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Selective masking: issue with mask opacity #34947

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mhugo opened this issue Mar 9, 2020 · 0 comments · Fixed by #36697
Closed

Selective masking: issue with mask opacity #34947

mhugo opened this issue Mar 9, 2020 · 0 comments · Fixed by #36697
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Bug Either a bug report, or a bug fix. Let's hope for the latter!

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@mhugo
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mhugo commented Mar 9, 2020 •
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Describe the bug

Playing with the opacity of selective masking sometimes give strange results.

Initially reported by @nirvn #33432 (comment) and on Twitter : https://twitter.com/klaskarlsson/status/1231518713804992512

image

image

How to Reproduce

I fail to reproduce the error on my system. But since two people reported it, there might be something ...

QGIS and OS versions

3.12
@mhugo mhugo added the Bug Either a bug report, or a bug fix. Let's hope for the latter! label Mar 9, 2020
obrix pushed a commit to obrix/QGIS that referenced this issue May 25, 2020
At the composition stage of the second pass introduced with selective…
a138022 
… masking, the unmasked elements located under the mask are copied into a temporary image buffer and then readded later to the final image.

This copy was done using the masks from selective masking with their full opacity informations. Unmasked informations were thus combined with opacity from the mask leading to the introduction of several artefacts :
* The "phantom lines" qgis#34650 (opacity information introduced by the antialising on mask drawing)
* Opacity applied on non masked surfaces in qgis#34947

To fix this issue a temporary mask is created for the copy of unmasked pixels, in this temporary mask the alpha channel is "binarized" (fully opaque on masking pixel, fully transparent on non masking pixel).

Should fix qgis#34650 and fix qgis#34947
@obrix obrix mentioned this issue May 25, 2020
Merged
nyalldawson pushed a commit that referenced this issue Jun 7, 2020
@nyalldawson
At the composition stage of the second pass introduced with selective…
708910a 
… masking, the unmasked elements located under the mask are copied into a temporary image buffer and then readded later to the final image.

This copy was done using the masks from selective masking with their full opacity informations. Unmasked informations were thus combined with opacity from the mask leading to the introduction of several artefacts :
* The "phantom lines" #34650 (opacity information introduced by the antialising on mask drawing)
* Opacity applied on non masked surfaces in #34947

To fix this issue a temporary mask is created for the copy of unmasked pixels, in this temporary mask the alpha channel is "binarized" (fully opaque on masking pixel, fully transparent on non masking pixel).

Should fix #34650 and fix #34947
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Fix selective masking composition stage obrix/QGIS
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@mhugo