BVH Frame Rate

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Theoretically, BVH animations for use in Second Life can have a frame rate between 1 and 1092 FPS (frames per second). Internally, the time that a key frame is displayed is stored as a unsigned short (a 16-bit integer), which has 65536 unique values (including 0). Animations can be up to 60 seconds long. For each key frame to have a unique value for the time it's displayed:
FPS = 65536/60

In practice, you should never use a frame rate above 45 FPS. Nothing else in the simulator can move at above 45 FPS.

It's important to understand that the FPS of a BVH animation only indicates how often the motion is sampled, not the number of images that will be shown when they are played.

The advantage of using a low frame rate is smaller animation files, which transfer to viewers faster. You need not worry that the animation will appear choppy. The viewer will smoothly interpolate between the key frames on computers with a display FPS higher than the FPS of your animation. That said, 60 second animations at 45 FPS are smaller than most images.

The advantage of using a high frame rate is more detail can be recorded. While Second Life lossily compresses animations, the motions dropped are so subtle as to be imperceptible. You should feel free to use 45 FPS for all animations.

The following is a table of frame rates with desirable qualities.

Frame Rate Factors Factor Of
5 prime 5 10 45 60 360
6 2 3 60 360
8 2 4 360
9 3 45 360
10 2 5 10 60 360
12 2 3 4 6 60 360
15 3 5 45 60 360
18 2 3 6 9 360
20 2 4 5 10 60 360
24 2 3 4 6 8 12 360
30 2 3 5 6 10 15 60 360
36 2 3 4 6 9 12 18 360
40 2 4 5 8 10 20 360
45 3 5 9 15 45 360

A frame rate with many factors is desirable. It is also desirable that the frame rate be a factor of numbers you often need to divide.

Numbers Second Life animators might want to divide:

  • 5 - pelvis offset limit in meters
  • 10 - pelvis offset axis length in meters
  • 45 - maximum useful frame rate
  • 60 - animation length limit in seconds; seconds in a minute; minutes in a hour
  • 360 - degrees in a circle

This makes it easy to place key frames such that the time and space is evenly divided, which is something animators often need to do.

You might find it easiest to place your key frames at 15 FPS, then change to 45 FPS before exporting to maximize the detail recorded. If you do this, be sure the program you're using compensates for the change in frame rate so your animation doesn't play three times faster. Selecting exactly the frame you want is easier at low frame rates. It should export perfectly, because each frame at 15 FPS falls exactly on a frame boundary at 45 FPS.