[webkit-dev] Do we need subpixel layout (Was: Should SATURATED_ARITHMETIC_LAYOUT be forced when enabling SUBPIXEL_LAYOUT ?)

Balazs Kelemen kbalazs at webkit.org
Wed Jul 31 15:26:09 PDT 2013

I think first we should clarify some more basic questions about subpixel 
1. Is it actually mantained?
2. Do any port really need it?

Please correct me if these questions are too obvious or have been 
resolved before. Note that I am not a fan of subpixel layout, neither an 
enemy of it - in fact I do not know or care much about it. But from what 
I know it seems like this is a feature that makes an essential 
difference on the engine and I don't understand how can we allow such 
huge divergence across WebKit ports. Isn't it a maintenance burden? 
Doesn't it result in that test results across ports will diverge a lot more?


On 07/31/2013 10:40 PM, Ryosuke Niwa wrote:
> Can't we encounter the same bug if we you multiplied the same height 
> by 64  even if the sub pixel layout is not turned off?  Or is there 
> some parser and other component that prevents such an overflow to happen?
> - R. Niwa
> On Wed, Jul 31, 2013 at 1:35 PM, Benjamin Poulain <benjamin at webkit.org 
> <mailto:benjamin at webkit.org>> wrote:
>     On Wed, Jul 31, 2013 at 4:34 AM, Javier Fernandez
>     <jfernandez at igalia.com <mailto:jfernandez at igalia.com>> wrote:
>         While looking into the bug #118595 I've found out that the
>         root cause
>         was an arithmetic overflow during the layout process, at least
>         in the
>         case of the WebKitGtk+ port, but I guess other ports have
>         similar root
>         cause. Notice that webkitgtk+ port enables the SUBPIXEL-LAYOUT
>         by default.
>         Later, I've concluded that the bug is not related to Regions
>         at all, but
>         something generic happening when the max-height CSS property
>         value is
>         set to a huge number. I've filled a new bug #119273 for this.
>         The arithmetic overflow is gone when enabling the
>         SATURATED_ARITHMETIC_LAYOUT flag, which actually, as far as I
>         could
>         understand, it's the purpose of such experimental feature.
>         However, I've provided a patch for the #119273 facing the
>         specific case
>         of the max-height. I don't think that's the right approach,
>         because I
>         think other CSS property might have the same problem, but I
>         thought it
>         was useful to understand the issue.
>         I think the right solution would be to enable the
>         enabled. Perhaps, eventually, both flags will become part of
>         the same
>         feature; that's one of the points to be discussed here.
>         I'm sending this email to this list in order to get an
>         agreement between
>         all the ports using the SUBPIXEL_LAYOUT feature, but perhaps
>         this should
>         be debated independently on each port's mailing list.
>     It looks to me like it is a good idea on the long term to have
>     saturated arithmetic enabled when you have subpixel layout.
>     I believe only GTK and EFL enable subpixel layout at the
>     moment(?). You can probably lead the way and mature the feature
>     and others will follow.
>     Cheers,
>     Benjamin
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