1717824 - GrayScale AA if css -webkit-text-stroke-width is applied

Status: NEW

(Core :: Graphics: WebRender, defect, P4)

Description

[Alice0775 White]

Attachment: -webkit-text-stroke-width - Varying_stroke_widths - code sample.html

See attached screenshot.
GrayScale if css -webkit-text-stroke-width is applied.

If disable WebRender, it is SubPixel AA.

Comment 1

[Alice0775 White]

Attachment: screenshot

Comment 2

[Brad Werth [:bradwerth] | Out until Feb 2025]

Jonathan, are we doing something incorrect with our text aliasing in WebRender?

Comment 3

[Jonathan Kew [:jfkthame]]

Curious - looking at the screenshot, it appears there are three different behaviors here. In all three examples, the unstroked glyphs have subpixel AA, but when the stroke is applied, the results in each case differ:

• WR: no subpixel AA on the stroked glyphs
• Basic: the stroked glyphs still have full subpixel AA
• D3D11: there's subpixel AA but it appears to be reduced, my guess would be that the glyph is being filled with subpixel AA, but then stroked without, so that as the stroke gets thicker, it obscures an increasing amount of the subpixel color fringing.

I don't know how glyph-stroke is implemented through webrender -- if it's done by getting the glyph path from the font, and then stroking this like any other path, presumably we don't do subpixel AA for generic path stroking. But in that case I'd kinda expect a result like the D3D11 screenshot, where we can still see a bit of the subpixel AA from painting the filled glyph "underneath" the stroke.

So it'd be interesting to know where the decision happens that seems to completely disable subpixel AA when a stroke is present. On the gfx/thebes side I'm not aware of a reason for this. Maybe Lee knows where that could be happening?

Comment 4

[Lee Salzman [:lsalzman]]

Stroking in WR causes a fallback to a blob image up in layout, since WR itself can't handle the glyph stroking (as with other path rendering). Once in blob image land, there is no subpixel AA...

Comment 5

[Jonathan Kew [:jfkthame]]

Ah, that makes sense. So this is expected, and probably not something we're aiming to fix any time soon, at least.