1997.03.04 01:38 "Questions on proposed replacement TIFF/JPEG Spec.", by Brian McDougle
I recently downloaded the README file located in the graphics/tiff section of the ftp site sgi.com and had some questions regarding the proposed replacement for the TIFF 6.0 TIFF/JPEG Specification.
You mention in the 'Overview of the solution' section that the JPEGTables field can be stored only once for a multi-segment TIFF file. First, is this 'field' also called a TIFF Tag? Second, am I correct that when you refer to a multi-segment TIFF file you are referring to a TIFF file with multiple images (i.e. a multipage TIFF file), or is "multi-segment" in reference to the JPEG compression? Third, if a multipage TIFF file is generated will each image that uses this proposed compression scheme require a separate JPEGTables field?
My concern is that this field is a TIFF tag and each page of a multipage TIFF file may contain a JPEGTables tag that references the same offset within the TIFF file. This use of "duplicate pointers" is prohibited by the TIFF specification as defined in Section 7 (Additional Baseline TIFF Requirements) of the TIFF 6.0 specification. If this is not the case then please forgive my misunderstanding.
However, while it is easy in most file systems to "seek" back to a position in a file, there are circumstances when this is not possible. In particular, I used to work for Xionics Document Technologies who developed a hardware product called XipPrint. This is an add-on for HP LaserJet printers that takes a compressed TIFF file and decompresses it within the printer and sends the uncompressed data to the printer engine. The latest version, XipPrint II, sits in the SIMM socket (like a postscript module) and converts the TIFF file to PCL data and inserts the PCL data back into the data stream.
XipPrint can print multipage TIFF files by processing each image separately. However, it usually has to free up previously used memory to receive and process another page of the file. If all the pages had a tag that referenced the same offset in the file and this part of the file is no longer in memory XipPrint cannot process the image and must skip to the next image.
Theoretically, though, given knowledge that this type of tag could appear only once in a file, XipPrint could allocate a block of memory that is not freed to hold the JPEGTables tag data. However, this leaves less data for TIFF or image data.
Thank you in advance for your clarifications.
Product Engineer, Imaging Products
ISV Support Group
Canon Information Systems, Inc.
3187 Red Hill Avenue
Costa Mesa, CA 92626 USA
Ph: (714) 438-7185 Fax: (714) 437-7272