RE: [Visualpython-users] GL issue

[David S. <dsc...@vy...>]

> 	When the .forward scene vector is set to (0,1,0) or 
> (0,-1,0), OpenGL initialization either fails (I've had two 
> machines do this) or the scene window will just be blank. Is 
> this a bug that anyone else can replicate?

More specifically, this happens when scene.forward == scene.up.  The
orientation of the scene cannot be fully constrained by only one vector.
The basis calculation in Display::refreshCache() promptly degenerates:

  Vector Z = forward.norm();
  Vector X = Z.cross( up ).norm();

The ambiguity chould be resolved arbitrarily using code similar to that
in Primitive::refreshCache:

    Vector W = axis->cross(*up);
    if (!W) {
      // Resolve degenerate cases:
      W = axis->cross(Vector(1,0,0));
      if (!W) W = axis->cross(Vector(0,1,0));
    }
    W = W.norm();
    Vector H = W.cross( *axis ).norm();
    Vector L = axis->norm();

Alternatively, we could make it an error to set scene.forward==scene.up,
and enforce that in Display::setattr.

    } else if (attr == "forward") {
      if (Vector(value)==up)
        throw ValueError("display.forward may not be the same as
display.up");
      forward = Vector(value);

...

    } else if (attr=="up") {
      if (Vector(value)==forward)
        throw ValueError("display.up may not be the same as
display.forward");
      up = Vector(value);      

Dave