11.2 The structures included to bitmap

At the beginning of a DIB file is the BITMAPFILEHEADER structure, which is defined by Windows as shown in Listing 11.1.

Listing 11.1 The BITMAPFILEHEADER Structure

typedef struct tagBITMAPFILEHEADER {

WORD bfType;

DWORD bfSize;

WORD bfReserved1;

WORD bfReserved2;

DWORD bfOffBits;

} BITMAPFILEHEADER;

Although this structure resides at the beginning of a DIB disk file, it need not be part of the DIB in memory. The first structure member, bfType, identifies the file as a DIB and should be the ASCII codes of the letters BM. In hex, the bfType word should be 4D42. Otherwise, the file is probably not a DIB. The second member, bfSize, is supposed to be the size of the DIB file in bytes. However, due to a mistake in the original Windows documentation, bfSize is not reliable and should be ignored. On the other hand, you can count on the bfOffBits member to contain the number of bytes from the start of the DIB file to the bitmap data. The BITMAPFILEHEADER structure is summarized in Table 3.1.

Table 11.1 The BITMAPFILEHEADER Structure

Member	Type	Description
bfType	WORD	Contains the ASCII values BM
bfSize	DWORD	The size of the file
bfReserved1	WORD	Always 0
bfReserved2	WORD	Always 0
bfOffBits	DWORD	The number of bytes from the beginning of the file to the bitmap

Following the BITMAPFILEHEADER structure is the BITMAPINFO structure, which is defined by Windows as shown in Listing 11.2.

Listing 11.2 The BITMAPINFO Structure

typedef struct tagBITMAPINFO {

BITMAPINFOHEADER bmiHeader;

RGBQUAD bmiColors[1];

} BITMAPINFO;

As you can see, this structure is made up of a header, represented by the BITMAPINFOHEADER structure and a color table, represented by an array of RGBQUAD structures.

A BITMAPINFOHEADER structure, also defined by Windows, looks like Listing 11.3.

Listing 11.3 The BITMAPINFOHEADER Structure

typedef struct tagBITMAPINFOHEADER {

DWORD biSize;

DWORD biWidth;

DWORD biHeight;

WORD biPlanes;

WORD biBitCount;

DWORD biCompression;

DWORD biSizeImage;

DWORD biXPelsPerMeter;

DWORD biYPelsPerMeter;

DWORD biClrUsed;

DWORD biClrImportant;

} BITMAPINFOHEADER;

The member biSize contains the size of the BITMAPINFOHEADER structure, which should be 40 bytes. The members biWidth and biHeight are the width and height of the bitmap in pixels. The member biPlanes is always set to 1. The biBitCount member, which indicates the number of bits per pixel, can be 1, 4, 8, or 24, which indicate monochrome, 16-color, 256-color, and 16.7 million color images. (In this book, you mostly use 256-color, or 8-bit, images.)

The biCompression member indicates the type of compression used with the bitmap image, where a 0 indicates no compression; a 1 indicates RLE-8 compression; and a 2 indicates RLE-4 compression. If you're not familiar with data compression techniques, don't worry about it. DIBs are rarely compressed. You'll usually find a 0 in the biCompression structure member.

The biSizeImage member is the size of the bitmap in bytes and is usually used only with compressed bitmaps. This value takes into account that the number of bytes in each row of a bitmap is always a multiple of 4. The rows are padded with blank bytes, when necessary, to ensure a multiple of 4. However, unless you're writing a program that creates DIBs, you don't need to deal with row padding and the code complications that arise from it.

The biXPelsPerMeter and biYPelsPerMeter members contain the horizontal and vertical number of pixels per meter of the intended display device, but are usually just set to 0. The biClrUsed and biClrImportant members, which contain the total number of colors used in the bitmap and the number of important colors in the bitmap, are also usually set to 0.

You might have noticed that BITMAPINFOHEADER structure members after biBitCount are likely to contain a 0, so after reading the structure from the disk file, you'll probably ignore the values stored in these structure members. In this chapter, you'll see how you can calculate any values you need such as the number of colors used in the image and store those values in the proper members for later retrieval. The BITMAPINFOHEADER structure is summarized in Table 11.2.

Table 11.2 The BITMAPINFOHEADER Structure

Member	Type	Description
biSize	DWORD	Size in bytes of this
		structure
biWidth	DWORD	Bitmap's width in pixels
biHeight	DWORD	Bitmap's height in pixels
biPlanes	WORD	Always 1
biBitCount	WORD	Number of bits per pixel
biCompression	DWORD	Compression type: 0 = None, 1= RLE-8, 2 = RLE-4
biSizeImage	DWORD	Bitmap's size in bytes
biXPelsPerMeter	DWORD	Horizontal pixels per meter
biYPelsPerMeter	DWORD	Vertical pixels per meter
biClrUsed	DWORD	Number of colors used
biClrImportant	DWORD	Number of important colors

The final data structure, RGBQUAD, is defined by Windows as shown in Listing 11.4.

Listing 11.4 The RGBQUAD Structure

typedef struct tagRGBQUAD {

BYTE rgbBlue;

BYTE rgbGreen;

BYTE rgbRed;

BYTE rgbReserved;

} RGBQUAD;

This structure simply contains the intensities of a color's red, green, and blue elements. Each color in a DIB is represented by an RGBQUAD structure. That is, a 16-color (4-bit) bitmap has a color table made up of 16 RGBQUAD structures, whereas a 256-color (8-bit) bitmap has a color table containing 256 RGBQUAD structures. The exception is 24-bit color images, which have no color table.

Following a DIB's BITMAPINFOHEADER structure is the bitmap's actual image data. The size of this data depends, of course, on the size of the image.