[bug] Losing transparency when converting png emoji to a static version

[rikkix]

Description

When uploading a custom emoji with transparency in png format, the static version of the emoji will lose transparency info (and actually will turn into black).

Example

Original file at https://example.ltd/fileserver/XXX/emoji/original/XXX.png looks like:

Processed static file at https://example.ltd/fileserver/XXX/emoji/static/XXX.png looks like:

Related code

I have done an experiment that confirms the code in internal/media/png-stripper.go will cause this issue.
However, limited by my ability, I cannot figure out how to fix the bug in that file currently.

And I am curious that, since png file can never be an animated image, why do we need to decode the png file and encode that back like the code below?
But I haven't deeply dived to the whole project yet, so I am not sure if there is something thoughtless in my idea.

gotosocial/internal/media/image.go

Lines 99 to 125 in 941893a

	func deriveStaticEmoji(r io.Reader, contentType string) (*mediaMeta, error) {
	var i image.Image
	var err error
	switch contentType {
	case mimeImagePng:
	i, err = StrippedPngDecode(r)
	if err != nil {
	return nil, err
	}
	case mimeImageGif:
	i, err = gif.Decode(r)
	if err != nil {
	return nil, err
	}
	default:
	return nil, fmt.Errorf("content type %s not allowed for emoji", contentType)
	}
	out := &bytes.Buffer{}
	if err := png.Encode(out, i); err != nil {
	return nil, err
	}
	return &mediaMeta{
	small: out.Bytes(),
	}, nil
	}

Related issue(s) & PR(s)

#425
#487

Version

v0.6.0

[tsmethurst]

png file can never be an animated image

It can be an animated image, there are animated pngs :) But yes indeed, losing transparency is a known issue, will have a look soon(tm) how this can be fixed.

[rikkix]

png file can never be an animated image

It can be an animated image, there are animated pngs :) But yes indeed, losing transparency is a known issue, will have a look soon(tm) how this can be fixed.

I thought PNG and APNG were two totally separated formats both in MIME type and filename extension.
Thank you so much for pointing out my mistake!