The curve will be a straight line from the left keyframe’s outgoing value, and will not match the right keyframe’s outgoing value. Unlike the hou.matchin() function, the left keyframe’s outgoing value can differ from its incoming value, so so hou.vmatchin() can produce curves whose left tangent matches the left keyframe but whose left values do not line up.
This function is one of the special animation functions that use the current playbar time and information in the parameter’s keyframes, such as in and out values, tangents, acceleration, etc., to evaluate themselves. Because the information needed to evaluate these functions comes from the keyframes, these functions take few or no parameters.
Calling this function from outside a parameter’s expression will raise a hou.NotAvailable exception.