A sequence of identical bytes (Replicate Run) is encoded as a two-byte code:
< -count + 1 > <byte value>, where
count = the number of bytes in the run, and
2 <= count <= 128
and a non-repetitive sequence of bytes (Literal Run) is encoded as:
< count - 1 > <Iiteral sequence of bytes>, where
count = number of bytes in the sequence, and
1 <= count <= 128.
The value of -128 may not be used to prefix a byte value.
Note: It is common to encode a 2-byte repeat run as a Replicate Run except when preceded and followed by a Literal Run, in which case it's best to merge the three runs into a Literal Run.
Three-byte repeats shall be encoded as Replicate Runs. Each row of the image shall be encoded separately and not cross a row boundary.