Biovision BVH

The BVH file format was originally developed by Biovision, a motion capture services company, as a way to provide motion capture data to their customers. The name BVH stands for Biovision hierarchical data. This format mostly replaced an earlier format that they developed, the BVA format which is discussed in the next section, as a way to provide skeleton hierarchy information in addition to the motion data. The BVH format is an excellent all around format, its only drawback is the lack of a full definition of the basis pose (this format has only translational offsets of children segments from their parent, no rotational offset is defined), it also lacks explicit information for how to draw the segments but that has no bearing on the definition of the motion.

Parsing the file

A BVH file has two parts, a header section which describes the hierarchy and initial pose of the skeleton; and a data section which contains the motion data. Examine the example BVH file called "Example1.bvh". The start of the header section begins with the keyword "HIERARCHY". The following line starts with the keyword "ROOT" followed by the name of the root segment of the hierarchy to be defined. After this hierarchy is described it is permissable to define another hierarchy, this too would be denoted by the keyword "ROOT". In principle, a BVH file many contain any number of skeleton hierarchies. In practice the number of segments is limited by the format of the motion section, one sample in time for all segments is on one line of data and this will cause problems for readers which assume a limit to the size of a line in a file.

The BVH format now becomes a recursive definition. Each segment of the hierarchy contains some data relevant to just that segment then it recursively defines its children. The line following the ROOT keyword contains a single left curly brace '{', the brace is lined up with the "ROOT" keyword. The line following a curly brace is indented by one tab character, these indentations are mostly to just make the file more human readable but there are some BVH file parsers that expect the tabs so if you create a BVH file be sure to make them tabs and not merely spaces. The first piece of information of a segment is the offset of that segment from its parent, or in the case of the root object the offset will generally be zero. The offset is specified by the keyword "OFFSET" followed by the X,Y and Z offset of the segment from its parent. The offset information also indicates the length and direction used for drawing the parent segment. In the BVH format there isn't any explicit information about how a segment should be drawn. This is usually inferred from the offset of the first child defined for the parent. Typically, only the root and the upper body segments will have multiple children.

The line following the offset contains the channel header information. This has the "CHANNELS" keyword followed by a number indicating the number of channels and then a list of that many labels indicating the type of each channel. The BVH file reader must keep track of the channel count and the types of channels encountered as the hierarchy information is parsed. Later, when the motion information is parsed, this ordering will be needed to parse each line of motion data. This format appears to have the flexibility to allow for segments which have any number of channels which can appear in any order. If you write your parser to handle this then so much the better, however, I have never encountered a BVH file that didn't have 6 channels for the root object and 3 channels for every other object in the hierarchy.

You can see that the order of the rotation channels appears a bit odd, it goes Z rotation, followed by the X rotation and finally the Y rotation. This is not a mistake, the BVH format uses a somewhat unusual rotation order. Place the data elements into your data structure in this order.

On the line of data following the channels specification there can be one of two keywords, either you will find the "JOINT" keyword or you will see the "End Site" keyword. A joint definition is identical to the root definition except for the number of channels. This is where the recursion takes place, the rest of the parsing of the joint information proceeds just like a root. The end site information ends the recursion and indicates that the current segment is an end effector (has no children). The end site definition provides one more bit of information, it gives the length of the preceding segment just like the offset of a child defines the length and direction of its parents segment.

The end of any joint, end site or root definition is denoted by a right curly brace '}'. This curly brace is lined up with its corresponding right curly brace.

One last note about the BVH hierarchy, the world space is defined as a right handed coordinate system with the Y axis as the world up vector. Thus you will typically find that BVH skeletal segments are aligned along the Y or negative Y axis (since the characters are often have a zero pose where the character stands straight up with the arms straight down to the side).

The motion section begins with the keyword "MOTION" on a line by itself. This line is followed by a line indicating the number of frames, this line uses the "Frames:" keyword (the colon is part of the keyword) and a number indicating the number of frames, or motion samples that are in the file. On the line after the frames definition is the "Frame Time:" definition, this indicates the sampling rate of the data. In the example BVH file the sample rate is given as 0.033333, this is 30 frames a second the usual rate of sampling in a BVH file.

The rest of the file contains the actual motion data. Each line is one sample of motion data. The numbers appear in the order of the channel specifications as the skeleton hierarchy was parsed.

Interpreting the data

To calculate the position of a segment you first create a transformation matrix from the local translation and rotation information for that segment. For any joint segment the translation information will simply be the offset as defined in the hierarchy section. The rotation data comes from the motion section. For the root object, the translation data will be the sum of the offset data and the translation data from the motion section. The BVH format doesn't account for scales so it isn't necessary to worry about including a scale factor calculation.

A straightforward way to create the rotation matrix is to create 3 separate rotation matrices, one for each axis of rotation. Then concatenate the matrices from left to right Y, X and Z.

vR = vYXZ

An alternative method is to compute the rotation matrix directly. A method for doing this is described in Graphics Gems II, p 322.

Adding the offset information is simple, just poke the X,Y and Z translation data into into the proper locations of the matrix. Once the local transformation is created then concatenate it with the local transformation of its parent, then its grand parent, and so on.

vM = vMchildMparentMgrandparentů