local-bone-to-world-bone

Calculate the world bone space matrices for a set of local bone space matrices

Given a set of pose bones (that came from a keyframe time), joint relationships, and joint indices (in order to look up pose bones) calculate and return the world bone matrices for this set of pose bones

Background / Initial motivation

I'm exporting some pose matrices from Blender and want to calculate the world bone transformations. I wrote some code over a year ago in collada-dae-parser to do this but it was a bit hacky so wrote something that I can make use of outside standalone.

Install

npm install --save local-bone-to-world-bone

Usage

var local2WorldBone = require('local-bone-to-world-bone')

// childBoneName: parentBoneName
var jointParents = {
  lower_back: null,
  upper_back: lowerBack,
  shoulder_left: upperBack,
  shoulder_right: upperBack
}

// childBoneName: indexInArrayOfPoseMatrices
var jointIndices = {
  lower_back: 0,
  upper_back: 1,
  should_left: 3,
  shoulder_right: 2
}

// An array of local bone matrices, ordered by the above jointIndices array
var localBoneMatrices = [
  // These probably won't all be identity matrices when using real data
  [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1],
  [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1],
  [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1],
  [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1]
]

var worldBoneMatrices = local2WorldBone(localBoneMatrices, jointParents, jointIndices)
console.log(worldBoneMatrices)
  /**
   * All of your bone world transformation matrices in the same order as your local bones
   * [
   *   [...],
   *   [...],
   *   [...],
   *   [...]
   * ]
   */

API

local2WorldBone(localPoseMatrices, jointParents, jointIndices) -> worldPoseMatrices

poseMatrices

Type: Array[numBones][16]

[
  [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1], // someOtherRootBone
  [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1], // childOfSecondRootBone
  [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1], // parentBone
  [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1], // childBone
  [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1]  // childChildBone
]

An array that has the pose matrix for each bone. Note that they won't typically be identity matrices as they are in this trivial example.

jointParents

Type: Object

{
  parentBone: null,
  childBone: parentBone,
  childChildBone: childBone,
  someOtherRootBone: null,
  childOfSecondRootBone: someOtherRootBone
}

An object that maps each bone to it's parent, or null if it is a root bone

jointIndices

Type: Object

{
  parentBone: 2,
  childBone: 3,
  childChildBone: 4,
  someOtherRootBone: 0,
  childOfSecondRootBone: 1
}

(returned) worldPoseMatrices

Type: Array[numBones][16]

[
  [...],
  [...],
  [...],
  [...],
  [...]
]

We return an array of the world bone poses. You typically want to then multiple each world matrix by the bone's inverse bind matrix before making use of it.

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License

(c) 2017 Chinedu Francis Nwafili. MIT License