Changes to <radialGradient> in SVG2 and WebKit

Oct 9, 02:35 PM

The web platform is evolving every day and so is SVG. I just wrote a blog post about new features and improvements coming in SVG2. The SVG2 specification contains both large changes and smaller ones, like the changes to the <radialGradient> element.

Defining a focal radius for radial gradients

Radial gradients are frequently used as paint servers for SVG content like shapes or text. Conceptually, radial gradients are an infinite number of circles drawn from a focal point to an outermost circle by interpolating radius, position and color of each circle. This results in colors that spread out smoothly from the focal point to the outermost circle. Just four different values are needed to define a radial gradient:

  1. The origin of the outermost circle with the attributes cx and cy,
  2. the radius of the outermost circle with the attribute r,
  3. the focal point of a gradient with the attributes fx and fy and
  4. the color stops that result into the smooth color fading.
<radialGradient cx="50%" cy="50%" r="50%" fx="50%" fy="50%">
    <stop stop-color="#FFDB80" offset="0%"/>
    <stop stop-color="#FF791F" offset="50%"/>
    <stop stop-color="#E09F00" offset="100%"/>

Radial gradient

But SVG radial gradients miss one argument in comparison to other gradient defintions (like CanvasGradient in HTML Canvas): the focal radius. In combination with the focal point, the focal radius defines the innermost circle. Color stops with the offset 0% start at the innermost circle, taking the focal radius as additional “offset”.

This changes with SVG2. The focal radius can be set with the attribute fr now.

<radialGradient cx="50%" cy="50%" r="50%" fx="50%" fy="50%" fr="20%">

Radial gradient with focal radius

This opens new possibilities. Not specified yet, but still plausible: A focal radius that is bigger then the gradient radius. The innermost circle becomes bigger then the outermost circle, which simply inverts the gradient fill operation.

<radialGradient r="20%" fr="50%">

Radial gradient where fr is bigger then r

The default value for fr of 0% makes radial gradient backwards compatible with existing content.

No restrictions to the position of focal points

To avoid a more complex definition for radial gradients, focal points can be positioned outside of the outermost circle now. A focal point outside the outermost circle draws a cone, touched by the innermost and outermost circles.

<radialGradient cx="50%" cy="50%" r="30%" fx="0%" fy="50%">

Radial gradient with focal point outside of outermost circle

Test it in WebKit now!

The extended behavior of <radialGradient> focal points and the new focal radius attribute make SVG radial gradients fully compatible with radial gradients in HTML Canvas. This takes us one step further to the interoperable, graphical web.

You can test both changes with one of the latest nightly builds of WebKit or a Chrome Canary build. Please test your content and give feedback on

Last but not least, an example of focal point and focal radius with SVG animation:


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