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BITMAPINFOHEADER info  Overview  Group

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

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; 

 

Members

biSize
Specifies the number of bytes required by the structure.
biWidth
Specifies the width of the bitmap, in pixels.
biHeight
Specifies the height of the bitmap, in pixels. If biHeight is positive, the bitmap is a bottom-up DIB and its origin is the lower left corner. If biHeight is negative, the bitmap is a top-down DIB and its origin is the upper left corner.
biPlanes
Specifies the number of planes for the target device. This value must be set to 1.
biBitCount
Specifies the number of bits per pixel. This value must be 1, 4, 8, 16, 24, or 32.
biCompression
Specifies the type of compression for a compressed bottom-up bitmap (top-down DIBs cannot be compressed). It can be one of the following values:

Value

Description

BI_RGB

An uncompressed format.

BI_RLE8

A run-length encoded (RLE) format for bitmaps with 8 bits per pixel. The compression format is a two-byte format consisting of a count byte followed by a byte containing a color index. For more information, see the following Remarks section.

BI_RLE4

An RLE format for bitmaps with 4 bits per pixel. The compression format is a two-byte format consisting of a count byte followed by two word-length color indices. For more information, see the following Remarks section.

BI_BITFIELDS

Specifies that the bitmap is not compressed and that the color table consists of three doubleword color masks that specify the red, green, and blue components, respectively, of each pixel. This is valid when used with 16- and 32-bits-per-pixel bitmaps.

biSizeImage
Specifies the size, in bytes, of the image. This may be set to 0 for BI_RGB bitmaps.
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 indices in the color table that are 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 the compression mode specified by biCompression.

If biClrUsed is nonzero and the biBitCount member is less than 16, the biClrUsed member specifies the actual number of colors the graphics engine or device driver accesses. If biBitCount is 16 or greater, then biClrUsed member specifies the size of the color table used to optimize performance of Windows color palettes. If biBitCount equals 16 or 32, the optimal color palette starts immediately following the three doubleword masks.

If the bitmap is a packed bitmap (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 either 0 or the actual size of the color table.

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

Remarks

The BITMAPINFO structure combines the BITMAPINFOHEADER structure and a color table to provide a complete definition of the dimensions and colors of a DIB. For more information about DIBs, see the description of the BITMAPINFO data 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 eight or four bits per pixel. Compression reduces the disk and memory storage required for the bitmap. The following paragraphs describe these formats.

When the biCompression member is BI_RLE8, the bitmap is compressed by using a run-length encoding (RLE) format for an 8-bit bitmap. This format can be compressed in encoded or absolute modes. Both modes can occur anywhere in the same bitmap.

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 

 

When the biCompression member is BI_RLE4, the bitmap is compressed by using a run-length encoding format for a 4-bit bitmap, which also uses encoded and absolute modes:

If biHeight is negative, indicating a top-down DIB, biCompression must be either BI_RGB or BI_BITFIELDS. Top-down DIBs cannot be compressed.

See Also

BITMAPINFO 

See:


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