[vtkusers] Coordinates on a surface

[James C. Robinson]

Hi All,

I have a very specific problem which I would like to solve.

I am using an MFC class derived window (CView) to view an unstructured grid
(3D FE mesh - 20 noded hexahedra converted to 8 noded). I wanted to have the
facility to output the coordinates of the point on the surface under the
mouse cursor as the mouse moves. I looked up the doc and saw the classes
derived from vtkAbstractPicker. I tried vtkWorldPointPicker, vtkPicker, and
vtkCellPicker. Here is some of the code I use in the WindowProc  of my
vtkMFCView derived CView:

Firstly, I try to output the coords when "p" is pressed (NB: I have
interrupted a lot of the default interactor functionality so that my program
conforms with an previous version) using the vtkInteractor picker:

        if (message == WM_CHAR) // Don't allow the toggle options for a
standard VTK window.
        {
           TCHAR chCharCode = (TCHAR) wParam;    // character code
// Look for "f" or "F" for the "fly" facility.
		   if ((chCharCode != 70)&&(chCharCode != 102))
// Look for "p" or "P" for the "pick" facility. (Hopefully to invoke the
vtkCellPicker
		   {   if ((chCharCode != 80)&&(chCharCode != 112))
			   {	return vtkMFCView::WindowProc(message, wParam, lParam);
		       } else
                         {	                  pPicker =
(vtkCellPicker*)(this->Interactor->GetPicker());
					wz = 0.0 ;
					wx = float(LOWORD(lParam));             /* Saves the current value
*/
					wy = float(HIWORD(lParam));

					pPicker->Pick(wx,wy,wz,Renderer) ;
					pPickPos = pPicker->GetPickPosition() ;
					coords.Format(" X:%7.2f   Y:%7.2f   Z:%7.2f",
								   *(pPickPos+0),*(pPickPos+1),*(pPickPos+2)) ;
					baseframe->p_wndStatusBar->SetPaneText(1,coords) ;
		       }
		   }
		}

I also try to do it dynamically as the mouse moves across the window using a
vtkCellPicker that I instantiate as part of my vtkMFCView derived CView:

                   if (message == WM_MOUSEMOVE)
		{ if (Panning||Zooming||Rotating)
		      { GetDocument()->ViewAltered() ;// Default title if view changes
                   } else
		  {
		    wz = 0.0 ;
                       wx = float(LOWORD(lParam));             /* Saves the
current value      */
                       wy = float(HIWORD(lParam));

		    pCellPicker->Pick(wx,wy,wz,Renderer) ;
		    pPickPos = pCellPicker->GetPickPosition() ;

                       coords.Format(" x:%7.2f   y:%7.2f   z:%7.2f",
			               *(pPickPos+0),*(pPickPos+1),*(pPickPos+2)) ;
                       baseframe->p_wndStatusBar->SetPaneText(1,coords) ;

		  }
		}

The behaviour of the output from these procedures is slightly different, but
both erroneous. (Almost) invariably the "x" coordinate outputted is correct.
However, the y coordinate is often incorrect and the orientation of the Y
axis is invariably inverted. The "z" coordinate is inconsistent. The quality
of the outputted coordinates seems to depend on the level of zoom and the
camera orientation. NB: I have set the Tolerance to 0.0001 for my own
vtkCellPicker, but I get a crash when I try to set the tolerance of the
vtkInteractor's picker.

I would appreciate any help. For this functionality to be useful it must be
consistently accurate.

Many thanks in advance,

Jim

______________________

James C. Robinson, PhD,
Chartered Engineer,
Kepler Simulation Systems Ltd.,
Unit 10,
Melbourne Business Park,
Model Farm Road,
Cork,
Eire

Tel:         +353-21-4822028
Tel:         +353-21-4817267
Tel:         +353-87-2393010
Fax:        +353-21-4822721
E-mail:     j.robinson at kepler-systems.com
______________________