PNG-Reading Demo

In order to provide a concrete demonstration of how to use libpng to read PNG images, I have written a complete (albeit basic) PNG viewer in Standard C. It consists of two main source files: a platform-independent one that includes all of the PNG- and libpng-specific code (readpng.c), and a platform-dependent file that contains all of the user interface and display code. The idea is that the PNG code (the ``back end'') is generic enough that it can be dropped into almost any image-reading C program, whether a viewer, editor, converter, or something else; it is the part that is of primary interest to us. The platform-dependent code (``front end'') is functional--yes, it really works!--but it is not complete or robust enough to be considered a final product.

The back-end code was written for libpng version 1.0.3, but it should work with any 1.x release of the library. Later releases of libpng may add new interfaces, but the functions used here are expected to remain available more or less indefinitely, for backward compatibility. As for the front-end code, two versions are currently available: one for the X Window System (rpng-x.c; mainly for Unix systems, but also potentially VMS and OS/2), and one for Windows 95/98 and NT (rpng-win.c). I will avoid getting into the details of these as much as possible, but where it is unavoidable, I will either use excerpts that are common to both or else point out the differences between the two versions. Complete source listings for both flavors can be found at http://www.libpng.org/pub/png/pngbook.html.

The basic PNG reader has the following features: it is file-based, it reads and displays a single image and then quits, and it is concerned only with reading and decoding that image--it has nothing better to do and can afford to wait on file input/output (I/O) and other potentially slow but non-CPU-intensive tasks. In other words, its characteristics are typical of standalone image viewers, converters, and many image editors, but not of web browsers. Browsers usually read from a network, which is often extremely slow compared to disk access (for example, due to limited modem bandwidth or just congested Internet sites), and they are usually busy formatting text and decoding several images at the same time--they do have something better to do than to wait for the rest of the file to show up. I'll address these issues in Chapter 14, "Reading PNG Images Progressively", with the second demo program.