Developing for Windows Phone and Xbox Live : Interacting with Objects

Let’s see how this works by creating an example of an avatar lifting a hand weight.

For additional member variables, add the following member variables.

 // Avatar bones in world space
List<Matrix> bonesWorldSpace;

Model handWeight;
Matrix handWorldMatrix;

The bonesWorldSpace stores the world space values for each of the bones that make up the avatar skeleton. You also need variables for the hand-weight model and the world matrix used to draw it.

In the LoadContent method, load the model and populate the list of bones in world space. Add the following lines of code to your game’s LoadContent method:

// Load the hand weight model
handWeight = Content.Load<Model>("HandWeight");

// Turn on default lighting
foreach (ModelMesh mesh in handWeight.Meshes)
{
    foreach (BasicEffect effect in mesh.Effects)
    {
        effect.EnableDefaultLighting();
    }
}

// Populate the world space bone list
bonesWorldSpace = new List<Matrix>(AvatarRenderer.BoneCount);
for (int i = 0; i < AvatarRenderer.BoneCount; i++)
    bonesWorldSpace.Add(Matrix.Identity);

Next, in the game’s Update method, calculate the world space transforms for the avatar animation. Add the following to your game’s Update method:

if (avatarRenderer.State == AvatarRendererState.Ready)
{
    ConvertBonesToWorldSpace(avatarRenderer, avatarAnimation);
}

// Find the position of the right hand bone
handWorldMatrix = bonesWorldSpace[(int)AvatarBone.SpecialRight];

Before you convert the bone transforms into world space, check that the AvatarRenderer.State property is set to Ready so you can verify that the avatar and its skeleton have completed loading. Then, call the ConvertBonesToWorldSpace method passing in the AvatarRenderer and AvatarAnimation we want to find the world space position of. After calculating the world space transforms, use the AvatarBone.SpecialRight bones transform for the world matrix for the hand-weight model. The SpecialRight bone is a piece of the avatar skeleton that is located near the inside of the forearm in front of the hand. It is a good bone to use when you want to place objects in the avatar’s hand.

To implement the ConvertBonesToWorldSpace helper method, add the following method to your game:

/// <summary>
/// Convert the avatar bones into world space
/// </summary>
private void ConvertBonesToWorldSpace(AvatarRenderer renderer, AvatarAnimation animation)
{
    // Bind pose of the avatar skeleton
    // Values are in local space and relative to the parent bone
    IList<Matrix> bindPose = renderer.BindPose;
    // Animation bone transforms are in local space and relative to the parent bone
    IList<Matrix> animationPose = animation.BoneTransforms;
    // Parent index values for each of the bones
    IList<int> parentIndexes = renderer.ParentBones;

    // Transform each bone into world space
    for (int i = 0; i < AvatarRenderer.BoneCount; i++)
    {
        // Partent world space matrix
        // The world matrix of the avatar renderer is used for the root
        Matrix parentMatrix = (parentIndexes[i] != -1)
                                ? bonesWorldSpace[parentIndexes[i]]
                                : renderer.World;

        // Calculate world space matrix for the bone
        bonesWorldSpace[i] = Matrix.Multiply(Matrix.Multiply(animationPose[i],
                                                                bindPose[i]),
                                                                parentMatrix);
    }
}

					  

To calculate the bone’s position in world space, you first need the bind pose of the avatar. The animation frames are relative to the bind pose transforms, so you need both the animation transforms and the bind pose transforms. Each of the bones is also relative to its parent bone, so you also need to multiply the transform by its parent world space matrix. For the root node, use the World property of the AvatarRenderer because this is the location in world space of the avatar. Again, you can loop over the bones linearly as you calculate the world space transforms because you know the parent bones are calculated before any of the children.

Finally, you can draw the hand-weight model using the world space transform you calculated. Add the following lines of code to your game’s Draw method:

// Draw the hand weight model
handWeight.Draw(handWorldMatrix, view, projection);

Running the sample displays an avatar with the weight model in his or her right hand (see Figure 1).