PNG: The Definitive Guide

Chapter 14. Reading PNG Images Progressively

As I noted in Chapter 13, “Reading PNG Images”, the basic style of PNG viewer that reads each image from a file in a single gulp is appropriate to some applications, but not all. In particular, web browsers and the like tend to read images from a network, and they often download more than one image at the same time. It is usually desirable for them to display whatever is available at regular intervals so the user can get some idea of the contents of the page as quickly as possible. The alternative--waiting the minute or more that some web pages take to download--went out of style almost as soon as Netscape Navigator became available late in 1994.

This style of display is known as progressive, and as one might imagine, it places strong constraints on the structure of the program. In fact, in many ways a progressive reader is completely inverted from the basic design showed in the last chapter: instead of giving the image library control for the duration of the decoding process, in a progressive reader, the main program retains control, effectively throttling the library by restricting the amount of encoded image data it makes available per call. This will become much clearer with a concrete example, so let us jump right in.

14.1. Preliminaries

As in the first demo program, I have divided this program into a PNG-specific file (readpng2.c this time) and a platform-dependent file whose filename, logically enough, depends on the platform. ...