Alpha artefacts when overlaying blend filtered objects

[ivan louette]

Wrong dark contouring appears around blurred objects when overlaying with a blended one with a Composite In or Out to Background image.

This is particularly visible when "screen" and "lighten" Blend mode are used.

It occurs even when the blurred object is white.

Gradients alpha isn't affected by this problem.

Inkscape Rev 10746

[su_v (suv-lp)]

Reproduced with Inkscape 0.48.2 and 0.48+devel r10754 on Mac OS X 10.5.8 (i386)

I'm not sure though how the filter is supposed to be rendered:
- Squiggle (Batik 1.7), Opera 11.52 render the file like Inkscape (stable + trunk)
- Firefox 3.6, Chromium (r111116) don't render the filtered group (id="g98") at all

[ivan louette (ivan-louette)]

My opinion is that as in blur the objects color fades to background like a transparency gradient this dark artefact should not happen.

[su_v (suv-lp)]

> My opinion is that as in blur the objects color fades
> to background like a transparency gradient this
> dark artefact should not happen.

It also happens when using a gradient simulating a straight blurred edge, or an object with a semi-transparent solid fill. The "artefact" is not related to using blurred objects in the background, it is related to having them fade into transparency: AFAIU (I could be wrong though) this is another instance of the known limitation in SVG 1.1 of the double-counting background as described in <http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Filters-Compositing.html>

(with either a white solid object in the background, or merging a flood fill as mentioned in the manual, the edges of the blurred object in the background do not show a darker color when fading into transparency)

[ivan louette (ivan-louette)]

Thanks. You are right and I was wrong about gradients alpha.