It might help to review the concept of an Alpha channel, and in particular look at the rationale for non-premultiplied alphas that the PNG format describes.
https://www.w3.org/TR/PNG-DataRep.html#DR.Alpha-channel https://www.w3.org/TR/PNG-Rationale.html#R.Non-premultiplied-alpha On Wednesday, August 20, 2025 at 7:55:34 PM UTC+1 SugarMGP 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 golang-nuts+unsubscr...@googlegroups.com. To view this discussion visit https://groups.google.com/d/msgid/golang-nuts/3de5794a-667a-42cf-9d9a-bc8d29a687ddn%40googlegroups.com.
