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The PNG File Format

Portable Network Graphic, or PNG (pronounced "ping") is a remarkable image format with a lot of cool features such as: "48-bit" (16 bits per channel) true color or 16-bit grayscale, lossless compression, gamma correction, two-dimensional interlacing, and best of all alpha transparency.

Originally designed to replace GIF (and to some extent, TIFF), PNG is the only truecolor, lossless image format that is both cross-platform and application independent?meaning that, unlike TIFF, any supporting application will be able to read a PNG created by another application. PNG is also patent-free and endorsed by the World Wide Web Consortium (W3C).

PNG Features

PNG has the capability for both binary transparency (like GIF) and variable transparency, more commonly referred to as alpha transparency or translucency. Stored as RGBA (Red, Green, Blue, Alpha), each pixel is assigned one extra byte of data (or palette entry for 8-bit images) that defines its transparency level from 0 (fully transparent) to 255 (fully opaque). What does all this mean? For one thing, anti-aliased images will look good on any background. Imagine, no more ugly image halos and fringing! You can even apply background-independent effects such as drop-shadows and glows.

PNG Transparency
Figure 1.3
The same PNG from above, displayed on a blue background

GIF Transparency

Figure 1.4
The same GIF from above, displayed on a blue background

GIF vs. PNG: a comparison of GIF and PNG compression for a typical screen capture (24-bit, 1280 x 1024)

	File Size (KB)	Size vs. GIF (%)
GIF (8-bit, dithered)	66.9	n/a
Photoshop CS: Save for Web, PNG (8-bit, dithered)	44.3	66.2
Photoshop 7: Save for Web, PNG (8-bit, dithered)	53.9	80.6
Photoshop: SuperPNG¹ (8-bit, dithered)	44.3	66.2