SAMPLE: Sample Code Uses PatBlt to Drag a Frame Rectangle

ID: Q68569

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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.

MORE INFORMATION

The following file is available for download from the Microsoft Software Library: 

 ~ DRAGRECT.EXE (size: 24507 bytes)
For more information about downloading files from the Microsoft Software Library, please see the following article in the Microsoft Knowledge Base: 

   ARTICLE-ID: Q119591
   TITLE     : How to Obtain Microsoft Support Files from
               Online Services
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.
When a window is moved by dragging the caption bar, Windows divides the window's rectangle into four parts: the bottom, top, left, and right lines. Each line is a rectangle because the border of a window is usually thicker than one pixel. The dragging process above is applied to all of these rectangles at the same time.

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.

Additional query words: softlib DRAGRECT.EXE 

Keywords          : kbsample kb16bitonly kbWinOS310 kbWinOS300 GdiDrw 
Version           : 3.00 3.10
Platform          : WINDOWS

Last Reviewed: June 16, 1999