D25611: [scene] Fix decoration texture bleeding

[Vlad Zahorodnii]

zzag created this revision.
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Herald added a project: KWin.
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REVISION SUMMARY
  Quite long time ago, window decorations were painted on real X11 windows.
  The nicest thing about that approach is that we get both contents of the
  client and the frame window at the same time. However, somewhere around
  KDE 4.2 - 4.3 times, decoration rendering architecture had been changed
  to what we have now.
  
  I've mentioned the previous decoration rendering design because it didn't
  have a problem that the new design has, namely the texture bleeding issue.
  
  In the name of better performance, opengl scene puts all decoration parts
  to an atlas. This is totally reasonable, however we must be super cautious
  about things such as the GL_LINEAR filter.
  
  The GL_LINEAR filter may need to sample a couple of neighboring texels
  in order to produce the final texel value. However, since all decoration
  parts now live in a single texture, we have to make sure that we don't
  sample texels that belong to another decoration part.
  
  This patch fixes the texture bleeding problem by padding each individual
  decoration part in the atlas. There is another solution for this problem
  though. We could render a window into an offscreen texture and then map
  that texture on the transformed window geometry. This would work well and
  we definitely need an offscreen rendering path in the opengl scene,
  however it's not feasible at the moment since we need to break the window
  quads API. Also, it would be great to have as less as possible stuff going
  on between invocation of Scene::Window::performPaint() and getting the
  corresponding pixel data on the screen.
  
  There is a good chance that the new padding stuff may make you vomit. If
  it does so, I'm all ears for the suggestions how to make the code more
  nicer.
  
  BUG: 257566
  BUG: 360549
  FIXED-IN: 5.18.0

REPOSITORY
  R108 KWin

REVISION DETAIL
  https://phabricator.kde.org/D25611

AFFECTED FILES
  decorations/decorationrenderer.cpp
  decorations/decorationrenderer.h
  plugins/scenes/opengl/scene_opengl.cpp
  scene.cpp

To: zzag, #kwin
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