[Bug 1280665] Re: libsdl1.2 always uses backingstore which causes tearing in modern X servers

[Andrejs Hanins]

I also confirm that 1.2.15-8ubuntu1.1 from trusty-proposed fixes tearing
bug in XBMC for me (Intel Haswell CPU). Original trusty version
1.2.15-8ubuntu1 definitely has this bug for me.

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https://bugs.launchpad.net/bugs/1280665

Title:
  libsdl1.2 always uses backingstore which causes tearing in modern X
  servers

Status in Simple Directmedia Layer:
  Unknown
Status in “libsdl1.2” package in Ubuntu:
  Fix Released
Status in “libsdl1.2” source package in Trusty:
  Fix Committed

Bug description:
  [Impact]
  Tearing in SDL applications regardless of vsync setting.

  [Test Case]
  Use eg. XBMC to play any video, or a particular video that more easily shows up the tearing of the images.

  [Regression Potential]
  This is a regression fix in itself, and restores old behavior.

  ---

  Fedora 21 has a similar patch applied
  https://bugzilla.redhat.com/show_bug.cgi?id=1073057

  libsdl1.2 always enables backingstore when it is present in the X
  server. For modern X servers, backingstore is implemented by
  redirecting through the composite extension. This causes tearing on
  SDL applications such as xbmc because the redirected rendering is not
  guaranteed to be synchronized.

  A discussion of the problem by Nvidia is here:
  https://devtalk.nvidia.com/default/topic/679688/linux/tearing-in-
  sdl-1-2-apps-unless-composite-extension-is-completely-
  disabled-/post/4111287/#4111287

  The upstream bug is here:
  https://bugzilla.libsdl.org/show_bug.cgi?id=2383

  I have a patch that restores a check for the
  SDL_VIDEO_X11_BACKINGSTORE environment variable before trying to
  enable backingstore.

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