27 Apr 2012

Improved appearance for the simplest Wayland client

Of the Wayland demo clients in the Weston repository, simple-shm is the simplest. All the related code is in that one file, and it interfaces directly with libwayland. It does not use GL or EGL, so it can be ran on systems where the EGL stack does not support the Wayland platform nor extensions. However, what it renders, is surprising:
The original simple-shm client on a Weston desktop.

The square with apparently garbage texture is the original simple-shm. To any graphics developer, who does not know any better, that immediately looks like something is wrong with the image stride somewhere in the graphics stack. That really is what it was supposed to look like, not a bug.

I decided to propose a different rendering, that would not look so much like a bug, and had some real diagnostic value.
The proposed appearance of simple-shm, the way it is supposed to look like.
The new appearance has some vertical bars moving from left to right, some horizontal bars moving upwards, and some circles that shrink into the center. With these, you can actually see if there is a stride bug somewhere, or non-uniform scaling. There is one more diagnostic feature.
This is how the proposed simple-shm looks like when the X-channel is mistaken as alpha.
Simple-shm uses XRGB buffers. If the compositor does not properly ignore the X-channel, and uses it as alpha, you will see a cross over the image. Depending on whether the compositor repaints what is below simple-shm or not, the cross will either saturate to white or show the background through. It is best to have a bright background picture to clearly see it.

I do hope no-one gets hypnotized by the animation. ;-)

1 comment:

Lycanthropist said...

Thank you for improving this simple example. But could you please write a lot more comments into the source code or maybe even a small tutorial (like http://xcb.freedesktop.org/tutorial/basicwindowsanddrawing/ only for wayland). Without some more documentation, this example is pretty much useless.

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