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Pictures technologiesThe Internet is packed with images. The problem is the prodigious quantity of digital space images require. A dot can take the same digital space as a single letter. It takes a lot of dots to make a screen. In very rough terms you can have a thousand pages of text for a one-page, quality colour photo. So pictures take a long time to load. The hunt has been on to reduce the size of pictures. Formats Images now come in different formats, whose purpose is to reduce the digital 'space' they require. Back in 1987, when a 4.8K modem was state of the art, CompuServe introduced the GIF (Graphics Interchange Format) format supporting 256 colours and 'dithering'. It is excellent for web buttons with simple design and a few colours. Now, the most popular picture format is JPEG (Joint Photographic Experts Group), pronounced 'jay-peg', which is a close relative of the MPEG format used in DVDs. To achieve this compression, the computer either samples and averages the content or uses some sophisticated maths to reduce the amount of storage space required. You can watch the process at work. Rather than building up the picture line by line, the image appears in a chunky format which is refined as more data is received. This keeps the watcher amused as they guess what the image will eventually be. Much advanced mathematics is employed to achieve compression. The words 'scalable' and 'vector' indicate that the image has undergone some sophisticated numerical analysis. GIFs have been given a new lease of life as the basis of animations. If the images keep moving, detail does not matter as much. To prove that, just freeze a frame on your video to appreciate how clever your brain is at filling in the gaps to compose a moving picture. Your brain turns the fuzzy images into a sharp action sequence. Other formats are better suited to paper-printed images. A normal screen has about 60 dots or pixels per centimetre (150 per inch). Printers can cope with resolutions that are 2, 4 or even 8 times as good as this, so you need better quality images. Bear in mind that you read a screen which is at least twice the distance from the eye that a photo would be, so the resolution is appropriate to the different uses. The TIFF (Tagged Image File Format) is a good format for saving all the details and is the standard in the publishing business where Mac computers dominate the desktop. Microsoft's answer is the Windows bitmap or BMP, which preserves the quality but offers no compression, with the result that it creates very large files. The MS graphics package also uses MIX format which is more compact. However, none of these are suitable for web publication as their quality is far greater than any computer screen. You might use these to send photos to friends and NASA has some high quality picture of the stars which are designed to be printed rather than viewed on screen. The PNG (Portable Network Graphics) format looks like the next advance as a format for print and screen. FlashPix file format has several images in the file so that a program can automatically select the best resolution for a particular job. This makes editing and display faster and more flexible. FlashPix cannot be used with 256-Colour output types. The technology does not stand still. JPEG files have been around since 1987, which is antique in computing terms. The JPEG 2000 format has been agreed. It provides better compression and higher quality. But you will not see many .jp2 images until all the browsers include the software to view them and all software packages can handle them. Practicalities The byte-size of a picture file is not just a function of the file type. You can squash a large image into a small frame but that will not make the file a single byte smaller. You have to actually make the picture itself smaller so that it has just enough information to fill the frame on the screen. A mug shot 150 by 200 pixels looks good on the screen and thumbnails as small as a 40 pixel square work well if the original photo has good colour contrast. Thoughtful designers will offer a thumbnail picture which you can click to download a screen-sized version. Always save the original picture in the best format and edit this image for use on the web, preserving the original. With JPEG, and particularly GIF, much of the picture data is dumped, so quality is lost if you try to expand the picture later. Most browsers allow you to capture images that appear on your screen and save them. Just put the cursor on the image and right-click and you can save the image to the directory of your choice. Interlacing is important Rather than building up the picture line by line, the image appears in a chunky format and is refined as more data is received. This keeps the watcher amused as the page loads and they guess what the image will eventually look like. Clever designers try to get the key text and navigation in place before they start to send the images. You often see a box with a little cross and some text to tell you what is coming. The future As the bandwidth or carrying capacity of the communications systems improves, so will the quality and quantity of the images. Already videos are reducing movies to a few per cent of the original data content by only sending the bits of each image that have changed. A clever and very fast processor puts the new dots on the screen and, provided it has a lead of a few frames, it can keep up with the action. It's amazing.
© Charles Jones 2001-3 |
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