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fact file2.14 Image formats

 

You may have already noticed that there are two distinctive types of graphic. Both are used extensively for print and for screen, but each has specific characteristics that determine which to choose for a particular job. 

Vector-based graphics
Vector graphics are resolution-independent, which means that they can be scaled up to very large sizes without losing quality and without increasing file sizes. This is because they are based on mathematical co-ordinates or equations. Serif DrawPlus is a vector-drawing program. It has many ready-made shapes that you can combine to create your own graphics, as with the icons used throughout this unit.

It also has freehand vector drawing tools, though these take more practice to get used to. Vector graphics are first choice for hard-edged, stylised images, strong shapes, flat colours, and geometric forms. They are ideal for cartoons, for typography, for designing logos, icons and web buttons.

Vector graphic

Bitmap-based graphics
Bitmap or raster (another word for computer screen) images, on the other hand, are resolution-dependent and lose sharpness when made larger. This is because a bitmap image is composed of small coloured dots called pixels: when the image is enlarged the same amount of information has to be stretched across a greater number of pixels, resulting in fuzzy and jagged edges. This is known as ‘pixelation’. A bitmap is literally a ‘map’ of binary digits (0 and 1), known as ‘bits’, which give each pixel on the screen a specific colour.

One of the basic features of bitmap images is that they not only have height and width, but also depth, because each pixel can have a number of bits stored to identify its colour information – the greater the number the higher the quality. A 1-bit graphic would have only two colours – black and white. With 8-bit colour a total of 256 colours are available; with 16-bit, a total of 65536 colours are available; while 24-bit colour has a total of 16777216 colours. Computer monitors also have bit-depth. You may find bitmap paint tools easier to use at first than vector ones because they respond more like traditional mark-making tools. They can also be pressure-sensitive if you use a graphics pad and stylus rather than a mouse.

Bitmap

One of the main benefits of bitmap images is that slight variations of colour and tone are accurately reproduced, so they are first choice when working with photographs. Serif PhotoPlus is a bitmap-editing program.   

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