BITMAPINFOHEADER

3.0 

typedef struct tagBITMAPINFOHEADER {    /* bmih */
    DWORD   biSize;
    LONG    biWidth;
    LONG    biHeight;
    WORD    biPlanes;
    WORD    biBitCount;
    DWORD   biCompression;
    DWORD   biSizeImage;
    LONG    biXPelsPerMeter;
    LONG    biYPelsPerMeter;
    DWORD   biClrUsed;
    DWORD   biClrImportant;
} BITMAPINFOHEADER;

The BITMAPINFOHEADER structure contains information about the dimensions and color format of a Windows 3.0 or later device-independent bitmap (DIB).

Members

biSize

Specifies the number of bytes required by the BITMAPINFOHEADER structure.

biWidth

Specifies the width of the bitmap, in pixels.

biHeight

Specifies the height of the bitmap, in pixels.

biPlanes

Specifies the number of planes for the target device. This member must be set to 1.

biBitCount

Specifies the number of bits per pixel. This value must be 1, 4, 8, or 24.

biCompression

Specifies the type of compression for a compressed bitmap. It can be one of the following values:

Value Meaning

BI_RGB Specifies that the bitmap is not compressed.
BI_RLE8 Specifies a run-length encoded format for bitmaps with 8 bits per pixel. The compression format is a 2-byte format consisting of a count byte followed by a byte containing a color index. For more information, see the following Comments section.
BI_RLE4 Specifies a run-length encoded format for bitmaps with 4 bits per pixel. The compression format is a 2-byte format consisting of a count byte followed by two word-length color indexes. For more information, see the following Comments section.

biSizeImage

Specifies the size, in bytes, of the image. It is valid to set this member to zero if the bitmap is in the BI_RGB format.

biXPelsPerMeter

Specifies the horizontal resolution, in pixels per meter, of the target device for the bitmap. An application can use this value to select a bitmap from a resource group that best matches the characteristics of the current device.

biYPelsPerMeter

Specifies the vertical resolution, in pixels per meter, of the target device for the bitmap.

biClrUsed

Specifies the number of color indexes in the color table actually used by the bitmap. If this value is zero, the bitmap uses the maximum number of colors corresponding to the value of the biBitCount member. For more information on the maximum sizes of the color table, see the description of the BITMAPINFO structure earlier in this chapter.

If the biClrUsed member is nonzero, it specifies the actual number of colors that the graphics engine or device driver will access if the biBitCount member is less than 24. If biBitCount is set to 24, biClrUsed specifies the size of the reference color table used to optimize performance of Windows color palettes.

If the bitmap is a packed bitmap (that is, a bitmap in which the bitmap array immediately follows the BITMAPINFO header and which is referenced by a single pointer), the biClrUsed member must be set to zero or to the actual size of the color table.

biClrImportant

Specifies the number of color indexes that are considered important for displaying the bitmap. If this value is zero, all colors are important.

Comments

The BITMAPINFO structure combines the BITMAPINFOHEADER structure and a color table to provide a complete definition of the dimensions and colors of a Windows 3.0 or later DIB. For more information about specifying a Windows 3.0 DIB, see the description of the BITMAPINFO structure.

An application should use the information stored in the biSize member to locate the color table in a BITMAPINFO structure as follows:

pColor = ((LPSTR) pBitmapInfo + (WORD) (pBitmapInfo->bmiHeader.biSize))

Windows supports formats for compressing bitmaps that define their colors with 8 bits per pixel and with 4 bits per pixel. Compression reduces the disk and memory storage required for the bitmap. The following paragraphs describe these formats.

BI_RLE8

When the biCompression member is set to BI_RLE8, the bitmap is compressed using a run-length encoding format for an 8-bit bitmap. This format may be compressed in either of two modes: encoded and absolute. Both modes can occur anywhere throughout a single bitmap.

Encoded mode consists of two bytes: the first byte specifies the number of consecutive pixels to be drawn using the color index contained in the second byte. In addition, the first byte of the pair can be set to zero to indicate an escape that denotes an end of line, end of bitmap, or a delta. The interpretation of the escape depends on the value of the second byte of the pair. The following list shows the meaning of the second byte:

Value Meaning

0 End of line.
1 End of bitmap.
2 Delta. The two bytes following the escape contain unsigned values indicating the horizontal and vertical offset of the next pixel from the current position.

Absolute mode is signaled by the first byte set to zero and the second byte set to a value between 0x03 and 0xFF. In absolute mode, the second byte represents the number of bytes that follow, each of which contains the color index of a single pixel. When the second byte is set to 2 or less, the escape has the same meaning as in encoded mode. In absolute mode, each run must be aligned on a word boundary.

The following example shows the hexadecimal values of an 8-bit compressed bitmap:

03 04 05 06 00 03 45 56 67 00 02 78 00 02 05 01
02 78 00 00 09 1E 00 01

This bitmap would expand as follows (two-digit values represent a color index for a single pixel):

04 04 04
06 06 06 06 06
45 56 67
78 78
move current position 5 right and 1 down
78 78
end of line
1E 1E 1E 1E 1E 1E 1E 1E 1E
end of RLE bitmap

BI_RLE4

When the biCompression member is set to BI_RLE4, the bitmap is compressed using a run-length encoding (RLE) format for a 4-bit bitmap, which also uses encoded and absolute modes. In encoded mode, the first byte of the pair contains the number of pixels to be drawn using the color indexes in the second byte. The second byte contains two color indexes, one in its high-order nibble (that is, its low-order four bits) and one in its low-order nibble. The first of the pixels is drawn using the color specified by the high-order nibble, the second is drawn using the color in the low-order nibble, the third is drawn with the color in the high-order nibble, and so on, until all the pixels specified by the first byte have been drawn.

In absolute mode, the first byte contains zero, the second byte contains the number of color indexes that follow, and subsequent bytes contain color indexes in their high- and low-order nibbles, one color index for each pixel. In absolute mode, each run must be aligned on a word boundary. The end-of-line, end-of-bitmap, and delta escapes also apply to BI_RLE4.

The following example shows the hexadecimal values of a 4-bit compressed bitmap:

03 04 05 06 00 06 45 56 67 00 04 78 00 02 05 01
04 78 00 00 09 1E 00 01

This bitmap would expand as follows (single-digit values represent a color index for a single pixel):

0 4 0
0 6 0 6 0
4 5 5 6 6 7
7 8 7 8
move current position 5 right and 1 down
7 8 7 8
end of line
1 E 1 E 1 E 1 E 1
end of RLE bitmap

See Also

BITMAPINFO