1260387 - Fuzzy line with Ahem on vertical writing modes

Status: NEW

(Core :: Layout: Text and Fonts, defect)

Description

[Xidorn Quan [:xidorn] UTC+11]

It seems to me that on horizontal writing mode, Ahem is reliable when font-size is a multiple of 5px. However, it is never reliable on vertical writing modes, a.k.a {vertical,sideways}-{lr,rl}, on Windows.

It seems Edge can render them prefectly, so I suppose it is somehow fixable.

When I was trying to import css writing modes tests from CSSWG's test repo (in bug 1258916), it seems most of the fuzzies on Windows are because of something like this.

Comment 1

[Xidorn Quan [:xidorn] UTC+11]

Attachment: testcase

Comment 2

[Xidorn Quan [:xidorn] UTC+11]

Attachment: Firefox vs. Edge

Comment 3

[Xidorn Quan [:xidorn] UTC+11]

jfkthame, any thought about this?

Comment 4

[Jonathan Kew [:jfkthame]]

This may mean that we're not device-pixel-snapping the text baseline or the line-spacing metrics (e.g. ascent) in the equivalent way for vertical text to what we do in horizontal mode.

Whether it's worth spending a lot of time to try and fix this is questionable, IMO -- unless of course we have an error that results in visible problems for real text, in which case it definitely needs fixing. Otherwise, accepting some antialiasing "fuzz" on the tests may be OK. A lot of those tests, IIRC, are inherently fragile in that they assume the rendering of a square <div> with a background color, for example, will match the rendering of an Ahem glyph -- but it's not at all surprising if a browser has different antialiasing behavior for glyphs vs rect fill, in which case mismatches at the edges are quite likely.

(The font sizes at which things "work" in horizontal mode, even, will probably be dependent on screen DPI, because it'll be the device-pixel size, not the CSS-px size, that is significant.)

Comment 5

[Gérard Talbot]

(In reply to Xidorn Quan [:xidorn] (UTC+10) from comment #0)
> It seems to me that on horizontal writing mode, Ahem is reliable when
> font-size is a multiple of 5px. However, it is never reliable on vertical
> writing modes, a.k.a {vertical,sideways}-{lr,rl}, on Windows.

Just for your information. 
It is not reliable on Linux. I can see the tiny vertical lines between glyphs.

I've set Platform and OS to ALL