Question

I’m terrible with matrix maths but I have a situation where I need to scale one. The matrix is an instance of a custom class defined here and my scaling object is a vector containing 3 floats (x,y,z). I’d like the actual code I need rather than a generic explanation because I’ve already gone down that path and just don’t understand the maths involved. Fortunately what I’m trying to accomplish is reasonably trivial once I can scale the matrix.

To clarify here is the code I’m updating. It iterates over a hierarchy of linked objects with relative transforms and updates mat& to an absolute transform:

void LocalModelPiece::GetPiecePosIter(CMatrix44f* mat) const
{
if (parent) {
    parent->GetPiecePosIter(mat);
}

if (pos.x || pos.y || pos.z) { mat->Translate(pos.x, pos.y, pos.z); }
// --> My problem is here. There is no Scale() method, I need one. <--
if (scale.x!=1.0f || scale.y!=1.0f || scale.z!=1.0f) { mat->Scale(scale.x, scale.y, scale.z); }
if (rot[1]) { mat->RotateY(-rot[1]); }
if (rot[0]) { mat->RotateX(-rot[0]); }
if (rot[2]) { mat->RotateZ(-rot[2]); }

}

Answer 1

I looked at the API link you provided, and based on that, this is how I would implement a scaling method for the matrix class so that it’s consistent with the existing Translate() and Rotate() methods. That is, it does a right side multiplication with a scaling matrix.

Though personally I find it a bit weird, because the API uses OpenGL style matrices. That means that a right side multiplication applies the new transformation before all the other transformations that exist in the matrix, where as left side multiplication would apply it after the other transformations. So I’m not sure if that’s what you want here. You might have to do all the transformations in the reverse order or do a left side multiplication yourself.

void CMatrix44f::Scale(float scalex, float scaley, float scaley) 
{
  /* the function should be equivalent to doing this:
  CMatrix44f scalemat;
  scalemat[0] = scalex;
  scalemat[5] = scaley;
  scalemat[10] = scalez;
  *this = Mul(scalemat);
  */

  m[0] *= scalex;
  m[1] *= scalex;
  m[2] *= scalex;
  m[3] *= scalex;

  m[4] *= scaley;
  m[5] *= scaley;
  m[6] *= scaley;
  m[7] *= scaley;

  m[8] *= scalez;
  m[9] *= scalez;
  m[10] *= scalez;
  m[11] *= scalez;
}