[webkit-dev] offsetWidth/Height/Left/Top int to float

[Levi Weintraub]

Hey Ion,

A few things...
- We didn't actually switch to floats for Layout, but with sub-pixel layout
enabled, we use integers that represent 1/60th of a pixel instead of one
(similar to Gecko).
- The CSSOM defines those properties to be longs
http://dev.w3.org/csswg/cssom-view/#extensions-to-the-htmlelement-interfaceso
changing this would violate the spec and potentially lead to
web-compat
issues.
- You can get floating-point-level precision values by using
getClientBoundingRect().
- IE uses a flag that switches between ints and floats. I don't advocate
this option (I think it adds complexity and doesn't solve a problem that
can't be solved using getClientBoundingRect) but it at least avoids web
compat issues.
http://msdn.microsoft.com/en-us/library/ie/hh673543(v=vs.85).aspx

Hope that helps,
-Levi

On Mon, Jun 18, 2012 at 7:56 AM, Ion Rosca <rosca at adobe.com> wrote:

> Hello,****
>
> ** **
>
> I have some questions regarding zooming and offsetWidth/Height/Top/Left*.
> *
>
> Currently, Element.offset* properties return imprecise values when zooming
> in/out. One of the bugs describing this problem would be Issue 104074:
> offsetWidth of element is shrinking when zooming in<http://code.google.com/p/chromium/issues/detail?id=104074>with a simple test
> case <http://jsfiddle.net/V4HQc/>. ****
>
> The product I’m working on also embeds WebKit and it would really need
> more precision from Element.offset*. We’ve done some testing by making
> offset*s to return floats and this approach appears to improve precision a
> lot.****
>
> ** **
>
> I found in archive<http://lists.webkit.org/pipermail/webkit-dev/2011-June.txt>an email from
> *Levi Weintraub <leviw at chromium.org>*, sent in June 2011, saying:****
>
> *… Changing rendering (and hit testing) to use floats does not
> necessarily mean*
>
> *that we need to expose floats through the dom api, however if we are to*
>
> *support sub-pixel layout (i.e. style=?left: 12.3px?) this would be
> required.*
>
> *Specifically we?d need to change the offsetLeft/Top/Width/Height
> properties*
>
> *to return floating point values [2]<https://bugs.webkit.org/show_bug.cgi?id=54018>
> .*****
>
> The meta bug 60318 <https://bugs.webkit.org/show_bug.cgi?id=60318>  (*Switch
> away from integers …)* Levi was working on is already fixed and the inner
> bugs (addressing this problem) bug 54018<https://bugs.webkit.org/show_bug.cgi?id=54018>
> * (Make offset* return doubles instead of ints) *and* *bug 39884<https://bugs.webkit.org/show_bug.cgi?id=39884>
> * - Full Page Zoom: rounding errors with element metrics *are still
> opened and theirs resolution is not clear.****
>
> ** **
>
> What’s your opinion on this subject? Were there some other discussions on
> it I couldn’t find? Is there a chance for making offset properties
> returning floats to be accepted? What are the implications of making this
> change in terms of specifications?****
>
> ** **
>
> Thank you,****
>
> Ion Rosca****
>
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> webkit-dev at lists.webkit.org
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>
>
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