Editing ILBM (section)

==== Compression ====
If an image is compressed, each row of data (but not each bitplane) is compressed individually, including the mask data if present.  The compression is a variety of [[RLE Compression]] using flags.  It can be decoded as follows:<ref name="ilbm_spec"/>

* Loop until we have [Final length] bytes worth of data (final length calculated from image size.)
* While [Decompressed data length] < [Final length]:
*# Read a byte [Value]
*# If [Value] > 128, then:
*#* Read the next byte and output it (257 - [Value]) times.
*#* Move forward 2 bytes and return to step 1.
*# Else if [Value] < 128, then:
*#* Read and output the next [value + 1] bytes
*#* Move forward [Value + 2] bytes and return to step 1.
*# Else [Value] = 128, exit the loop (stop decompressing)

For the compression routine, it's best to encode a 2 byte repeat run as a replicate run except when preceded and followed by a literal run, in which case it is best to merge the three into one literal run. Always encode >3 byte repeats as replicate runs.<ref name="ilbm_spec"/>