DIGITAL IMAGES BASIC

Before you begin improving your images that you have created on your digital camera, there are just a few other things that you will need to be familiar with.

When working on images in Photoshop, you will need to consider different file formats, based on what your images are and what you plan to do with them. For example, you may want to place your images on a website or

email them, in which case they need to have a small file size so that they load quickly. This page describes four file formats and how and when they should be used.

Photoshop (PSD) File Format

The PSD file format is essentially the only file format that supports Photoshop features such as layers (which is discussed in another chapter). Most other file formats flatten all layers in the image into a single layer during the conversion. Once flattened, you cannot edit any changes that you made on the individual layers. So, although PSD files are quite large, you should always keep a copy of the PSD image just in case you need to edit the image later. You can then open this file, edit it and save it to the desired file format, such as JPEG.

Joint Photographic Experts Group (JPEG) File Format

JPEG is a standardised image-compression mechanism that is designed for compressing full colour or greyscale images of natural, real-world scenes. As a result, it works well on images with a subtle blend of colour such as portraits, photographs and gradients, but not so well on lettering, simple cartoons and line drawings. Because JPEG was designed for photos it can handle as many colours as there are in the photo. It can actually support up to 16 million colours.

JPEG utilises lossy compression which means that some information is removed during the compression. Normally, the fact that information is discarded during a lossy compression is not a problem because it is discarded in such a way that the changes are too subtle to be picked up by the human eye. But, the changes can become apparent when images are edited and resaved several times in the JPEG format. Each time a JPEG image is resaved, dirty marks (known as artefacts) are added to the image. To overcome this, work on the file in the PSD format and only save it in JPEG format once you have finished making all changes.

Graphics Interchange Format (GIF) File Format

GIF is another file format ideal for the web. However, as the GIF file format restricts you to 256 colours, it is ideal for compressing line drawings such as cartoons and logos, and illustrations or images with large areas of similar colours. GIF is faster to decode and on a small, limited colour image, GIF compression makes a much smaller file size compared to JPEG because it only needs to carry information about the colours that are actually present in the image.

The GIF file format utilises lossless compression which means that the image is compressed with no loss of data. The GIF file format supports transparency. For example, if you want to create a heading image for a web page which has a textured background, you can create the heading with a transparent background. The web page’s background texture will then appear around and between the letters of the heading, rather than the heading image appearing as a rectangular shape.

Portable Network Graphics (PNG) File Format

PNG (pronounced PING) is the ideal file format if you want to use your images in Macromedia Flash.

PNG is a newer file format that has been designed to eventually replace the GIF file format (it is not intended to replace the JPEG file format). PNG uses lossless compression, similar to GIF, but typically a PNG file can be 10 – 30% more compressed than in a GIF format. The PNG file format supports transparency (without jagged edges) and up to 16 million colours. Be aware, however, that some browsers do not support PNG images.