W3C home > Mailing lists > Public > www-style@w3.org > August 2009

Re: [CSS3, Backgrounds and Borders Module] some questions about border-radius

From: Brad Kemper <brad.kemper@gmail.com>
Date: Sat, 22 Aug 2009 22:35:09 -0700
Cc: www-style <www-style@w3.org>
Message-Id: <2AAE8453-2A09-45C3-96AC-C51781E99B4A@gmail.com>
To: Andrew Fedoniouk <news@terrainformatica.com>

On Aug 22, 2009, at 9:41 PM, Andrew Fedoniouk wrote:

> Brad Kemper wrote:
>> On Aug 22, 2009, at 8:30 PM, Andrew Fedoniouk wrote:
>>> Reading this document
>>> http://www.w3.org/TR/2008/WD-css3-background-20080910/#the-border-radius
>>> got couple of questions.
>>> Phrase after [Example XXI]:
>>> "(In such cases its center might not coincide with that of the  
>>> outer border curve.)"
>>> Not clear what it means. Center of circle having radius zero? It's  
>>> a point then. Has no center... Or is this about something else?
>> That is how I read it. A point does have a center (it's the same  
>> point).
> Mathematically speaking - yes, someone can say that point is a  
> circle of
> radius zero but that is pretty unusual way of saying "point".
>> That point would no longer align with the center of the center of  
>> the circle describing the outside of the border, when the border  
>> thickness is thicker than the corner-radius. There may be a better  
>> way to say this...
> In case of borders of different thickness centers of outer and inner  
> circles are different. Is this what you are trying to say?

No. But given that fact, if you have, say a 20px border and a 10px  
border-radius, the inner "circle" (actually a corner), will not be at  
the center of the 10px circle. I believe that's what it is saying,  
although it seems kind of self evident to me. I can't really imagine  
it some other way.

>>> [...]
>>> And the main question:
>>> "Other effects that clip to the border or padding edge (such as  
>>> ‘overflow’) also must clip to the curve."
>>> Clipping of content on such border may lead to information lost.
>>> Text behind rounded corner will not be seen at all [top-left corner,
>>> case #8 above].
>> Text that goes outside its boundaries is also "information lost"  
>> when overflow is 'hidden'. Thus it is within the author's ability  
>> to obscure text with overflow, as always. If I don't want the text  
>> to be clipped by the corners, then I can add padding.
> It is about overflow:auto rather than hidden.
> You cannot see that portion even with overflow:auto. That is the  
> point.

If I consider that a problem when I'm authoring, then I will add  
padding to prevent it. CSS does not prevent authors from hiding  
content that users can't get to. It happens all the time with negative  
absolute positioning, for instance, or can happen with  
overflow:hidden. It can also happen with black text on top of a black  
border with border-radius. So instead of trying to prevent authors  
from creating unreadable text, let us just be responsible for our own  
design decisions. I think it is completely reasonable and expected  
that any border that would clip content should also do so at the  
curved corners.

> I think we just need to remove "Other effects that clip to the  
> border or padding edge (such as ‘overflow’) also must clip to the  
> curve."
> statement. It does not create solutions - just problems.

I disagree.

> Technically it is even not possible to do such clipping if  
> antialiasing is used. Border should be drawn on top of content but  
> not underneath as the spec mandates.

I don't see how that prevents you from clipping the content (including  
antialiasing) to the same path as what is under the content.
Received on Sunday, 23 August 2009 05:35:46 UTC

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