image/png: unexpected behaviour while encoding paletted image with transparency

[prl900]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.10 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
...

What did you do?

This simple program defines a palette with increasing alpha from 0 to 255 for the blue color. A paletted image using this palette is defined with a horizontal gradient from 0 to 255. This image is then encoded as a png on disk.

https://gist.github.com/prl900/b94fc4d71a25185ff962154e5209c563

What did you expect to see?

What did you see instead?

[odeke-em]

/cc @nigeltao @horgh

[nigeltao]

Your gist has this line:

palette[i] = color.RGBA{0x00, 0x00, 0xff, uint8(i)}

The RGBA should be NRGBA. As https://golang.org/pkg/image/color/#RGBA and https://golang.org/pkg/image/color/#NRGBA say, RGBA is alpha-premultiplied and that "An alpha-premultiplied color component C has been scaled by alpha (A), so has valid values 0 <= C <= A" and NRGBA is non-alpha-premultiplied.