Extended Exercise: The PNG Format
In previous sections of this tutorial we applied DaeDaLus to parsing the PPM text file format. In this section we’ll apply DaeDaLus to a binary format to explore the binary format parsing features DaeDaLus has while also providing an opportunity to practice with the features that we already introduced. The format we’ll be specifying is PNG, the Portable Network Graphics image format, which provides lossless raster-graphics compression.
The PNG specification is quite long, so we won’t be implementing it in its entirety in this exercise. In particular, we’ll focus primarily on the structure of PNG chunks and we will not validate the order or number of chunks other than the header and end. Furthermore, we will not perform any of the typical CRC validation, a criticial error detection/correction system that contributes to the reliability of PNG.
We recommend reading Validation in Parsing on some high-level guiding principles one should have in mind when deciding what to include in the parsing stage of a format-processing application. This will directly address some of the things we’re omitting from our development of a PNG parsing specification.
Before we get started with the PNG specification itself, we’ll introduce some useful definitions from the DaeDaLus standard library that we did not use at all in the PPM specification. The standard library (which is all implemented in DaeDaLus itself) provides a number of convenient types for handling binary formats in particular.