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;
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. |
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.
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.
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 offsets of the next pixel from the current position. |
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:
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
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:
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