does the DNG licensing formula allows for extensions to the format ? I'm currently investigating the possibility to add pyramid images and georeferencing to what is, basically, a TIFF offspring.
It is however not clear to me how proprietary or 3rdparty extensions to the format are valid from a legal viewpoint, if they build upon the existing specification (18.104.22.168 - I guess that would make any extended DNG a kind of 22.214.171.124)
N.B. I did note recently the appearance 'in the wild' of a bunch of non-compliant DNG files, which 1.3-compatible DNG software cannot open - so I guess this must be possible to some extent...
It is certainly possible to store extra images in other IFDs in a DNG/TIFF, without officially extending the spec. These could be considered preview/optional images, in additional to the (required) main image.
As an example, starting with recent versions of ACR/LR/DNG Converter, the Adobe tools can embed a low-res preview image that we call Fast Load Data. It is effectively a cache image that is used for fast initial preview purposes, building thumbnails, etc. It is completely optional, not required, and existing DNG readers are free to ignore it. (The main image data is unchanged.)