In the early days of desktop publishing - basically when [then] Aldus PageMaker was the only and [then] main player in DTP - the scanners that could capture images for use in desktop programs were few and far between.

Then, in 1986, several different manufacturers started either modifying existing scanners for the desktop market or creating new ones.

While the designers loved the ability to capture graphics, each scanner captured images in a slightly different way. The net result was, when the image would not import into PageMaker correctly, they called Aldus.

As you can imagine, Aldus soon got tired of this, so they partnered with Apple and Microsoft to develop a standard called TIFF (Tag Image File Format).

This format separated the image file into at least two parts: the header and the body of the data. The header consists of a series of numeric tags that identify specific pieces of information. For example, tag number 256 is always the width of the image.

Private Tags in TIFF
This greatly improved the situation with desktop scanners and the original developers of TIFF had the foresight to add private tags. These tags allow inclusion of any information that was missed in the original specification.

In order to use these private tags, a developer had to register them with Aldus (now with Adobe) to make sure no one else used the same tag.

Imagine one vendor using tag 256 for image width and another using it for image height - anarchy!

Enter TIFF/IT
Let's fast forward now to 1992 when DDAP developed their requirement statement for digital ad delivery. This was presented to CGATS (Committee for Graphic Arts Technology Standards) for development of an accredited file format standard for the delivery of digital ads.

When CGATS reviewed their alternatives, TIFF (then Version 6) seemed like the ideal candidate, except for the fact that it could not handle certain functionalities of the requirement.

In order to circumvent this issue, CGATS asked Aldus for a block of their own private tags in order to implement what eventually became TIFF/IT. One simple example of the functionality required was the ability to identify the sequence of the colors which is handled by tag 34017, the Color Sequence Tag.

For more information on this block of tags and their uses, you can contact NPES, secretariat for CGATS, at their website: http://www.NPES.org

Private tags in TIFF/IT
As the old saying goes, if you don't learn from history, you are bound to relive it. So the developers of TIFF/IT left the door open for developers to utilize private tags.

The DDAP verification application (TIFF/IT-P1 Checker) is designed to process the private tags that CGATS specified as part of the TIFF/IT specification, but will flag any other private tag as a warning because it does not "recognize" what these tags are to be used for.

If you get a warning about the private tag that you are processing, you should inform the supplier of the file that your verification software has detected this, in case it is an error on their part. You should also let the vendor of the software you are using to process the TIFF/IT files that the warning has appeared.

While the TIFF/IT specification is very precise regarding how these private tags should be treated (they should be parsed, but ignored), it is only fair to make your vendor aware that their equipment will be receiving this type of data.

TIFF/IT-P1 Private Tags: Q & A

Why do we need private tags?
In the development of the TIFF/IT -P1 specification, private tags were originally intended to provide developers with ways to add specific funtionality for specific applications - like a secret keys.

What if two companies decide to use the same private tag number, but assign different meanings to them?
All private tags must be registered with Adobe. This procedure has been used for many years now, dating back to when TIFF was maintained by Aldus and the system has worked quite well.