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Bitmaps
Without graphics images, Windows-based applications would be pretty dull. Some applications depend on images for their usefulness, but any application can be spruced up with the addition of decorative clip art from a variety of sources. Windows bitmaps are arrays of bits mapped to display pixels. That might sound simple, but you have to learn a lot about bitmaps before you can use them to create professional applications for Windows. In the following sections, you'll learn how to create device-independent bitmaps (DIBs). By using DIBs, you'll have an easier time with colors and with the printer. In some cases, you'll get better performance. The Win32 function CreateDIBSection gives you the benefits of DIBs combined with all the features of GDI bitmaps.
You'll also learn how to use the MFC CBitmapButton class to put bitmaps on push buttons. (Using CBitmapButton to put bitmaps on pushbuttons has nothing to do with DIBs, but it's a useful technique that would be difficult to master without an example.)
GDI Bitmaps and Device-Independent Bitmaps
In this section, we'll spend more time looking at DIBs. The best place to check is the Platform SDK available through the MSDN help system. Windows has two kinds of bitmaps: GDI bitmaps and DIBs. GDI bitmaps have been around for quite a while, and you can find a great deal of information about them elsewhere. GDI bitmap objects are represented by the MFC library CBitmap class. The GDI bitmap object has an associated Windows data structure, maintained inside the Windows GDI module, that is device-dependent. Your program can get a copy of the bitmap data, but the bit arrangement depends on the display hardware. GDI bitmaps can be freely transferred among programs on a single computer, but because of their device dependency, transferring bitmaps by disk or modem doesn't make sense. A GDI bitmap is simply another GDI object, such as a pen or a font. You must somehow create a bitmap, and then you must select it into a device context. When you're finished with the object, you must deselect it and delete it. You know the drill. There's a catch, though, because the "bitmap" of the display or printer device is effectively the display surface or the printed page itself. Therefore, you can't select a bitmap into a display device context or a printer device context. You have to create a special memory device context for your bitmaps, using the CDC::CreateCompatibleDC function. You must then use the CDC member function StretchBlt or BitBlt to copy the bits from the memory device context to the "real" device context. These "bit-blitting" functions are generally called in your view class's OnDraw function. Of course, you mustn't forget to clean up the memory device context when you're finished.
In Win32, you're allowed to put a GDI bitmap handle on the Clipboard for transfer to another process, but behind the scenes Windows converts the device-dependent bitmap to a DIB and copies the DIB to shared memory. That's a good reason to consider using DIBs from the start.
DIBs offer many programming advantages over GDI bitmaps. Because a DIB carries its own color information, color palette management is easier. DIBs also make it easy to control gray shades when you print. Any computer running Windows can process DIBs, which are usually stored in BMP disk files or as a resource in your program's EXE or DLL file. The wallpaper background on your monitor is read from a BMP file when you start Windows. The primary storage format for Microsoft Paint is the BMP file, and Visual C++ .NET uses BMP files for toolbar buttons and other images. Other graphic interchange formats are available, such as TIFF, GIF, and JPEG, but only the DIB format is directly supported by the Win32 API.
Color Bitmaps and Monochrome Bitmaps
Windows deals with color bitmaps a little differently from the way it deals with brush colors. Many color bitmaps are 16-color. A standard VGA board has four contiguous color planes, with one corresponding bit from each plane combining to represent a pixel. The 4-bit color values are set when the bitmap is created. With a standard VGA board, bitmap colors are limited to the standard 16 colors. Windows does not use dithered colors in bitmaps. A monochrome bitmap has only one plane. Each pixel is represented by a single bit that is either off (0) or on (1). The CDC::SetTextColor function sets the "off" display color, and SetBkColor sets the "on" color. You can specify these pure colors individually with the Windows RGB macro.
