Spooky transparency influence at a distance

[thorwil]

Some very hard to pinpoint combination of shapes, layers and filters result in rectangular areas rendered with increased or reduced transparency.

I will attach a file that triggers this bug. Look at the right rectangle with gradient. Its left side is rendered darker. If you move the right blue shape (layer b) sideways, this area of darker rendering is affected. If you delete the left blue shape and zoom one step, the rendering bug disappears.

Inkscape 0.48.3.1 r9886

[su_v (suv-lp)]

To be expected, the trigger is layer blend mode which affects semi-transparent objects (rect with gradient which uses transparency): because the content of the layer which has a special blend mode is spread apart, the FER (filter effects region) is much larger than individual objects, and renders larger areas of the semi-transparent objects darker due to the effect of the 'double counting background'.

Known limitation of SVG filter effects (SVG 1.1), see
"Double counting background"
<http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Filters-Compositing.html>

Proposing to link as duplicate to
- Bug #180890 “Page background is not an SVG element”
  <https://bugs.launchpad.net/inkscape/+bug/180890>

[thorwil (t-w-)]

I can confirm a opaque background "solves" the issue. Only that I need the transparency. A workaround is to place an object further right on each layer with special blend mode.

Thanks ~suv, for the great job you do reviewing reports and making the right connections!