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BI_RLE4 rgb pixel format

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The following definition and example are quoted from the Windows 3.1 API online help file.

"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 u_int16 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"

Duplicate formats: RLE

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