RazrFalcon
Global rotation inconsistent with chrome
I've got a small SVG reproduction of an issue I've seen where feOffset is used to create a drop shadow, but it turns out to be inconsistent when rotated - the global transform doesn't seem to be applied to the offset, so the positioning of the offset image is wrong.
<svg height="1000" width="1000" transform="rotate(45)" xmlns="http://www.w3.org/2000/svg">
<rect x="400" y="100" width="200" height="100" fill="black" filter="url(#myFilter)" />
<defs>
<filter id="myFilter" x="0" y="0" width="800" height="600" filterUnits="userSpaceOnUse">
<feOffset in="SourceGraphic" dy="200" result="offsetImage" />
<feBlend in="SourceGraphic" in2="offsetImage" />
</filter>
</defs>
</svg>
I've done the above with a couple of different values of rotate(N), across a couple of different renderers. Image were rendered to 400px, and scaled down to 200px square in the table markdown below:
| rotation | resvg | Gimp (rsvg) | Chrome |
|---|---|---|---|
| 0 | |||
| 20 | |||
| 45 | |||
| Seemingly rotating each item independently then applying the offset | The offset seems correctly modified, but centre of rotation isn't | I assume this is correct - and that it is rotated around the centre of the image? |
There's probably a second bug/inconsistency relating to the centre of rotation. I think all of the above are identical if transformed with rotate(N 0 0), so have different ideas of what 0,0 is.
I haven't quite worked out which bit of code should be handling such adjustments - assuming it's part of is it something that should be done by convert_offset, or does this relate to the TODO: Wrong! Does not account rotate and skew in resolve_primitive_region?
What general changes would be needed to support them? And do you have an idea of which rotation origin is correct?
I have no idea how to implement them. Would probably require lots of significant changes to how filters are rendered. It's a very old issue: #254
And do you have an idea of which rotation origin is correct?
Filters with transform is effectively undocumented in SVG, so I have absolutely no idea either. But in general we always follow Chrome, since it has the best SVG support in general.