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Re: [svg2] radialGradient @fr constraints

From: Jasper van de Gronde <th.v.d.gronde@hccnet.nl>
Date: Wed, 29 Aug 2012 19:16:54 +0200
Message-ID: <503E4E86.9010903@hccnet.nl>
To: www-svg@w3.org
On 29-08-12 15:53, Erik Dahlstrom wrote:
> On Wed, 29 Aug 2012 14:20:28 +0200, Dr. Olaf Hoffmann
> <Dr.O.Hoffmann@gmx.de> wrote:
> ...
>> What could be the intended effect for a negative 'fr'?
>
> Not sure, I don't think we allow circles to have a negative radius
> anywhere else in svg.
>
>> Define, that the absolute value has to be used, if there is no intended
>> effect known.
>> This is better as to disallow, because if the viewer fixes this, the
>> authors
>> cannot fail by accident anymore ;o)
>
> I agree.

If you take the absolute value before interpolating, then you get the 
weird situation that a cone (for example) first gets "thinner" and then 
thicker again as you change the radius of one end from positive to negative.

It would make more sense to have a logical progression, where at fr=0 
you get a cone that ends/starts in a point, and for fr<0 you essentially 
get a "double cone". This is also the typical picture you have when 
defining a circle as one of the conic sections:
http://en.wikipedia.org/wiki/Conic_section
It also makes perfect sense if you represent a circle by x^2+y^2=r^2, or 
using a parametric representation. I uploaded some images of what this 
would look like:
http://home.hccnet.nl/th.v.d.gronde/negativeRadius/tube.png
http://home.hccnet.nl/th.v.d.gronde/negativeRadius/cone.png
http://home.hccnet.nl/th.v.d.gronde/negativeRadius/double%20cone.png
The first goes from radius 5 to radius 5, the second from 5 to 0 and the 
last from 5 to -5. The images were drawn by solving 
(x-cx(t))^2+(y-cy(t))^2-r(t)^2 and taking the largest t within the 
interval [0,1], blending the colour between red and blue.
Received on Wednesday, 29 August 2012 17:17:25 GMT

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