3.1.6.1 Bitmaps with 4 Bits per Pixel

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When the Compression field of a BitmapInfoHeader Object (section 2.2.2.3) is BI_RLE4, an RLE compression algorithm is used to compress a 4-bit bitmap. This format specifies encoded and absolute modes, and either mode can occur anywhere in a given bitmap.

Encoded mode involves two bytes. If the first byte of a pair is greater than zero, it specifies the number of consecutive pixels to be drawn using the two color indexes that are contained in the high-order and low-order bits of the second byte.

The first pixel is drawn using the color specified by the high-order 4 bits, the second is drawn using the color in the low-order 4 bits, the third is drawn using the color in the high-order 4 bits, and so on, until all the pixels specified by the first byte have been drawn.

If the first byte of a pair is zero and the second byte is 0x02 or less, the second byte is an escape value that can denote the end of a line, the end of the bitmap, or a relative pixel position, as follows.

Second byte value

Meaning

0x00

End of line

0x01

End of bitmap

0x02

Delta

When a delta is specified, the 2 bytes following the escape value contain unsigned values indicating the horizontal and vertical offsets of the next pixel relative to the current position.

In absolute mode, the first byte is zero, and the second byte is a value in the range 0x03 through 0xFF. The second byte contains the number of 4-bit color indexes that follow. Subsequent bytes contain color indexes in their high- and low-order 4 bits, one color index for each pixel. In absolute mode, each run is aligned on a word boundary.

The following example shows the hexadecimal contents 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 is interpreted as follows:

  • 03 04: Encoded mode, specifying 3 pixels with the values 0x0, 0x4, and 0x0.

  • 05 06: Encoded mode, specifying 5 pixels with the values 0x0, 0x6, 0x0, 0x6, and 0x0.

  • 00 06 45 56 67 00: Absolute mode, specifying 6 pixels with the values 0x4, 0x5, 0x5, 0x6, 0x6, and 0x7, padded to a word boundary.

  • 04 78: Encoded mode, specifying 4 pixels with the values 0x7, 0x8, 0x7, and 0x8.

  • 00 02 05 01: Encoded mode, specifying a new relative position 5 pixels to the right and one line down.

  • 04 78: Encoded mode, specifying 4 pixels with the values 0x7, 0x8, 0x7, and 0x8.

  • 00 00: Encoded mode, specifying the end of a line.

  • 09 1E: Encoded mode, specifying 9 pixels with the values 0x1, 0xE, 0x1, 0xE, 0x1, 0xE, 0x1, 0xE, and 0x1.

  • 00 01: Encoded mode, specifying the end of the bitmap.

Now, suppose the bitmap has a width of 32 pixels and a height of 4 lines, and pixel values that are not specified are 0x00 by default. The resulting expended bitmap would be as follows. 

  04 00 60 60 45 56 67 78 78 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 07 87 80 00 00
  1E 1E 1E 1E 10 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00