Processing Pictures

The image produced on a VDU screen or printer is made up of a large number of dots. The dots are close enough together to give the impression of a continuous image. A typical laser printer would be capable of forming 600 dots per inch. The number of dots per inch is used as a measure of the best quality possible from the device and is called its resolution. If the final image is to be formed as a series of dots then it is possible to store and process the image as a series of numbers. The simplest way of doing this would be to have a 0 for white and a 1 for black, thus allowing a black and white image to be stored as a series of ones and noughts. These are the binary digits and an image stored in this way is called a bitmap-image - with bit standing for binary digit.

Additional data can be used to code information about the colour of each of the dots, so that colour images can be stored. In the case of a colour image stored in monochrome a grey-scale is used where different shades of grey represent different colours.

Picture data can be processed by various applications packages created for the purpose. The processing is usually carried out interactively with the user viewing the current state of the image on a VDU screen. Since a typical image could consist of several megabytes of data, this type of processing can make heavy demands on both the CPU and its central memory. Normally fast processors with large memory are required. Problems can also occur if the screen resolution is not fine enough to display the image or if there is a mismatch between the screen and printer resolutions so that the image, as seen on the screen, does not match the final printout.

Typically a user might want to rotate or straighten an image, resize it or to retouch areas by shading or deleting sections of the original image. Most packages will also allow the user to view the image in different magnifications so that the user can work on small sections to carry out detail work. The ultimate magnification is to view the image as a series of dots where the user is working directly at the bit image level.

More sophisticated packages will allow the user to process the image to increase its sharpness or to change the brightness and contrast in the picture. In almost every case the initial processing of an image is a step towards using the image in another application. This has led to the development of a number of standard formats to store image data. All packages will allow the user to save the processed image in one or more of these standard formats. One example is the Tagged Image File Format (TIFF) which is recognised by most systems. Other formats have become standard because of the importance of the company that manufactures related hardware that makes use of the format. Hewlett Packard Graphics Language and Kodak Photo CD formats are examples of this.

I recommend theuse of PNG files for cartoon like graphic images and JPG files for photographs. . There is an important distinction between Bitmapped images and vector encoded images. These are both discussed in the GCSE section.

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