[whatwg] Canvas - createRadialGradient

Ian Hickson ian at hixie.ch
Mon May 14 18:49:43 PDT 2007

On 5/4/07, Philip Taylor <excors+whatwg at gmail.com> wrote:
> For radial gradients, I've been looking into what happens when the two
> circles that define the gradient are not fully overlapping (i.e. not
> one entirely inside the other) - this appears to not be implemented or
> specified accurately.

No kidding. Wow.

> Negative radius values don't sound very useful or sensible to me, and
> they're not implemented in a useful or compatible way, so I think they
> ought to be handled as either:
> * Always take the absolute value of r0 and r1.
> or
> * Throw INDEX_SIZE_ERR when passed a negative r0 or r1 (for similarity
> with arc).
> I don't know which would be better. I'll guess the first for now,
> since it matches Firefox.


> So, I would suggest defining something like:
> <<<<
> Once a gradient has been created (see below), stops must be placed
> along it to define how the colors are distributed along the gradient.
> Between each such stop, the colors and the alpha component must be
> linearly interpolated over the RGBA space to find the color to use at
> that offset. Before the lowest stop, the color of the lowest stop must
> be used. After the highest stop, the color of the highest stop must be
> used.
> [Clarified to "linearly interpolated". Changed what happens outside
> the range of the stops - since this text is shared with linear
> gradients, I'm assuming they are both changed to be the same.]


> Radial gradients must be rendered as follows:
> * If one of the start circle and end circle is enclosed within the
> other circle, and their circumferences touch in at least one point,
> then the gradient must be rendered as the color at offset 0.
> * Otherwise, a cone or cylinder must be created from the circles, such
> that at the circumference of the starting circle the color at offset 0
> is used, that at the circumference of the ending circle the color at
> offset 1 is used, that the circumference of a circle drawn a certain
> fraction of the way along the line between the two origins with a
> radius the same fraction of the way between the two radii has the
> color at that offset (interpolation happening as described above) for
> all offsets such that the radius of the circle at that offset is
> positive; that the end circle appear to be above the start circle, and
> that any points not described by the gradient are a transparent black.

I did basically what you described as your second bulletpoint.

The only case this doesn't cover is the x0,y0,r0=x1,y1,r1 case, but
that doesn't really make any sense to me from a radial gradient point
of view. I've made it paint nothing. It could raise an exception, I
guess. Or paint a circle of the second colour against a background of
the first colour.

Ian Hickson

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