SAMPLE: DRAGBMP Demonstrates Dragging Bitmaps on the Screen

• Microsoft Windows Software Development Kit (SDK) for Windows version 3.1

SUMMARY

DRAGBMP shows how to drag images smoothly on the screen. This is achieved through the use of several bitmaps and a series of BitBlt operations.

Before you can drag a bitmap, you must store the following information:

1. A bitmap of the desired image, such as the cards in Solitaire or the domino used in this sample.

2. A bitmap of what lies underneath the desired image. This information is necessary to redraw the underlying area when the image is dragged elsewhere.

3. The current position of the image (you can also store or retrieve the width and height of the image using GetObject() in the image's bitmap).

In this sample, the image is a yellow domino. Choose Draw Image! to draw the domino, then drag the domino using the mouse. Hit-testing in WM_LBUTTONDOWN determines when the domino is selected. Once selected, the dragging process begins. On each WM_MOUSEMOVE, a new background bitmap is created, the old background bitmap is restored, and the domino is moved to its new location. The WM_LBUTTONUP message ends the dragging process.

(x',y') +------------------------- new position

|\                       |
| \(dx,dy)               |
|  \                     |

|   .(x,y)               |            .
|   .                    |            .
|   .                    |            .
----.---------------------            .
    .                                 .
    .                                 .
    . . . . . . . . . . . . . . . . . .

1. Calculate the delta x and delta y in the mouse movement (dx,dy).

2. BitBlt() the screen at the new position onto a new bitmap. This corresponds to a rectangle with origin at (x',y') in the figure above, and becomes the new background bitmap.

3. Since the new background bitmap still contains part of the domino's image starting at point (x,y) in the figure above, BitBlt() the old background bitmap to that point on the new background bitmap, offsetting it by dx and dy. The resulting bitmap contains the correct background for the new position.

4. BitBlt() the domino's bitmap to the screen at its new location: origin at point (x',y') in the figure above.

5. The domino is now in the correct position on the screen and we have a bitmap of what is underneath, but a portion of the domino still appears at the lower right. To erase this portion, first BitBlt() the bitmap of the domino onto the old background's bitmap, offsetting it by -dx and -dy, and then BitBlt this modified old background bitmap to the screen at point (x,y). This erases the old portion of the domino without flashing.

6. Dragging is now complete. Store the new background bitmap and delete the old one.

DRAGBMP was built and tested under Microsoft Windows version 3.1.

Download DRAGBMP.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 DRAGBMP.EXE (size: 41906 bytes) 
  

• Internet (anonymous FTP)

        ftp ftp.microsoft.com
        Change to the \SOFTLIB\MSLFILES directory
        Get DRAGBMP.EXE (size: 41906 bytes)