| Summary: | Treat color glyph alpha as mask instead of source alpha | ||
|---|---|---|---|
| Product: | cairo | Reporter: | Jan Alexander Steffens (heftig) <jan.steffens> |
| Component: | freetype font backend | Assignee: | David Turner <david> |
| Status: | RESOLVED MOVED | QA Contact: | cairo-bugs mailing list <cairo-bugs> |
| Severity: | normal | ||
| Priority: | medium | CC: | andyrtr, apodtele, freedesktop, wl |
| Version: | unspecified | ||
| Hardware: | All | ||
| OS: | Linux (All) | ||
| Whiteboard: | |||
| i915 platform: | i915 features: | ||
| Attachments: | [PATCH] Hacky hack: Use color glyph alpha as mask | ||
|
Description
Jan Alexander Steffens (heftig)
2017-09-11 18:04:45 UTC
I think using alpha as the mask but leaving it in the source would do weird things to the destination alpha, as it becomes src * src + dst * (1 - src), i.e. you have a quadratic curve. (In reply to Jan Alexander Steffens (heftig) from comment #2) > I think using alpha as the mask but leaving it in the source would do weird > things to the destination alpha, as it becomes src * src + dst * (1 - src), > i.e. you have a quadratic curve. Yeah I agree we are double-multiplying... Reopening to discuss double-multiplying. I'm willing to let it go though. Note that the proposed hacky hack patch that separate alpha and rgb suffers from the same problem IMO. Does it? My patch turns the color surface into RGB24 format, so the alpha is dropped. (In reply to Jan Alexander Steffens (heftig) from comment #5) > Does it? My patch turns the color surface into RGB24 format, so the alpha is > dropped. Yes, but RGB that get in there are premultiplied... Ah, I missed that detail. And the colors coming out of FreeType are already premultiplied. Well, there goes the easy solution. I now agree this is probably not worth worrying about. (In reply to Jan Alexander Steffens (heftig) from comment #7) > Ah, I missed that detail. And the colors coming out of FreeType are already > premultiplied. Well, there goes the easy solution. Exactly. The PNG is NOT premultiplied, but we premultiply in FreeType before returning... > I now agree this is probably not worth worrying about. Ahh. I wish I knew about premultiplication and objected as much as I could. Now we are about to release layered fonts with the premultiplication... Screw everyone who cares about gamma correction too! At least Direct2D offers a choice https://docs.microsoft.com/en-us/windows/desktop/api/dcommon/ne-dcommon-d2d1_alpha_mode Cairo was designed to act like the letters were the *mask*. No pixels outside the letters should be changed no matter what the compositing operation is or what character is drawn. Users expect that by default (which means the compositing op is OVER) the emoji are filled in with their colors. It is also useful to be able to fill the shapes of the emoji with solid colors, for effects such as drop shadows. This cannot be done in one step right now. My recommendation: * If the op is OVER, then it does what it is doing now. * If the op is not OVER, then the color of the emoji is ignored. Instead the alpha of each of them is merged into the mask with the rest of the glyphs, and then font rendering continues as normal. In particular this allows SOURCE to be used to fill with a solid color or image. A different recommendation. This is more consistent, but much harder to implement, and may be less useful. To draw an emoji produce exactly the same result as if you did this: * Intersect the mask with the alpha of the emoji * Set the source to an opaque *unpremultiplied* version of the emoji's color * Fill a rectangle that is bigger than the mask. This produces the same result for OVER as it does now. SOURCE also produces the same result. -- GitLab Migration Automatic Message -- This bug has been migrated to freedesktop.org's GitLab instance and has been closed from further activity. You can subscribe and participate further through the new bug through this link to our GitLab instance: https://gitlab.freedesktop.org/cairo/cairo/issues/296. |
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