If I want to scale the convex hull of a rigid body, what is the correct operation?

[CeSun]

Transform all vertices and reconstruct the shape?

[notgiven688]

Transform all vertices and reconstruct the shape?

Yes, transforming all vertices and reconstructing the shape would be the method of choice. However, there's an exception: if you have many instances of convex hulls that only differ in their scaling factor. In such cases, you can wrap your ConvexHullShape inside a TransformedShape. To perform the scaling transformation, modify the diagonal entries of the "orientation" matrix in the constructor. I will rename the matrix, as it supports not only orientation but also scaling. In fact, all affine transformations (see Affine Transformation) are supported by TransformedShape.

Here's an example from JitterDemo, in ConvexDecomposition.cs:

Change:

foreach (ConvexHullShape s in shapesToAdd)
{
  body.AddShape(s.Clone(), false);
}

To:

foreach (ConvexHullShape s in shapesToAdd)
{
  body.AddShape(new TransformedShape(s.Clone(), JMatrix.Identity * 0.2f, JVector.Zero), false);
}

This will scale your object by a factor of 0.2.

[CeSun]

modify the diagonal entries of the "orientation" matrix in the constructor

Orientation does not seem to take effect on scaling, my code is:

RigidBody = Owner.CurrentLevel.PhyWorld.CreateRigidBody();
RigidBody.Position = new JVector(WorldLocation.X, WorldLocation.Y, WorldLocation.Z);
foreach(var shape in StaticMesh.Shapes)
{
    RigidBody.AddShape(shape);
}
var Matrix = MatrixHelper.CreateTransform(Vector3.Zero, WorldRotation, WorldScale);
RigidBody.Orientation = new JMatrix
{
    M11 = Matrix.M11,
    M12 = Matrix.M12,
    M13 = Matrix.M13,
    M21 = Matrix.M21,
    M22 = Matrix.M22,
    M23 = Matrix.M23,
    M31 = Matrix.M31,
    M32 = Matrix.M32,
    M33 = Matrix.M33,
};

[CeSun]

Modifying the shape every time the scale changes is very resource intensive, and will also generate a lot of objects that generate GC pressure, so I hope to directly modify the matrix of the rigid body.

[notgiven688]

You need to wrap your hull-shape in a TransformedShape - see my example from above. You can then change the transforms "on the fly" without much cost, by modifying the "Transformation" property of the TransformedShape.

[CeSun]

Good job! at first, I thought TransformedShape could only transform the shape of the input parameter during construction...

[notgiven688]

I found a small bug in the code: When you set the transformation matrix via the Transformation property, all mass properties (geometric center, mass, inertia) of the TransformedShape get recalculated by recalculating the mass properties of the child shape (in your case a PointCloudShape). For a PointCloudShape this can be rather slow. #42 fixes this problem.