Message-ID: AIB.HDY
To: tiff@olympiakos.com
Subject: Extremely large images in TIFF
From: chanson@totalint.com (Chris Hanson)
Date: Tue, 28 Mar 2000 13:54:02 -0600
Sender: owner-tiff@olympiakos.com


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

--
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.
chanson@totalint.com     |

