With the KWin desktop effects in KDE SC 4.5 they are beginning to use the GL Shading Language. Initially this GLSL usage is limited to KWin's blur effect and lanczos filter, but stressing the GLSL code paths is exposing some Linux graphics card driver bugs, primarily with the open-source hardware drivers.
KWin checks to ensure the drivers advertise GLSL support and runs other checks, but there's been issues with the Intel driver handling some operations (slowly) on the CPU via software emulation rather than the Intel IGP, frame-buffer object bugs, multi-monitor problems for some drivers, the Intel driver rendering some windows upside-down, and other bugs.
Now why are we using something that new? Because it is quite old: this is OpenGL 2 we are speaking about, a standard specified in 2004! Btw. Microsoft made use of blur by default when they introduced Vista, that was in 2006. So we are talking about functionality specified since six years and used by default by our competition for four years. Oh and please note: the same hardware runs fine in Vista or Windows 7 – at least that’s what we can see from the bug reports.
With even the OpenGL 2.1 support in the open-source drivers causing grief for a number of users on the KDE desktop, it may be tough to hope for an even modest level of OpenGL 3.x support in them by the time KDE SC 4.7 rolls out next year. While Mesa is making progress towards OpenGL 3.x functionality, this still needs to be hooked in within the different drivers and/or the long-awaited OGL3 state tracker to emerge for Gallium3D.
Martin's blog post concerning these graphics driver issues with KWin found in KDE SC 4.5 can be read on this page.