An Introduction to Image Processing

An image is an array, or a matrix, of square pixels (picture elements) arranged in columns and rows.

In a (8-bit) grayscale image each picture element has an assigned intensity that ranges from 0 to 255. A grey scale image is what people normally call a black and white image, but the name emphasizes that such an image will also include many shades of grey.

A normal grayscale image has 8 bit color depth = 256 grayscales. A "true color" image has 24 bit color depth = 8 x 8 x 8 bits = 256 x 256 x 256 colors = ~16 million colors.

Some grayscale images have more grayscales, for instance 16 bit = 65536 grayscales. In principle three grayscale images can be combined to form an image with 281,474,976,710,656 grayscales.

There are two general groups of ‘images’: vector graphics (or line art) and bitmaps (pixel-based or ‘images’). Some of the most common file formats are:

* GIF – an 8-bit (256 color), non-destructively compressed bitmap format. Mostly used for web. Have several sub-standards one of which is the animated GIF.
* JPEG – a very efficient (i.e. much information per byte) destructively compressed 24 bit (16 million colors’) bitmap format. Widely used, especially for web and Internet (bandwidth-limited).
* TIFF – the standard 24 bit publication bitmap format. Compresses non-destructively with, for instance, Lempel-Ziv-Welch (LZW) compression.
* PS – Postscript, a standard vector format. Has numerous sub-standards and can be difficult to transport across platforms and operating systems.
* PSD – a dedicated Photoshop format that keeps all the information in an image including all the layers.