DRAGBMP Demonstrates Dragging Bitmaps on the Screen

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.

The following files are available for download from the Microsoft Download Center:


Dragbmp.exe (http://download.microsoft.com/download/platformsdk/demo2/3.1/w31/en-us/dragbmp.exe)

For additional information about how to download Microsoft Support files, click the following article number to view the article in the Microsoft Knowledge Base: Microsoft scanned this file for viruses. Microsoft used the most current virus-detection software that was available on the date that the file was posted. The file is stored on security-enhanced servers that help to prevent any unauthorized changes to the file.



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).

Once the information is stored, the image is ready to drag. You can select the image by clicking the left mouse button when the mouse pointer is on the image, drags the image by moving the mouse while pressing the mouse button. When the selection has been moved to the appropriate position the mouse button is deselected.

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. The basic steps to perform the bitmap dragging are as follows:

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 prcedding figure 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. There is a bitmap of what is underneath, but a portion of the domino 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. Save the new background bitmap and delete the old one.

The Drag.c file contains the code for dragging and hit-testing. The Mainwind.c file contains the code for processing the mouse and paint messages.

DRAGBMP was built and tested under Microsoft Windows version 3.1.