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Otherwise the TurboJPEG 3.2 API would allow callers compiled against TurboJPEG 3.1 and prior to set TJPARAM_SUBSAMP to a value that, from the caller's point of view, is undefined. tj3DecompressHeader() would also potentially set TJPARAM_SUBSAMP to a value that would overflow the caller's static tjMCUWidth[] and tjMCUHeight[] arrays. The trick here is to redefine tj3Init() in the TurboJPEG header as a macro that wraps a new undocumented function, tj3InitVersion(), and passes it an API version constant (TURBOJPEG_VERSION_NUMBER = 3002000) that is also defined in the header. (This is a similar trick to the one that the libjpeg v6a+ API uses for jpeg_create_*compress().) The old tj3Init() function is still exposed in the library, and it now calls tj3InitVersion(..., 3001000) internally. Based on the API version, the API then sets and enforces an internal limit on the number of chrominance subsampling options (TJ_NUMSAMP if the caller was compiled against TurboJPEG 3.2+ or 7 otherwise.) Thus, if an application is compiled against TurboJPEG 3.1-, it will still call the old tj3Init() function, which sets the API version to 3.1 and doesn't expose TJSAMP_410 or TJSAMP_24. If an application is compiled against TurboJPEG 3.2+, then it will call the new tj3InitVersion() function via the tj3Init() macro, which sets the API version to 3.2 and exposes TJSAMP_410 and TJSAMP_24. Ramifications: - Applications compiled against TurboJPEG 3.2+ will not be able to use the TurboJPEG 3.1- API library at run time, because the tj3InitVersion() function will not be available. - Applications compiled against TurboJPEG 3.1- will work the same regardless of whether they use the TurboJPEG 3.1- or 3.2+ API library at run time. - Applications that weren't specifically developed for TurboJPEG 3.2+ will not know about TJSAMP_410 or TJSAMP_24. Thus, when decompressing or transforming without transposition, such applications will gain the ability to obtain the iMCU width/height of a 4:1:0 or 2:4 JPEG image, which is useful when adjusting the cropping region. However, they will not gain any other abilities.