MS-Windows .bmp RLE4

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