2000.03.28 19:54 "Extremely large images in TIFF", by Chris Hanson
Here's an idea that I had for supporting large raster images in a TIFF-like file format.
Step 1: Increment the magic number. What I'm suggesting is an incompatible change to the format, so it probably shouldn't use the same magic number as TIFF 6.0.
Step 2: After the magic number in the TIFF file header, add a two-byte field that tells readers the *type* of an offset field. This is the same sort of type enumeration used in an image file directory entry; the size of the given type is the size that should be used for the file's initial IFD offset, for subsequent IFD offsets, and for the Value/Offset field within an IFD. Some types (BYTE, ASCII, RATIONAL) should probably be illegal in this field.
Step 3: Define value types for signed and unsigned 64-bit quantities.
That's it! A TIFF-like file format that can handle 64-bit image sizes and file offsets and so on.
On the one hand, this complicates readers slightly because they can no longer rely on IFD entries being 12 bytes (they may be 10 bytes, or 16 bytes, or even larger in the future). On the other hand, it makes the file format much more extensible; small files can save a minor amount of space by using 16-bit offsets, while extremely large files can use 64-bit offsets.
On the other hand, existing TIFFs would be convertible to this new format very easily, and as long as it was written with extensibility in mind existing TIFF code should be readily adaptable to this new format.
Any suggestions or comments?
Chris Hanson | Disclaimer: I do not speak for Total
Software Engineer | Integration, Inc. These are my opinions.
Total Integration, Inc. | Treat them as such, please.