Isofs transparent compression
Mkisofs transparent compression is what we're calling a certain extension of ISO 9660 format that supports per-file compression. It is designed as an extension of Rock Ridge, characterized by the signature "
ZZ". It is based on gzip format.
The only known software to support it are old versions of mkisofs/cdrtools, and patches that add support to the Linux kernel. It was apparently made obsolete by zisofs in 2001. There might be multiple incompatible versions of it.
Transparent compression was new in mkisofs 1.1 (a.k.a. 1.01) (1994). Around 2001, mkisofs was incorporated into the cdrtools software. The feature was present in cdrtools 1.10 (2001). It was removed and replaced with zisofs support in cdrtools 2.0 (2002).