## Saturday, November 12, 2011

### Why Gamebryo was "fun"

If you didn't catch it in the title, there was some sarcasm in there. Gamebryo (the game engine used to make LEGO Universe as well as other titles such as Fallout 3, Oblivion, Epic Mickey, and more) had some very annoying quirks that made my life a little bit more difficult, as well as some of my coworkers'.

Both the Matrix and the Quaternion classes had multiple ways to create rotations, and naturally Gamebryo used no naming convention between these two classes.

Create a Rotation from the X axis and an angle?
Matrix function: MakeXRotation(float angle)
Quaternion function: FromAngleAxisX(float angle)

Gamebryo included a lot of functionality in either the Matrix or Quaternion classes, but not both. It's like a cruel joke.

Want to make your matrix an identity matrix? Call 'MakeIdentity( )'. Want to do that with a Quaternion? Do it yourself, there is no function.

Want to lerp (linearly interpolate) a Quaternion? Go for it. Want to lerp a Matrix? Sorry, convert it to a Quaternion, lerp it, then convert it back to a matrix. On top of that, the Slerp (spherical linear interpolation) did not even work properly with Gamebryo, often causing objects to rotate 350 degrees to get to the target, rather than 10 degrees. We ended up discontinuing the use of Gamebryo's Slerp method altogether and calling to Havok to have it do the rotations for us. Alternatively we could have just rolled our own Slerp code or altered Gamebryo's version to use ours, but Gamebryo made calls internally to its Slerp method so we didn't want to break something by fixing something.

Gamebryo also thought it would be cute to use opposite rotation directions for matrices vs. quaternions. Those two rotation functions I listed earlier, if you want to represent the same rotation with a matrix as well as a quaternion you pass a positive angle into one, and a negative angle into the other. It's always fun to try and remember which way you need to rotate which type depending on the case you were using it in. Gamebryo likes to make sure the programmer is paying attention I guess.

Here's another little chestnut that Gamebryo left me to deal with. Their camera coordinate system has a 'forward' vector of X, rather than Z. Seriously? What game has EVER done that? For those new to game programming, about 90% of games use the X-axis as a 'right' vector, and Z as a 'forward', the other 10% use the Y-axis as the forward and the Z-axis as the 'up'. But I had never seen an X-axis be used for a 'forward' before. There was no way we were going to use the X-axis as the forward for LEGO Universe, so we ended up having to deal with Gamebryo's wacky camera coordinate system. It was always a fun task when trying to do something that involved aligning the camera based on a rotation that was in another system, you'll set them to the same rotations and then rotate the camera 90 degrees to line it up.

I might go more into some annoyances using Gamebryo later, but for now I'm still bound to an NDA on most subjects.

#### 1 comment:

1. Glad I don't have to work with Gamebryo.
Although I think they used X for forward because most 2D graphs have X as the horizontal axis and Y as the up axis.
Then Z'd be the depth. Just my guess though.