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Web images and colours

Setting your screen to 256 colours
About images and file formats

Setting your screen to 256 colours

It will be best for this section of instruction if your Video display is working in 256 colours (8 bit video).

To check if your video card is working in 256 colours, just

If your computer refuses to run in 256 colours, and you can only see the 16 colour setting with a Desktop area of 640 by 480 pixels, your computer may not have the right video drivers installed. If you are on a network, ask your IT support person (nicely) to do something about this. If at home, find someone locally who knows about video drivers and cards, and ask them to check your installation.

Images and file formats

There are two main ways of storing and processing graphical information on a computer

Bitmap images, also called raster images

  Each pixel has one colour, and the image is built up like a roman mosaic. Bitmap images show 'jagged edges' if you look closely - this is often more obvious when you print the image out. Editing a bitmap is akin to painting, you can only change the colour of pixels. The image file contains no information about the 'objects' shown in the picture.

Vector images, also called drawings

  The instructions used to make the objects in the picture are stored as entries in a database. When you alter the image, you are changing the objects - you can work on just the Sun in an image of a sunset - and the other objects will remain. The image can be re-constructed at any size, and will have smooth edges when printed or enlarged.

The Web uses bitmaps at present for all kinds of graphical information as standard. There are 'plug-in' extension programs for the popular browsers which enable you to use vector format material, the most common systems include

There are two bitmap formats which are widely used on the Web. Both formats use image compression, to reduce the size of the files downloaded to your machine, but the way the compression is done is different in each case:

CompuServe Graphic Interchange Format (GIF)

  Best used for solid colour pictures like charts, diagrams and cartoons. Also used for icons and buttons. A particular feature of the GIF format is the presence of a 'transparent colour', so images can appear to have irregular outlines. The images are really rectangles with a transparent background. The GIF format is limited to 256 logically different colours, although the appearance of those colours can be arbitrary, i.e. you can have a greyscale. GIF compression is reversible - you get the full detail in the original image back at the other end.

Joint Photographic Experts Group (JPEG) format

  000Used for photographs and other images (such as a scanned pastel sketch) where colours are mixed in a 'fractal' way. The compression used in JPEG images can be controlled (see a later activity), and is lossy. If you save a JPEG image to a high compression or low quality, you can't get the information back. Increasing JPEG compression is like turning the treble down on your HiFi, the rapidly varying parts of the image are smoothed out.

A crude and basic summary of this section is

All Web graphics are bitmap files of two main kinds: GIF which has 256 colours and JPEG, which can store 24 bit colours (millions of colours).

Use GIF for solid colour images, use JPEG for photographs and other images with complex colours.

You can get further information about Web graphics from the following Web pages:

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