1421041 - WebRender on OS X forces LCD antialiasing even when disabled in system settings

Status: RESOLVED FIXED

(Core :: Graphics: WebRender, defect, P1)

Description

[Antti Lankila]

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.59 Safari/537.36

Steps to reproduce:

1. I installed nightly (2017-11-27) on OS X.

2. I turned on webrender settings: gfx.webrender.blob-images and gfx.webrender.enabled

3. I started browsing random sites, e.g. reddit, LWN, and so on.


Actual results:

I immediately observed that subpixel antialiasing was turned on. However, I have told OS X to not enable LCD rendering even if it's available, under the system's general settings. Applications typically respect this choice, and only show grayscale antialiased text. 

The underlying reason for why I do this is that turning off LCD rendering seems to make glyphs much thinner. As I understand it, OS X uses some kind of outline fattening in LCD modes. Whatever the justification for distorting the glyph shapes like that, I think it is misguided and looks ugly. With today's high DPI screens, we don't even get much of a benefit from the increased horizontal resolution over just doing grayscale.


Expected results:

Firefox should respect system settings and only use grayscale antialiasing when user has expressed such preference.

Comment 1

[Lee Salzman [:lsalzman]]

Does this happen when WebRender is disabled?

Comment 2

[Antti Lankila]

No, Firefox switches to grayscale antialiasing.

Comment 3

[Lee Salzman [:lsalzman]]

(In reply to Antti Lankila from comment #2)
> No, Firefox switches to grayscale antialiasing.

Are you actually getting LCD anti-aliasing in WebRender? Even though the text may be thicker, it might still be grayscale. Can you take screenshots with both WebRender on and off and attach them here?

Comment 4

[Antti Lankila]

Attachment: Screenshot without webrender

Comment 5

[Antti Lankila]

Attachment: Screenshot with webrender

The only difference between these two firefox renderings is the about:config change in gfx.webrender.enabled.

Comment 6

[Markus Stange [:mstange]]

It looks like the Skia code only calls CGContextSetShouldSmoothFonts, but the webrender code additionally calls set_should_smooth_fonts.

https://searchfox.org/mozilla-central/rev/be78e6ea9b10b1f5b2b3b013f01d86e1062abb2b/gfx/skia/skia/src/ports/SkFontHost_mac.cpp#923
https://searchfox.org/mozilla-central/rev/be78e6ea9b10b1f5b2b3b013f01d86e1062abb2b/gfx/webrender/src/platform/macos/font.rs#570

Comment 7

[Markus Stange [:mstange]]

Er, I mean: webrender additionally calls set_allows_font_smoothing.

Comment 8

[Antti Lankila]

Tangentially, if you analyze the webrendered screenshot that I provided, you can see an additional reason why I keep LCD smoothing off in Macintosh computers. E.g. Look at the vertical stem of the D character in the SPDX word. See that this stem's left edge is red/yellow and its right edge is light blue, and there are multiple completely black pixels between these colored edges, and the pixels outside of those mentioned are 100% white.

This will appear color fringed because the red and blue are too far apart to appear gray in human eye. All locally correct LCD filtering technology must generate reddish and a corresponding bluish pixel at neighboring pixels if the result is to appear gray, no matter the thickness of the outline. This is usually achieved by applying a 3 subpixel wide averaging filter on the alpha coverage bitmap. Evidently OS X hasn't figured out that it needs to do this. I suspect those fat glyphs are someone's hack to hide the problem.

Comment 9

[Lee Salzman [:lsalzman]]

(In reply to Markus Stange [:mstange] from comment #7)
> Er, I mean: webrender additionally calls set_allows_font_smoothing.

Looks like we probably do not want use set_allows_font_smoothing nor set_allows_antialiasing? I guess this is how the OS is getting involved here. Docs state that these are not part of the graphics state, whereas the should_ versions are the ones it says we should be using.

Comment 10

[Lee Salzman [:lsalzman]]

Added WR PR https://github.com/servo/webrender/pull/2152 to address this.