The PM file format is an obsolete Mesh file format supported by Auran Jet. It provided support for an unusual form of LOD that was designed for high efficiency and flexibility on the early GPU hardware in existence at the time of development. Meshes could scale over a wide range of polygon counts by following a simple pre-defined triangle splitting/merging sequence. Unfortunately it never entirely lived up to its promise of automatic LOD generation or fully scalable scene detail, offering worse visual quality than hand-created LODs.
In the subsequent period, the format suffered a number of additional blows:
- Hardware optimisation requirements changed such that the Progressive Mesh technique was no longer particularly optimal.
- 3D modelling tools began to offer high-quality automatic LOD mesh generation which exceeded the capabilities of the PM file format.
- Users began to demand a consistent visual quality which was beyond that offered by the Progressive Mesh technique.
Full support for PM files within Trainz was dropped in favour of the LM.txt file technique. Trainz now ignores the progressive mesh data, and simply loads PM files as if they were a standard IM file with a single (maximum) level of detail.