e9b-87ff8ee7453fn%40googlegroups.com
>>
>> <https://groups.google.com/d/msgid/golang-nuts/9b7cb846-622c-44c1-9e9b-87ff8ee7453fn%40googlegroups.com?utm_medium=email&utm_source=footer>
>> .
>>
>
--
You received this message because you are subscribed to the Google Groups
"golang-nuts" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To view this discussion visit
https://groups.google.com/d/msgid/golang-nuts/f8cb79d3-4c35-4910-b3a3-d4b8ad08ac1en%40googlegroups.com.
