Web Publishing for Genealogy

Designing Your Web Site

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Image Formats

There are two main image formats used on the Web: gif and jpeg. Images in any other format will need to be converted to gif or Jpeg if your readers are to see them on screen; otherwise they will only be able to download them. Any graphics editor should be able to carry out conversion to these formats.§ A further format called PNG has been designed specifically for the Web, but is not yet widely used.

See examples

Both GIF and jpeg formats use compression to make files smaller than they would otherwise be, but the two formats have different strengths and weaknesses. GIFs can have only a limited number of colours (up to 256), but the advantage is that the fewer colours they have, and the more blocks of identical colour, the smaller the file. Conversely, the more different colours and the more subtle the colour variation, the larger the gif file. This makes the GIF format ideal for icons, line drawings and the like, but less suitable for photographic images.

An important advantage of the GIF format is that it uses "loss-less" compression, that is, the compression process does not destroy any information and the image will not lose any of its original colour or detail. But 256 colours is not enough to show the subtle skin-tones in a colour portrait, and the dithering effect which comes from not having enough colours available can be seen in the rightmost image in Fig. 10.

JPEG images can contain up to 16 million colours (but bear in mind that not all computer displays can show all of these) and can be created with different amounts of compression. However, the more the image file is compressed, the more image quality is compromised - this is called "lossy" compression - and the JPEG format really only makes sense for photographs or graphic images where subtlety of colour and shading are important. The amount of compression used for an individual jpeg image can be chosen to achieve the best compromise between file size and image quality.

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4. Designing Your Web Site