Graphics File Formats(Windows的.BMP .CUR .ICO文件的格式，有范例)

4 bitmap has a maximum of 16 colors. each pixel in the bitmap is
represented by a 4-bit index into the color table. for example, if the first
byte in the bitmap is 0x1f, the byte represents two pixels. the first pixel
contains the color in the second table entry, and the second pixel contains
the color in the sixteenth table entry.

8 bitmap has a maximum of 256 colors. each pixel in the bitmap is
represented by a 1-byte index into the color table. for example, if the first
byte in the bitmap is 0x1f, the first pixel has the color of the
thirty-second table entry.

24 bitmap has a maximum of 2^24 colors. the bmicolors (or bmcicolors)
member is null, and each 3-byte sequence in the bitmap array represents the
relative intensities of red, green, and blue, respectively, for a pixel.

the biclrused member of the bitmapinfoheader structure specifies the number
of color indexes in the color table actually used by the bitmap. if the
biclrused member is set to zero, the bitmap uses the maximum number of colors
corresponding to the value of the bibitcount member. an alternative form of
bitmap file uses the bitmapcoreinfo, bitmapcoreheader, and rgbtriple
structures.

bitmap compression

windows versions 3.0 and later support run-length encoded (rle) formats for
compressing bitmaps that use 4 bits per pixel and 8 bits per pixel.
compression reduces the disk and memory storage required for a bitmap.

compression of 8-bits-per-pixel bitmaps

when the bicompression member of the bitmapinfoheader structure is set to
bi_rle8, the dib is compressed using a run-length encoded format for a
256-color bitmap. this format uses two modes: encoded mode and absolute mode.
both modes can occur anywhere throughout a single bitmap.

encoded mode

a unit of information in 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. the first byte of the pair can be set to zero
to indicate an escape that denotes the end of a line, the end of the bitmap,
or a delta. the interpretation of the escape depends on the value of the
second byte of the pair, which must be in the range 0x00 through 0x02.
following are the meanings of the escape values that can be used in the
second byte:

second byte 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.

absolute mode

absolute mode is signaled by the first byte in the pair being set to zero and
the second byte to a value between 0x03 and 0xff. the second byte represents
the number of bytes that follow, each of which contains the color index of a
single pixel. each run must be aligned on a word boundary. following is an
example of an 8-bit rle bitmap (the two-digit hexadecimal values in the
second column represent a color index for a single pixel):

compressed data expanded data

03 04 04 04 04
05 06 06 06 06 06 06
00 03 45 56 67 00 45 56 67
02 78 78 78
00 02 05 01 move 5 right and 1 down

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