Tuesday, 27 November 2012

Crash! Boom! Bang!

The aforementioned freezes are back and getting a bit ridiculous now. The problem's not limited to OpenGL, it sometimes happens shortly after boot before I've run anything. Fortunately I now occasionally get useful error messages, so I've been able to do better Google/forum searches and apparently this is quite a widespread issue. Setting the USB speed to 1.0 seems to help quite a bit, performance still seems acceptable, but it's making the whole Raspberry Pi experience a bit frustrating at the moment.

I don't see any point in working with the Raspberry Pi in this state, definitely not any hardware project where there are likely to be power issues obscured by the USB problem. So it's with heavy heart that I'm moving my OpenGL coding over to Windows, which is a crying shame. I'll come back to the Raspberry Pi one day, hopefully soon, but for now I'm left feeling that I got mine a bit too early, more so now there's a rev 2 board and more recently it's being shipped with 512MB as standard. Maybe I'll blow mine up with a hardware project and have an excuse to buy a new one?

OpenGL is intended to be cross-platform, and in past projects I've had it up and running on Windows and Linux very quickly. The first problem with OpenGL in Windows is that the maximum version supported is OpenGL 1.1, which was released way back in January 1997 when the likes of the 3dfx VoodooMatrox Mystique and PowerVR series 1 were all the rage, as indeed was the Rage. v1.1 has been fine for me in the past, but if I want to use the same features that are mandatory for OpenGL ES 2.0 (primarily shaders, introduced to desktop OpenGL in v2.0) then I need something more up to date.

You can't upgrade Windows to a newer version of OpenGL as far as I can tell, to get more up-to-date feature support you have to add individual features as extensions. Thankfully this can be handled by the GL Extension Wrangler Library (GLEW). It's a bit of a pain to set up, and when I thought I'd managed it both the static and the dynamic library refused to link no matter what I did, so I ended up importing the GLEW source into my project.

And then I think I found a bug in Visual C++. I've got a square matrix class template which takes a value type and a dimension. Its only member variable is an array which contains the elements, and there are member functions to assign values, do multiplication of two matrices, etc. The default constructor does nothing and, as I'm not ready for the brave new world of C++11 yet (given that VC++ has enough trouble getting C++98 right), I assign values with a redefined operator= which copies data out of an array, or another constructor which takes an array. When I created some arrays to do this, and then declared the matrices, I found some really weird stuff going on. If I just did the matrix declarations, no copying, all of the matrices had the same pointer. If I passed the arrays to the matrix constructors, or assigned them with operator=, then each matrix would have the same pointer as one of the arrays, but not the array that was assigned to it. If I made the arrays static (which is perhaps the right thing to do anyway) then everything was fine. What on earth could cause this? Just my own incompetence? The same code worked OK in g++. As soon as I've found a minimal example of this going wrong I'll submit it to MS.

After I'd worked around that, and remembered to actually call the function which initialised my OpenGL shaders (took me two days to work that one out), worked out how to use a class method as a custom message handler, tried GDI+, failed to get it working and reverted to OLE (about a month on that, admittedly much of it spent being too frustrated to progress and playing Skyrim instead) I was back to where I'd got to on the Raspberry Pi. I was doing a simple rotation about the X-axis, but when I set up the perspective projection matrix properly I got oscillation in the Y direction in time with the rotation. This didn't happen with orthographic projection, so surely I'd done something wrong with the projection matrix? Turns out it was fine, but GLSL stores matrices in column-major format whereas C arrays are effectively row-major. Transpose the final Modelview-Projection matrix and hey presto... everything working beautifully.

Sorry Chloe, Little Teddy turned out to be an intergalactic
criminal mastermind so we had to send him to the Phantom Zone
I've now moved away from "yayy, it works!" and started structuring things a little better for Offender (still need a better name). Rather than continuously drawing a load of triangles, I've got object classes with drawing and moving methods, and separate drawing routines for terrain. Now I can build up a list of objects, it's actually starting to look like the beginnings of a game. However, now I'm putting in more stuff I've found that it bellies-up and dies at around 17,000 triangles. At 60Hz that's about a million a second, which seems a bit low. Admittedly there's still a lot of room for improvement - I'm not using vertex buffers for example - but sorting that out is secondary as I don't need huge numbers of triangles on-screen (yet). All I really need is a single object and some terrain for context, hence the rather psychadelic effort shown here.

In spite of being a lot more complex under the hood, on the surface it's still a bit "Hello Triangle!". Next steps:
  • Maybe use someone else's matrix library, for all the usual reasons people use standard libraries. Why go to the trouble of implementing a matrix inverse when someone's got a tried-and-tested implementation already? The ever-dependable Boost has a matrix library, but I don't think it's quite what I want.
  • Do object positions by coordinate and rotations by quaternion, rather than matrix, so it's easier to move things around. I've already got much of the code for this in my OpenGL screensaver.
  • Add mouse input and player control. Easy for Windows, I'll leave Linux to another day.
  • Add a chase camera to follow the player object. Should be dead easy once I've done all of the above.
  • Add collision detection. Though it's not hard to knock together a crude algorithm, it's difficult to do collision detection accurately and not slaughter your CPU in the process. I've had loads of ideas about this, found a guide on the subject and looks like I was definitely thinking along the right lines. I'll start with something pretty crude though - if I could just make the terrain solid so I can't fly through it, that'd be a start.
  • Switch to using vertex buffer objects, maybe use index buffers too, as I'm going to need more triangles sooner or later. I'll probably want texture buffers too.
Once I've done all that, I think I'll have all of the major boilerplate in place and can actually start building up the interesting stuff.

And finally, you may have noticed that I'm a bit of a David Braben fanboy, so please support Elite: Dangerous. 'Tis a worthy project.