Designing Your Own Format

We find it hard to imagine why anyone would think that the world needs another graphics file format. And, in fact, we don't want to give the impression that we're encouraging such behavior. But given the fact that people can and will create new formats, we'd like to leave you with some pointers.

Why Even Consider It?

The truth is that this book does not even begin to include all of the hundreds of more obscure formats, some of which are used only privately and remain inside company walls. Companies wishing the output of their products to remain proprietary will always find a way to make it so and thus will continue to develop new formats.

Designing your own format also will help you avoid trouble should the use of someone else's format one day be restricted through legal action. The use of the GIF file format has recently come under licensing restrictions requiring that a royality fee be paid for software that reads or writes the GIF file format. Payment of this fee has been actively enforced through the threat of legal action both by both the owners of the GIF format and the owners of the Lempel-Ziv-Welch (LZW) compression algorithm used by GIF. Remember that even though many formats appear to be freely and publicly available, very few actually are.

Of course there are functional reasons for designing your own format. You may decide that an appropriate format doesn't yet exist, for instance, and thus feel compelled to create a new one. Reasoning leading to this decision is always suspect, however, and sending yet another format out into the world might even decrease your market share in this era of increasing interoperability and file sharing. The unfortunate reality is that file formats are usually created to support applications after the fact. In the modern world, marketing decisions and speculation about the future evolution of the supporting operating system and hardware platform play large parts in the development of program specifications from the very start. So we urge you to consider designing your application around a set of existing formats, or at least a format model.

But If You Must...

With that said, consider the following guidelines if you persist in designing your own:

Study everybody else's mistakes. No matter what you think, you're not smart enough to avoid them all, unless you see them first.
Plan for future revisions. Give plenty of room for future expansion of, and changes to, data. Avoid building limitations into your format that will one day force you to make kludges to support further revisions.
Keep it simple. The last thing the world needs is another "write-only" format. The object is to make it easy to read, not easy to write.
Document everything! Use consistent terminology that everyone understands and will continue to understand until the end of time. Number your documentation revisions with the format revisions; that way, it will be obvious if you "forget" to document a new feature.
Write the format before, not after, your application. Build the application around it. Don't make convenience revisions no matter what the provocation.
Avoid machine dependencies; at the same time, don't add complications designed to support portability.
Find some unambiguous means by which a reading application can identify the format.
Write a detailed file format specification and make it available. Do not discourage people who are interested in your format by refusing to supply them with information simply because they are not registered users of your product. The more widely distributed your information, the greater the potential acceptance of your format will be.
If your format is truly superior, market, market, market! Write software applications that use your format. Formats gain currency almost entirely through the marketing power of companies and their software. If your format is unique in the way it stores information, and you feel that it fills a niche that other formats don't, then advertise that fact.
Explicitly place the format in the public domain. If that is too threatening in the context of your company model, allow use with attribution. Do not discourage the spread of your format by including threats about copyright infringement and proprietary information in the specification. This only prevents your format from becoming widely accepted, and it alienates programmers who would otherwise be happy to further your company's plan of world domination for free.
Develop canonical test files and make them freely available. Mark them as such in the image, with your company name, the format type, color model, resolution, and pixel depth at the very least. They will be a good form of advertising for you and your company and are sure to be widely distributed with no effort on your part.

One Last Word

Remember that a lot of code is already out there and plenty of libraries are available in source form that may be able to supply your needs. Consider this statement from the FAQ (Frequently Asked Questions list) from the comp.graphics.misc newsgroup on the Internet:

Format documents for TIFF, IFF, BIFF, NFF, OFF, FITS, etc. You almost certainly don't need these. Read the section on free image manipulation software. Get one or more of these packages and look through them. Chances are excellent that the image converter you were going to write is already there.