ROP Codes, Rubber Bands, Clip Regions '& Coordinate Transforms

Environment: VC6, 95/98/NT

Skill Level: Beginner, Intermediate  

This sample shows how to:

1. track the mouse using CWnd messages

  •     WM_LBUTTONDOWN -- use this message to start tracking the mouse motions, and save the starting point for drawing a rubber band rectangle
  •     WM_LBUTTONUP -- use this message to end mouse tracking and end drawing
  •     WM_MOUSEMOVE -- use this message to draw a rectangle and update the x-y coordinate readout

2. draw a "rubber band" rectangle

On occasion you will need to draw a selection rectangle to select a group of drawing objects. This example project shows one way to do this efficiently in the WM_MOUSEMOVE handler. As the rectangle is stretched with the mouse, the old rectangle is erased and new one drawn using a special ROP2 code for the device context.

3. use ROP2 drawing codes

There are several ROP2 device context drawing codes available, and you can experiment with each code by selecting from the Mode menu. The most interesting codes for rubber band rectangles are R2_NOT and R2_NOTXORPEN. Play with all of them to see the results of the logical operations combining new and existing pixels.

4. draw in logical units
5. reorient the coordinate system and logical origin

Occasionally you may want to plot data using you own coordinate system. The sample changes the device context mapping mode to a 1st quadrant cartesian coordinate system. It's a useful technique if you want to plot x-y graph data in user units. MM_ISOTROPIC is used so that the drawings aspect ratio is preserved as the window is resized.


void CRopFunView::SetDCProperties(CDC *pDC)
{
   CRect r;
   GetClientRect(r);

   pDC->SetMapMode(MM_ISOTROPIC );
   pDC->SetWindowExt( XEXTENTS_LOGICALUNITS, -YEXTENTS_LOGICALUNITS );
   pDC->SetViewportExt( r.Width(), r.Height() );
   pDC->SetViewportOrg( 0, r.Height() );
   pDC->SetWindowOrg( -XORIGIN_LOGICALUNITS, -YORIGIN_LOGICALUNITS );
} 

6. clip drawing outside a window region  

Some of the MFC classes and methods demonstrated are:

CDC:

  • SetMapMode
  • WindowExt
  • SetViewportExt
  • SetViewportOrg
  • SetWindowOrg
  • MoveTo
  • LineTo
  • Rectangle
  • SetBkMode
  • TextOut
  • SelectObject
  • DPtoLP
  • GetClipBox

CRgn:

  • CreateRectRgn
  • CreateRectRgnIndirect

Running the sample: Use the left mouse button to drag a rectangle inside the gray rectangle. Select different ROP2 codes from the menu.  

Download demo project - 21 Kb

Download source - 9 Kb