SAMPLE: Sample Code Uses PatBlt to Drag a Frame Rectangle

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.0 and 3.1

SUMMARY

The following information describes a procedure to smoothly drag the window frame; this procedure is similar to the one that Windows uses when a window is moved by dragging its caption bar. When the user moves a window with the mouse or with the keyboard, Windows shows the movement of the window by a moving a gray-patterned frame. The dragging looks smooth because Windows updates only the portions of the frame that need to be erased or drawn, leaving all other areas untouched.

One technique to implement smooth dragging for a single rectangle is described below. An actual frame in Windows consists of four thin rectangles dragged together.

DRAGRECT is a file in the Microsoft Software Library that demonstrates dragging a single rectangle and a frame.

Download DRAGRECT.EXE, a self-extracting file, from the Microsoft Software Library (MSL) on the following services:

• Microsoft Download Service (MSDL)

        Dial (206) 936-6735 to connect to MSDL
        Download DRAGRECT.EXE (size: 24507 bytes) 
  

• Internet (anonymous FTP)

        ftp ftp.microsoft.com
        Change to the \SOFTLIB\MSLFILES directory
        Get DRAGRECT.EXE (size: 24507 bytes) 
  

MORE INFORMATION

To smoothly drag a rectangle, redraw only the invalid portions of the rectangle. For example, if a rectangle is dragged up by 3 pixels, only the top 3 pixels need to be drawn and the bottom 3 pixels erased. All other parts of the rectangle are left alone. The same principle applies when a rectangle is moved on a diagonal--two edges are drawn, two edges are erased, and the bulk of the middle of the rectangle remains the same.

To implement smooth dragging of a single solid rectangle, perform the following eight steps:

1. When dragging begins, save the coordinates of the rectangle.

2. Draw the initial rectangle using a pattern brush and the PatBlt function. Windows uses its own gray brush made with a bitmap of alternating 0x55 and 0xAA values. In the PatBlt call, use the PATINVERT option to combine the pattern brush and the screen using the XOR operation. The XOR option is very convenient because another XOR operation restores the screen to its original state.

   Alternatively, use the DSTINVERT option, which performs a NOT operation on the destination rectangle, ignoring the current selected brush. A second NOT operation restores the screen to its original state. A NOT operation is slightly faster than an XOR operation; however, it is not nearly as flexible with respect to the nature of the rectangle drawn.

3. Each time the mouse is moved, save the new coordinates. The old coordinates must be retained for the time being.

4. Create an intersection rectangle of the old and new positions using the IntersectRect function. If this rectangle is empty, call the PatBlt function twice: to erase the old rectangle and to draw the new rectangle. This is allowed because the old and new positions have no overlapping region.

5. If the two rectangles intersect, update only the nonoverlapping regions--the edges of the old and new rectangles that are not in the intersection.

6. In most cases, the rectangle moves in both horizontal and vertical directions, which creates two regions that need updating:

         XXXXXXXXXXXX    The union of X, x, and . is the original
         XXXXXXXXXXXX       rectangle.
         x...........o   The union of O, o, and . is the new rectangle.
         x...........o
         x...........o   Erase the areas marked with an X or x.
         x...........o   Draw the areas marked with an O or o.
         x...........o
          OOOOOOOOOOOO
          OOOOOOOOOOOO
   
   

7. Each update region contains two smaller rectangles: one that makes up the horizontal section along the top (marked with uppercase characters in the diagram above) and one that makes up the vertical section along the side (marked with lowercase characters).

   Once the application determines the coordinates of these rectangles, call the PatBlt function to draw or erase them as appropriate. Do not draw overlapping areas twice; that is, when dealing with areas marked x or o, do not draw in the areas marked X or O, or vice versa.

   At this point, the rectangle has been moved without affecting the intersecting region (marked with periods) at all. Because the intersection was not erased or drawn, no flicker occurs and the dragging appears smooth.

8. When dragging stops (the mouse button is released), call the PatBlt function to erase the current rectangle because it is no longer needed.

The DRAGRECT sample program available in the Microsoft Software Library demonstrates the dragging process for both a single rectangle and a frame made of four separate rectangles. Note that the sample program is not optimized to drag a frame with the same efficiency as Windows; it is meant only to demonstrate the concepts involved.