Going down the programmable pipeline road

As you might know OpenGL comes in two flavors: fixed functionality and programmable pipeline. With the fixed functionality you have to use API calls to influence the execution of each stage of the rendering pipeline. It is a very powerful API allowing you to do most of the stuff we use in KWin. The programmable pipeline allows to directly execute code (called a "Shader") to do vertex and fragment processing. For example we are able to saturate a complete window as a whole with fixed functionality, but we need a fragment shader to be able to change the color of each pixel depending on the input color. This is for example used in the invert effect. A vertex shader can be used to influence the geometry. E.g. we could use it to transform a cube into a sphere. OpenGL 1.x is completely fixed functionality, in OpenGL 2 the programmable pipeline was introduced to exchange parts of the rendering stack, but fixed functionality was still around. With OpenGL 3 everyone expected the fixed functionality to be removed, but it was only deprecated and you can still use it. All the modern calls have been moved into a "core profile".

KWin uses OpenGL 2, that is most of our code is fixed functionality as it was around with OpenGL 1, but we can use shaders to exchange parts of the rendering pipeline to e.g. invert a window. As our shaders are used to emulate fixed functionality the code they use is mostly deprecated as well, KWin does not yet use the core profile.

To make it all more complicated there is also an OpenGL version for Embedded Systems (ES/EGL), which also comes in a flavor for fixed functionality (1.1) and programmable pipeline (2.0). In opposite to the desktop version of OpenGL, the ES version does not include the fixed functionality API calls in ES 2. This means that our source code cannot be compiled directly for ES.

Up to now I only blogged about that I want to port KWin to ES. I started working on it and in 4.6 there is some code already which was written to ease the porting effort. The rendering code has been partly split into legacy painting and modern painting, which makes it easier to just #ifdef away the unsupported calls when compiling for mobile. My initial plan was to first port KWin to ES 1.1 as the code base is closer to 1.1 than to 2.0. The work on it is quite boring and basically stalled. So I decided that this is no solution and instead want to add a core profile compatible rendering path to KWin – this would work both with OpenGL 3 and OpenGL ES 2. Of course no existing code will be removed and most likely it will become opt-in as I expect the changes to break some current effects when using the core profile (at least I can see that cube is partly broken in my special kwin version I’m running here).

With the feature freeze for 4.6 behind us, it’s the perfect time to play around with a new rendering path as it can mature on my systems before it is made available to a broader developer testing. So yesterday I set down and implemented a core profile compatible shader for basic rendering without transformations. This is to be used when no effect is active and only parts of the screen need to be repainted (visible with show paint effect). The first results of this change look promising as our EffectFrame overlays can already be rendered with this new coding path. I would like to present a screenshot, but it is boring as it just looks as always. If you want beautiful pictures, go to Nuno’s blog 😉 As soon as the window rendering is also based on this new shader I have a base to use it in the ES porting. Luckily I can reuse the work I spent for porting to ES 1.1, as the same code has to be removed there. So the initial version for mobile will be based on this code and by that just support basic compositing of windows without any effects as those might do transformations or use own GL calls, which have to be ported, too.

Unfortunately the state of the OpenGL API with all the deprecated function calls make it unnecessary difficult to port to the core profile. When passing the geometry to the graphics card we have to either use the legacy code path (for OpenGL 1) or a modern one. Now the modern one we already have, is also deprecated and the code path which has to be used with the core profile and core shaders is incompatible with our existing shaders replacing fixed functionality and the no shader case (but not legacy). Changing all shaders is a lot of work without any gain and the risk to break them. This means that the easy check based on is a shader used, is impossible to decide which code path to use and we have to introduce states into our rendering code to decide which path to use.

These changes will allow us to optionally use OpenGL 3 in KWin. I would like to have at least one effect using a geometry shader in 4.7. And it is required to get KWin on OpenGL ES 2. While that used to be a nice to have feature with a low priority up to now, it looks like it will become very important in the future as Wayland requires ES. This is a very sane decision by the wayland developers. As everybody is writing about wayland nowadays I had to mention it at least once 😛 I am very interested in the topic and hope that it will happen.

And now to something completely different: There is an old saying that something didn’t happen, till you blogged about it. So… Lubos stepped down as KWin Maintainer this week and asked me to become the new Maintainer. I want to thank Lubos for the long time as maintainer and all the work he has done for KWin to make it the extraordinary window manager it is today.

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