/// <summary>
/// Adjusts child transform, making it relative to new parent node's transform.
/// Is recursive, looking for parents that also implement IRenderableNode.</summary>
/// <param name="parent">Parent node</param>
/// <param name="child">Child node</param>
public static void AddChild(ITransformable parent, ITransformable child)
{
Path <DomNode> path = new Path <DomNode>(parent.Cast <DomNode>().GetPath());
Matrix4F parentToWorld = TransformUtils.CalcPathTransform(path, path.Count - 1);
// We want 'child' to appear in the same place in the world after adding to 'parent'.
// local-point * original-local-to-world = world-point
// new-local-point * new-local-to-parent * parent-to-world = world-point
// ==> new-local-to-parent * parent-to-world = original-local-to-world
// (multiply both sides by inverse of parent-to-world; call it world-to-parent)
// ==> new-local-to-parent = original-local-to-world * world-to-parent
Matrix4F worldToParent = new Matrix4F();
worldToParent.Invert(parentToWorld);
Matrix4F originalLocalToWorld = child.Transform;
Matrix4F newLocalToParent = Matrix4F.Multiply(originalLocalToWorld, worldToParent);
// The translation component of newLocalToParent consists of pivot translation
// as well as the child.Translation. So, start with the original child.Translation
// and transform it into our new space.
Vec3F newTranslation = child.Translation;
worldToParent.Transform(ref newTranslation);
// There's only one way of getting rotation info, so get it straight from matrix.
Vec3F newRotation = new Vec3F();
newLocalToParent.GetEulerAngles(out newRotation.X, out newRotation.Y, out newRotation.Z);
child.Rotation = newRotation;
// Likewise with scale.
Vec3F newScale = newLocalToParent.GetScale();
child.Scale = newScale;
// We can compose together all of the separate transformations now.
Matrix4F newTransform = CalcTransform(
newTranslation,
newRotation,
newScale,
child.ScalePivot,
child.ScalePivotTranslation,
child.RotatePivot,
child.RotatePivotTranslation);
// However, the composed matrix may not equal newLocalToParent due to rotating
// or scaling around a pivot. In the general case, it may be impossible to
// decompose newLocalToParent into all of these separate components. For example,
// a sheer transformation cannot be reproduced by a single rotation and scale.
// But for common cases, only the translation is out-of-sync now, so apply a fix.
Vec3F desiredTranslation = newLocalToParent.Translation;
Vec3F currentTranslation = newTransform.Translation;
Vec3F fixupTranslation = desiredTranslation - currentTranslation;
Matrix4F fixupTransform = new Matrix4F(fixupTranslation);
newTransform.Mul(newTransform, fixupTransform);
// Save the fix and the final transform. Storing the fix in RotatePivotTranslation
// is done elsewhere, as well.
child.Translation = newTranslation + fixupTranslation;
child.Transform = newTransform;
}