Bug 77205 – <feImage> doesn't work with local references when using primitiveUnits="objectBoundingBox"

• Component: SVG
• Status: RESOLVED FIXED

[Nikolas Zimmermann]

<feImage> doesn't work with local references when using primitiveUnits="objectBoundingBox".

It's quite tricky, consider following SVG document:
<svg width="1000" height="500">
<defs>
    <circle id="c" cx="50%" cy="50%" r="10%"/>
    <filter id="f" filterUnits="userSpaceOnUse" x="0" y="0" width="100" height="100" primitiveUnits="objectBoundingBox">
        <feImage xlink:href="#c"/>
    </filter>
</defs>
<rect width="100" height="100" filter="url(#f)"/>
</svg>

The <svg> has a viewport of 1000x50. The <circle> in the <defs> element is resolved as <circle cx="500" cy="250" r="79"/>, as the context for this element (looking at it isolated!) is the viewport of the <svg>.
We then setup a 0x0 - 100x100 rect in user space, which gets filtered, by a filter, also defined in user space (for simplicity), but with primitiveUnits="objectBoundingBox".
That means that the default subregion of the filter effect <feImage/> which has no inputs, is defined in the SVG 1.1 spec to be equal to the filter region, which is 0x0-100x100 in user space!
This <feImage/> is supposed to produce a 100x100 image, containing a circle in the middle (aka. <circle cx="50" cy="50".../>), but it doesn't, as the <circle> its trying to paint, is laid out at 500x250, and thus outside the 100x100 sized image buffer.

So how to resolve this?
The RenderSVGShape thats owned by the <circle> generates its Path value by calling cx().value(lengthContext) and the SVGLengthContext here resolves to the <svg>. That happens on _layout_.
If we would want to change the SVGLengthContext in this case (what I originally planned!) to carry a custom 100x100 viewport, we'd need to relayout. Unfortunately this is not an option, as this would mean that SVGImageBufferTools::renderSubtreeToImageBuffer, would need to relayout its children first, but we produce the filter images when painting. This is very dangerous and has just recently been fixed so that SVGImageBufferTools can ASSERT(!item->needsLayout()) when painting the subtree to an image buffer.

The only sane solution I see, that does not require relayouts, is to make a map between the <circle> bounding box in user space (500x250 center point) to the filter primitive subregion space (here: 100x100 center point), and concat that transformation before painting the subtree to the image buffer.
Of course this approach only works if all values of the <circle> are relative. If someone uses <circle id="c" cx="50%" cy="666"> the transformation that I'm looking for is undefined. We'd really need to create a new Path here, to resolve only cx against the new viewport, and not cy. Though in practice it turns out this is not a problem. All use cases of feImage + primitiveUnits="objectBoundingBox" link to elements that are completely defined in percentual values, as this is really the only thing which makes sense - otherwise you can always switch back to primtiveUnits="userSpaceOnUse".
Anyhow, I'm going to fix all known wild-life test cases by my approach, and say we don't support using mixed length units when referencing those elements from feImages with primitiveUnits="objectBoundingBox".

[Nikolas Zimmermann]

Created attachment 124296 [details]
Patch

[Nikolas Zimmermann]

Committed r106113: <http://trac.webkit.org/changeset/106113>