MS-Windows .bmp RLE4

In principle RLE4 coding is based on the same scheme as RLE8. The primary difference is the resolution of the colour index using 4 bit and a range of values between 0 and 15 (16 colours).

RLE4 coding scheme:

Within a run two colour indices are stored together in a byte value. If direct repititions of a single index appear, both the lower and the most significant 4 bits of the run value has to be identical. The run counter relates to the number of pixels and not to the entire byte.

  encoded   decoded
  01 0F     0
  02 0F     0 F
  03 0F     0 F 0
  04 0F     0 F 0 F
  04 00     0 0 0 0

As defined for 8 bit RLE, the coding in absolute mode has to be extended to word boundaries too:

  encoded             decoded
  00 03 12 30         1 2 3
  00 04 12 34         1 2 3 4
  00 05 12 34 50 00   1 2 3 4 5
  00 06 12 34 56 00   1 2 3 4 5 6
  00 07 12 34 56 70   1 2 3 4 5 6 7
  00 08 12 34 56 78   1 2 3 4 5 6 7 8

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