2005.12.22 22:46 "Re: [Tiff] filtering TIFF tags", by Bob Friesenhahn
It should be pretty easy to make the code that reads custom (ie. generic) tags do so in a lazy fashion. But it could be pretty hard for the tags that are loaded directly into the tiff directory structure as other code then just assumes they can be accessed at will.
Maybe memory-mapping can help? With memory mapped files there is no hit until the data is accessed. Unfortunately, memory mapping places more expectation on the execution environment.
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/