397423 - Enable Cleartype "natural widths" mode

Status: RESOLVED WONTFIX

(Core :: Graphics, defect)

Description

[Robert O'Callahan (:roc) (email my personal email if necessary)]

http://msdn2.microsoft.com/en-us/library/aa480221.aspx

By default Cleartype uses "compatible widths", which means they force glyphs to have to have the same advance widths when hinted as when not hinted. According to the above page, this can be disabled in Windows XP1 and later. We should disable it because we do not need that compatibility and using "natural widths" should produce better looking text.

Comment 1

[Robert O'Callahan (:roc) (email my personal email if necessary)]

I mean Windows XP, SP1.

Comment 2

[Jonathan Kew [:jfkthame]]

I tried setting CLEARTYPE_NATURAL_QUALITY in the logFont structure that's created in FontEntry::FillLogFont (gfxWindowFonts.cpp) on Vista.

The result was, umm, "interesting". I do indeed get different spacing than with the DEFAULT_QUALITY setting, but the result is most definitely not an improvement; the glyph spacing is quite poor. To me, it looks as though I'm getting exactly the same glyphs as with the DEFAULT setting, but spaced differently (usually tighter, but irregular), as if the flag has affected the metrics but has not altered the actual rasterization of the glyphs.

Further investigation needed; there may be additional places where we need to set different flags to control the rendering properly. Maybe somewhere within Cairo....

Comment 3

[Robert O'Callahan (:roc) (email my personal email if necessary)]

sounds likely yes.

Comment 4

[Jonathan Kew [:jfkthame]]

Yes, Cairo handles this stuff internally in _win32_scaled_font_create. Doesn't look like it offers any API to access the "natural widths" setting at the moment, but I'll try hacking something in locally to see if the results are worthwhile. If so, we can propose a patch for Cairo to add the support we'll need.

Comment 5

[Masayuki Nakano [:masayuki] (he/him)(JST, +0900)]

Should we use the compatible mode if the font style is system font? UI by XUL should emulate the native UI rendering/behavior.

Comment 6

[Jonathan Kew [:jfkthame]]

Attachment: Calibri font on Vista, ClearType with compatible widths

Comment 7

[Jonathan Kew [:jfkthame]]

Attachment: Calibri font on Vista, ClearType with natural widths

Comment 8

[Jonathan Kew [:jfkthame]]

Attached screenshots providing a comparison between ClearType in "compatible" and "natural widths" modes, using the Calibri font on Vista. (I have also looked at a number of other fonts, and could post more images if desired.) The difference is often slight, though with some sizes/fonts it is more obvious.

Doing this involves a patch to Cairo, to support an additional font quality setting on Windows. I will tidy up my patch and send it upstream to see if they'd like to adopt it.

Comment 9

[Vladimir Vukicevic [:vlad] [:vladv] (needinfo me, slow to respond)]

Should be easy to add the option to upstream; though from looking at the screenshots, the compatible widths mode looks better to me in a lot of cases.  The UI font issue in particular is going to be tricky, e.g. look at the "H" in History/Help and the T in Tools.

Comment 10

[Jonathan Kew [:jfkthame]]

Yes, I agree; sometimes one looks better, sometimes the other. Which suggests to me that this should probably be controlled by a preference (unless we simply decide it's not worth doing it at all).

Comment 11

[Jonathan Kew [:jfkthame]]

Attachment: Times New Roman on Vista, ClearType with compatible widths

Comment 12

[Jonathan Kew [:jfkthame]]

Attachment: Times New Roman on Vista, ClearType with natural widths

Comment 13

[Jonathan Kew [:jfkthame]]

Attachment: wikipedia.org, compatible widths

Comment 14

[Jonathan Kew [:jfkthame]]

Attachment: wikipedia.org, natural widths

Comment 15

[Jonathan Kew [:jfkthame]]

Added sample screenshots from wikipedia.org as well. In this case, I think that the "natural" mode gives better results for some things; for example, the M and N in the language lists look better to me, as does the spacing of the word "search". But the option doesn't improve everything, as already noted.

Comment 16

[Robert O'Callahan (:roc) (email my personal email if necessary)]

Sounds to me like this is not worth doing.

Comment 17

[Jonathan Kew [:jfkthame]]

Attachment: patch cairo and thebes to enable ClearType natural widths (WIP)

Yes, I'm inclined to agree. Attaching my current patch for safe-keeping in case we ever want to revisit this, but for now there doesn't seem to be a clear benefit. When I posted the first screenshots, I didn't want to prejudice other people's opinions by saying much about them right away, but overall I was pretty disappointed with the results.

Comment 18

[Jonathan Kew [:jfkthame]]

Note: If we do ever come back to this, and want to use the natural-widths patch to Cairo, it's unfinished: it adds a new enumeration value for cairo_antialias_t, so this needs to be handled by the other font backends as well as the win32 one. (They'd just fall back to CAIRO_ANTIALIAS_SUBPIXEL if they don't have a separate mode for this.) Also should do the same in cairo_set_antialias().

This doesn't affect the current usage in Gecko, but would be needed for the patch to be appropriate for Cairo in general.

Comment 19

[Ali Juma [:ajuma]]

Marking WONTFIX based on comments 16 and 17.