Capturing the screen (or a specific window) into a DIBWINCAP: Captures Screens Using DIB API
WINCAP demonstrates how to capture, save, and print the entire screen, portions of the screen, or specific windows. The program also defines routines that accomplish common device-independent bitmap (DIB) functions; these routines form the DIB application programming interface (API). WINCAP uses the DIB API functions for most of the screen capture, printing, and saving work. See the DIBAPI.TXT file for a description of the DIB API functions.
WINCAP illustrates the following techniques:
Capturing the screen (or a specific window) into a DIB
Capturing the screen to a device-dependent bitmap (DDB)
Printing a DIB using banding
Loading and saving a DIB to a disk file (.BMP file)
Converting between DIBs and DDBs
Displaying the captured screen DDB
All of these techniques call the DIB API. WINCAP includes the source code for these functions. You can call these functions easily from a different application by simply compiling the enclosed DIBAPI.DLL, and linking with the DIBAPI.LIB import library.
The preferred way to capture a screen in Microsoft(R) Windows(TM) is to copy the screen pixels into a DIB, and then to use this DIB in subsequent operations (for example, to save the bitmap to a file or print the bitmap).
If you do not use a DIB in the intermediate step, the results of the screen print may be less than desirable. If you use a DIB to hold the screen image, device-dependent information is removed from the bitmap. This simplifies the process of displaying the image on devices with different display capabilities. For instance, capturing a screen from a 24-bit display adapter and printing it on a 1-bit (monochrome) printer can produce exceptional results if DIBs are used. Additionally, many printer drivers implement gray-scale dithering; the output on these printers is also excellent when DIBs are used.
NOTE: Because bitmaps are device-dependent, you should avoid the BitBlt function when printing bitmaps. The type of bitmaps that the BitBlt function requires are normally in the format of the display driver rather than the printer driver. Depending on the drivers involved, the results of using BitBlt to print a bitmap can vary from extremely poor output quality to no output at all.
Although all printer drivers are able to BitBlt a monochrome bitmap to the printer, this technique generally produces poor results because the printer cannot apply gray-scaling to the image.
This technique of using a DIB to convert a bitmap between display devices with different capabilities can also be used to convert bitmaps while preserving the original color information (for example, loading a 256-color bitmap from a .BMP file and printing it to a 3-color printer or displaying it on a 24-bit display).
The file DIBAPI.TXT contains more information on function parameters and usage. A Windows Help file for the DIB APIs is also included.
WINCAP was built and tested under Microsoft Windows version 3.1.
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