Friday, June 12, 2009

ATI graphics fixes

The last month saw both the release of Ember 0.5.6 and the start of the Google Summer of Code.
The Ember release included the really nice addition of real time shadows, thanks to the work done by Alexey. It also included some really nifty area editing tools for world authors, as well as the usual slew of polish and bug fixes.
After the release was done Alexey and me did some additional work on the shadow system, to fix some of the bugs and artifacts that existed in the release. Amongs them was some issues with shadow texture fidelity at certain distances and a bug where lights that were created in the world never disappeared. Another issue we found was that lights which were attached to entities which a character was wielding did't get updated as the character moved. The prime example here was the torch, which in turn had a "fire" child entity.
All of these issues has now however been fixed in git master.
I've also added a new nifty snap-to-move feature. It's activated when you move an entity in the world, and allows you to easily make it snap to another entity. The snapping is done by matching the bounding boxes of the entities. This is really useful when building fences or walls, since each wall section then can snap into the other.

However, the bulk of my time the last couple of weeks has been spent on graphic glitches, mainly those found on ATI cards. We've known for a little while that there are a couple of artifacts on ATI cards, but since none of the main developers use ATI cards (we use Nvidia, since at least historically their Linux support has been orders of magnitudes better than ATI) we haven't really known how bad these artifacts are.
After upgrading to Mandriva 2009.1 I began to run into X server segfaults when running OpenGL applications. I therefore had to abandon my Nvidia card until the issue has been fixed, and it just so happened that I was able to borrow an ATI cards from work.
Turns out there were all kinds of issues with ATI cards, amongs them the issue with random segfaults within the OpenGL driver. Just this issue had me spend more than a week trying to track down. Intially it wasn't clear that the crashes were random, since they seemed to occur near certain materials. However, after disabling more and more features it became apparent that they were indeed completely random. Thanks to cdleonard, developer of the Caelum environment system, we concluded that the issue was that using hardware generated mipmapping on ATI cards, as supplied by the SGIS_generate_mipmap extension, resulted in random memory corruption withing the (closed source) GL stack. Yep, pretty much the worst possible kind of bug to track down. Once we did track it down it down and disabled the crashes went away.
There are however still some artifacts on ATI cards. The paged geometry has some issues, since it in places mixes vertex shaders with fixed function fragment processing. And the high detail glsl shaders doesn't produce any output, so when running Ember in high detail level all meshes are hidden when viewed up close. But all of these issues are easily fixable and it shouldn't take long until we can do a new release, which will work much better on ATI cards.

The Summer of Code is also progressing nicely. Manuel reports on the progress over at the wiki. We're now mainly in the discovery stage, where we're identifying where in the code base it would be suitable to add threading support. Nonetheless Manuel has already submitted a series of patches for various code cleanup issues which I've merged into Ember trunk. Git is truly a wonderful tool!

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