Apart from the scripting interface KWin will not ship any major new feature in 4.6. Most of the work I did for 4.6 was to improve the overall performance of our compositing engine. One of the first improvements I added was for the text and icon overlays in our effects. Now the texture and geometry is always cached and Texture from Pixmap (TFP) is used instead of costly QPixmap/QImage conversations.

Another area which saw great improvements are effects which transform one window. These effects used to cause the whole screen to repaint even if only a small region changed. Most prominent visible is this with the Show Paint effect enabled and for example Sliding Popups or Wobbly Windows. Those effects are now repainting only the changed area which is a great improvement, especially considering that there is less blurring involved. The changes seem to make a difference as developers running trunk asked in #kwin if we changed something in Wobbly Windows. Of course not each effect does benefit from this change. It is an ongoing issue and one effect after another has to be improved.

Another area for optimization is not directly in the compositing engine but more in making use of the advantages of compositing in general. 4.6 will include a new effect to render the startup notification ("bouncing cursor"). The current way involves resizing and shaping a window and moving this window. With compositing enabled this can be a costly operation (for me the performance was so bad, that I disabled the notification completely). The new effect does the same animation as the legacy system without using a window at all. It is just a texture rendered on the screen. This means the whole window transformations are not needed and also the costly TFP operations are not required. Another nice side-effect is that the icon can now use an alpha channel – the old animation transformed each icon to a RGB-only icon. If compositing is not used the system will fall back to the legacy rendering which is btw. not part of KWin.

During the last week I worked on another optimization for our Lanczos filter. Especially in Present Windows the filter was causing some performance regressions. Highlighting one of the windows causes the complete screen to repaint (optimization opportunities see above) which caused a new Lanczos filtering for each visible window. The highlighting animation lasts for something like 200 msec with constant fullscreen repaints. It is obvious why this caused a performance regression. My changes cache each generated texture and use this whenever a repaint without change of content is required. So the highlighting does not require a recreation of the texture (the highlighting is added in a later step). So Present Windows in 4.6 will have the performance as it used to be in 4.4 with the improved thumbnails from 4.5. Of course the cache is useful to all places where the Lanczos sampling is used – be it Box Switch or Taskbar Thumbnails.

I hope that with this change the blacklist is no longer required for performance problems but can be used for driver problems only as intended. Also this allows to integrate the Lanczos improved thumbnails in more effects – e.g. Desktop Grid as in the image.

Improving the performance is of course an ongoing issue and there are still some areas where we can get some clever caching in place. For example blur might be a good candidate for improvements. But this is topic for a blog post "Optimization in KWin 4.7".

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