mProjectPP performs a plane-to-plane transform on the input image, and is an adaptation of the Mopex algorithm and developed in collaboration with the Spitzer Space Telescope. It provides a speed increase of approximately a factor of 30 over the general-purpose mProject. However, mProjectPP is only suitable for projections which can be approximated by tangent-plane projections (TAN, SIN, ZEA, STG, ARC), and is therefore not suited for images covering large portions of the sky. Also note that it does not directly support changes in coordinate system (i.e. equatorial to galactic coordinates), though these changes can be facilitated by the use of an alternate header.
One situation that has happened often enough to warrant special note: It is possible to define a FITS header with the reference location off the image. In particular, people often reference cylindrical projections (e.g., CAR, CEA) to location (0.,0.) (e.g., the Galactic center). This is not wrong in itself but does lead to extremely large pixel offsets (e.g., in the hundreds of thousands). Related to this, if you extract a header from an image with large offsets of this type, it is unlikely that you can simply change projection types without also adjusting the reference location/offsets. Most likely, you will end up with the reproject data all being off-scale.
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