PNG: The Definitive Guide

Chapter 15. Writing PNG Images

Writing PNG images is both simpler and more complex than reading them. Weighing in on the side of simplicity is the fact that there is no need for a lot of platform-specific code, particularly platform-specific graphical code--unless, of course, the application already is graphical. In general, there is also no need for a special progressive mode; writing a PNG file, or almost any image format, for that matter, is more or less progressive by nature, although some complexity creeps in when the image is interlaced.

Writing PNGs is more explicitly complex when it comes to dealing with ancillary information like text annotations, timestamps, and so forth. A simple PNG viewer can ignore all of that; its only concern is with displaying the pixels correctly and in a timely manner. But a PNG-writing application should be prepared to preserve any existing textual information and to give the user the option of adding new information--for example, a title, the author's name, and copyright information. One wants to avoid adding too much baggage to the image, but the user should also be given the option of adding a timestamp (e.g., the tIME chunk for time of last modification, or perhaps a tEXt chunk indicating the creation time).

When it comes to the actual image data, at a minimum, the application should be able to detect when there are no more than 256 colors or color-transparency pairs, including a possible background color, and write a palette-based image ...