Bug in project() function? [GFXD3DDevice::project()

void GFXD3DDevice::project(   Point3F &outPoint,
                              Point3F &inPoint,
                              MatrixF &modelview,
                              MatrixF &projection,
                              RectI &viewport )
{
   D3DVIEWPORT9 D3Dviewport;
   D3Dviewport.X = viewport.point.x;
   D3Dviewport.Y = viewport.point.y;

   //[b]DOESN'T[/b] work; original code
   D3Dviewport.Width = viewport.extent.x  - viewport.point.x;
   D3Dviewport.Height = viewport.extent.y - viewport.point.y;

   //[b]DOES[/b] work; commented-out code
   D3Dviewport.Width = viewport.extent.x; [b]// - viewport.point.x;[/b] [b]// isn't 'extent' the width/height[/b]

   D3Dviewport.Height = viewport.extent.y; [b]//- viewport.point.y;[/b]


   D3Dviewport.MinZ = 0.0;
   D3Dviewport.MaxZ = 1.0;

   //...

I found that my bitmaps were not getting plotted because this function is setting the width & height incorrectly & returning very 'wrong' numbers. I'm just calling project() with world coordinates of (a shapeBase*) shape->getPosition().

For example, if I have a TS control on an 800x600 screen at (622,0) with an extent of (178, 178), the width would be set as 178 - 622, or -444. I commented out the subtraction and simply set the width & height as the extent parameters and everything worked properly. Is this a bug or am I not using the function correctly?

Edit: of course if the position of the control is @ (0,0), it'll work fine...is that it? project() expects the control to be positioned at (0,0)? If so, there would be no need to have the viewport.point.x subtraction.