BVHplay FAQ

BVHplay FAQ version 0.1

WATCH OUT -- this is unlikely to be the most update FAQ -- please see www.cgspeed.com for the most current version.

Q: How do I control the camera?
A: See help-->command list within BVHplay. Here's the summary:
a: move camera left
d: move camera right
s: move camera back
w: move camera forward
q: rotate camera right
e: rotate camera left
r: move camera up
f: move camera down

The use of ASDW for camera moves is designed to mimic the common use of these keys in video game control. In other words, many of us are already trained to use ASDW for camera move, so BVHplay just uses that convention.

Q: What are the red, green, and blue lines that show up in the viewport?

A: These axis lines show you where the X (red), Y (green), and Z (blue) axes are. These 3 line segments meet at the origin (0,0,0) of worldspace. If you don't want to see them, you can turn them off from the Settings menu.

Q: What are the coordinate systems in use within BVH, and how is the camera set up?

A: Worldspace is a right-handed space with the Y axis pointing up. This is a very common setup used by many popular animation programs, though it isn't universal. The default camera position faces forward along the NEGATIVE world Z axis, so that your camera sits at a positive Z value and looks towards the scene. Each time you load a new BVH file, BVHplay resets the camera position to a rotation and position that attempts to give you a reasonable initial view of the animated figure. Internally, the camera uses a left-handed coordinate system with the camera-space Z axis pointed in the view direction. This means that in the initial camera position, the camera-space Z axis points in the opposite direction from the (blue) worldspace Z axis. However, the camera's coordinate system is entirely internal to BVHplay and has nothing to do with the Camera position readout shown on the screen.

Q: When I load a BVH file, the animated figure is far above (or below) the grid. What's wrong?

A: Some BVH files have the hips at a very high or very low position, such that the feet are either far above the ground (Y=0) or below the ground. BVHplay simply follows the dataset of the BVH file and positions the figure wherever the BVH file says. If your BVH file says that the figure should have its hips up in the clouds, that's where BVHplay will put it.

Q: How can I (fix the figure height / change the figure orientation / change the motion / eliminate motion capture artifacts / etc)?

A: Within BVHplay, you can't. BVHplay is designed to be a quick and easy BVH playback tool, not a motion editor. However most modern character animation software has the ability to make these sorts of adjustments to animation clips. You could also try using bvhacker, from http://davedub.co.uk/bvhacker/ , although bvhacker only runs on Windows and has a strong emphasis on prepping BVH for SecondLife.

Q: The grid doesn't extend forever -- it's just a finite rectangle. What's up with this?

A: BVHplay draws a grid large enough to cover the XZ (horizontal plane) space that the BVH figure traverses throughout its motion. To avoid complexity and the need to manage a lot of line segment objects, BVHplay doesn't support an "infinite mesh" that covers the entire world no matter where you move the camera. The mesh will always cover

the physical floorspace that your figure travels throughout its motion, which is generally enough to get the animation visibility that BVHplay is designed to provide.

Q: When I play back an animation, it doesn't play back in realtime - it's slower than it should be. What's going on?

A: Testing on a modern 2.4GHz Intel Linux box suggested that the Python code of BVHplay runs fast enough to play 30fps animations in real-time, but doesn't typically run fast enough to play 60fps or 120fps animations in realtime. The internal timing system of BVHplay is set up to be more conservative about animations faster than 30fps, and errs on the side of always displaying each frame rather than take the risk of the animation locking up, which is what happens if your CPU can't keep up with the playback rate. The limitation here is presumably due to Python (which is a quasi-interpreted language) and Tkinter, neither of which are designed to be high-speed graphics engines.

To make your animation play back slightly faster, play it through from start to finish all the way once. The second and future playbacks should be slightly faster. BVHplay does the internal trigonometric computations for each keyframe on a per-keyframe basis the first time it plays the keyframe, and then saves them for future use. So the first time you play your animation, it might run a bit slower than it will the second time and beyond.

I've also observed that even 30fps animations don't play back in realtime on a dual-core Athlon 4600 box, although CPU doesn't get high at all. More Windows speed testing will be necessary -- it's possible that Python's internal timing routines, which bvhplay uses to compute the 1/30-second time intervals between frames, aren't working quite as they're supposed to under Windows.

Q: How can I determine the frame rate at which my BVH animation SHOULD play back?

A: Use a text editor or word processor to look at the text of the BVH file and find the "Frame Time" line. Take the reciprocal of the value to determine fps (frames per second):

A Frame Time of 0.033333 is 30fps
A Frame Time of 0.016667 is 60fps
A Frame Time of 0.008333 is 120fps

Q: My animation playback really crawls, 3x or 4x slower than it was recorded (captured) in the real world. What's up?

A: Some BVH files were captured at 60fps or 120fps, but have the Frame Time value set to 0.033333 (30fps) in the BVH file. This is an error with the BVH file, not with BVHplay. If you like, you can manually edit the BVH file and adjust the Frame Time value to something more appropriate, the reload the file in BVHplay. Remember that BVHplay doesn't play 60fps or 120fps animations quite at realtime. Testing on my own 2.4GHz machine showed on the second playback, BVHplay played a 1029 120fps animation in 12 seconds, which is a playback rate of about 85fps instead of the desired 120fps. So the 120fps animation ran at about 2/3 normal speed.

Q: I can't move the slider to frame 0 - why not?

A: BVHplay numbers animation frames starting at 1, not 0. There's no animation data at frame 0. The reason why the slider bar labeling starts at "0" instead of "1" is because of the behavior of the auto-numbering system of Tkinter's "slider" widget. If I started the fixed slider numbering at "1", then the displayed numbers would be numbers such as "1, 101, 201, 301..." instead of 1, 100, 200, 300... So the slider label shows a "keyframe 0" position even though you can't actually move to that position.