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Re: [css3-images] Probably Editorial: radial-gradient() and concentric ellipses

From: Øyvind Stenhaug <oyvinds@opera.com>
Date: Wed, 29 Feb 2012 17:58:06 +0100
To: "Boris Zbarsky" <bzbarsky@mit.edu>, "Tab Atkins Jr." <jackalmage@gmail.com>
Cc: www-style@w3.org
Message-ID: <op.waf9e4i6bunlto@oyvinds-desktop>
On Tue, 28 Feb 2012 17:16:28 +0100, Tab Atkins Jr. <jackalmage@gmail.com>  

> On Tue, Feb 28, 2012 at 8:07 AM, Tab Atkins Jr. <jackalmage@gmail.com>  
> wrote:
>> On Tue, Feb 28, 2012 at 5:12 AM, Boris Zbarsky <bzbarsky@mit.edu> wrote:
>>> On 2/28/12 3:01 AM, Kang-Hao (Kenny) Lu wrote:
>>>> I am not sure this is well-defined as I don't know if concentric
>>>> ellipses are by definition necessarily having the same eccentricity.
>>> They're not.
>>>> I don't mean to use www-style as a Math forum but oddly enough I
>>>> couldn't find the definition of "concentric ellipses" on Google.
>>> When people say "concentric ellipses" they seem to mean  
>>> non-intersecting
>>> ellipses with shared center and major axes pointing in the same  
>>> direction.
>>>  Or something.  There is no commonly accepted definition I know of,  
>>> other
>>> than "I know it when I see it".
>> Huh.  I thought there was a simple definition (linearly scale both
>> axises).  I'll do some quick testing to see what impls do and ensure
>> it matches what I assume, then specify that more clearly.
> Yup, everyone does the obvious thing.  I've added the word
> "proportional" in front of "concentric ellipses".  That sound good?

I think you'd need a further restriction. As I understand it, a radial  
gradient consists of all ellipses E such that
1) E and the ending shape are concentric - that is, they have the same  
2) E and the ending shape have the same eccentricity - I assume this is  
what "proportional" means, though using that word when talking about  
two-dimensional shapes sounds unfamiliar to me (I didn't use the term  
"similar" since that has a non-mathematical meaning which is much more  
3) the major semiaxes/radii of E and the ending shape coincide (otherwise  
you'd include ellipses that are rotated by any arbitrary amount)

(I also think that the orientation of the ending shape's axes is only  
implicitly given near the very end of "4.2.1.  radial-gradient() Syntax" -  
before that it seems to be taken for granted that they are  

Øyvind Stenhaug
Core Norway, Opera Software ASA
Received on Wednesday, 29 February 2012 16:58:44 UTC

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