Image Optimisation Tutorial
Part 1 - Understanding Image Compression

Who should read this tutorial?

This tutorial is aims to help anyone who finds it hard to work within the constraints of their gallery space or would like to learn how to optimise (compress) their images for Internet/portable use.

Introduction

We live in a world where space is at a premium, whether its the size of the back garden or the sock drawer that is overloaded and won't close. In the virtual world there is no difference; whether it is running out of space on a cd or floppy disk or needing a bigger hard drive to store your growing mp3 collection, space is always at a premium.

You have probably at some time in your computer usage opened or downloaded a program that was 'zip' compressed. Once downloaded the file can then be unziped (uncompressed) using suitable software before use. Image files are no different. However other ways exist for compressing images to appropirate formats in order to lose the need for uncompression tools. The JPEG image format is one such example, and one we will focus on during this tutorial.

The JPEG (Joint Picture Expert Group) format was developed to provide a portable (i.e small) image format that could be used to quickly transfer color photographs and images over computer networks without compromising noticable quality. JPEG files view well on a computer screen but are less suitable for printing. A JPEG is a lossy format which means it loses (permantly) picture information after the compression into the JPEG format has taken place. A JPEG file can be recognised by its file extension '.jpg'.

Here is a common question,

'Why when I download a JPEG file from my digital camera is the image and file size so big?'

The answer? The file has not been fully optimised when it was compressed into the JPEG format. This is an advantage of the JPEG format though, in that you can specify how much to compress the picture when creating the JPEG file. As has been previously said once compressed, picture information lost during compression of a JPEG image is lost forever, therefore it make sense that from sources of creation (e.g. digital camera) JPEG files are as un-optimised as possible. In photographic terms think of these as your 'negatives'. You then make clone images (to optimise) from the negative.

An Interesting comparison

Perhaps a good analogy for how JPEG compression works would be to compare it to doing a jigsaw puzzle.

Pretend for a moment you are given two identical 1000 piece jigsaw puzzles to complete - the only difference is one has had 100 strategic pieces removed (the bits that don't really do much for the image, e.g. parts of a skyline).

It is likely that the jigsaw with only 900 pieces will be completed faster because there are less pieces to place - think of this as your JPEG load time.

It goes without saying that the 900 piece jigsaw will take up less space in two identical sized boxes - think of this as the image byte size.

However, it is more than likely when showing someone the 900 piece jigsaw the viewer will still easily be able to discern the jigsaw image. JPEGs share many traits with the above analogy.

Identical images that are very different…

Take a look at the pictures below you will be hard pressed (at a glance) to see the differences in image quality between these two photos. Now press and hold the <Ctrl> key on your keyboard whilst pressing the <F5> key did you see which image loads faster?

So the left image loads faster and it is also the smallest file approx (10k). Whereas the image on the right hand side takes longer to load and is bigger in size, approx 50k. Well everthing makes sense so far. However when comparing visually the two images - from normal screen viewing distance - their differences in quality is almost unnoticable. You have to admit that's an incredible reduction in size for just 2 minutes work!

The program used to compress the file on the right is called 'JPEG Resizer'. You can find out more about this FREE program at,

http://www.virtualzone.de/resizer/

This simple program will allow you to shrink large JPEG files to the seemingly impossible 'byte' sizes wihout great losses in image quality. Part two of this tutorial shows you how to use JPEG resizer to optimise your '.jpg' images.

To get the most from part two of this tutorial you should first download 'JPEG Resizer' and then install it to your computer.