In this blog post I want to give an overview on what I am planning and working on for KWin in KDE SC 4.6. The big topic for 4.6 is performance – in 4.5 we introduced the blur effect and our designers want to extend the usage of blur to all windows. This is currently not yet recommendable (yes there are widget styles on kde-look which offer this function, but KWin is not ready for it!), so we have to work on it.
In general there are three topics I will address in the next cycle:
- Mobile edition: port compositing stack to OpenGL ES 1.1 and/or 2.0
- Improve performance: render less, render faster and more frames per seconds during animations
- Stabilize the effect ABI to be able to provide BC latest in 4.7 (if it makes sense)
These three topics are very much related to each other. For a mobile port we need to improve the performance and port to OpenGL ES. This requires to cleanup the rendering code which gives us a more modern rendering code resulting (in theory (I do not trust drivers any more)) in better performance (render less and faster). As part of this I am aiming to use forward-compatible OpenGL Shaders by default for rendering. Yesterday OpenGL 4.1 was released and it would be nice to be able to use OpenGL 2 functionality in a real world application on Linux. So this is an urge to the driver developers: please get your drivers ready for 4.6 – we will stress them. In a next step I want to go to OpenGL 3 in 4.7, but the free drivers do not yet support this generation. (That’s perhaps a point to work together with Compiz developers)
Thanks to the improvements on the rendering stack our API gets into a state where it makes sense to think about guaranteeing BC. Currently we do not and break the ABI in each cycle several times (up to now I broke BC in trunk more than five times). Our API does not yet use d-pointers and we have many pure virtual classes, which will make it difficult. But the API itself has not changed much during the last cycles, so it’s pretty stable already. With 4.6 the compositing stack will have been used three years, with two years enabled by default, so it’s really time to think about it. But this is just an idea I have and is not yet discussed with other KWin developers and it will require lots of boring work.
So where are we? I started with improving the rendering code of our Plasma styled frames and moved the rendering code into the concrete implementations for XRender and OpenGL. That made it possible to get something like the following screenshot:
Boxswitch with blurred background
Also the rendering code is improved. Instead of recreating the icons in each rendered frame by doing a QPixmap -> QImage -> GLTexture conversation, the icon texture is reused and generated using Texture From Pixmap (TFP) as used for our window pixmaps (there are still some effects forcing a recreating in each frame, but this will soon be fixed). For passing the geometry to the GPU a vertex buffer object (VBO) is used instead of the very, very old glBegin/glEnd. This VBO is also cached, so each geometry is only created once and passed to the GPU once. My abstraction for VBO is able to fall back to legacy rendering if VBOs are not available (very unlikely), which gives us just one high level API call to do legacy, normal and modern (forward compatible) rendering. The old methods are deprecated although still used by the window rendering (this requires some more useful changes as they currently stream the vertices in each frame). For rendering the effect frames (and all other textures) triangles are now used instead of quads (unavailable on ES and OpenGL 3 forward compatible profiles).
And what next? We still have many things to improve. Especially window rendering has to be changed to cache the geometries (when it makes sense) and the clip regions. The clip regions are one of the reasons why I want to switch to Shaders (other reasons are Nuno’s wishes for effects – if you want to work on awesome effects get in touch with one of us). The API still needs some more cleanup and our compositing stack has to be split into parts for GLX and EGL. Nevertheless I think the API is already in a state where I think of daring to compile KWin effect library this weekend on Maemo to see what breaks (effects would not yet compile).
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