First Draft

My progress with the Chunk has been slower than I had expected. I have always had difficulty with knowing when to cease reading and commence writing. Then there is the tendency to embellish and re-embellish the written prose. All of which of course slows me down. Anyway, the fruits of labour to-date are as posted below.

CHUNK 22

Introduction

The previous chapter, Chapter 21, described the concept of a coordinating system and the use of pixels for the production of digital images. In this chapter we explain firstly, how the pixel-based approach to digital imaging is used to produce the so-called Raster graphics, and how Raster graphics differ from the other type of digital imagery known as Vector graphics. Secondly, the chapter explains how the techniques of computer imaging are further extended to achieve that more exotic type of computer graphic, - animation.

Raster Graphics

In Raster graphics, an image may be conceptualised as a grid of pixels. which coalesce together to form a smooth, continuous image. Imagine each pixel as a single light bulb which is capable of acquiring individually, a wide range of different colours by virtue of being controlled by some clever software or some clever programmable graphics card, to which all the bulbs in the matrix are connected. If this grid of bulbs were set up to display, say, concentric red, white and blue circles on a white background, then close up, an observer may only notice different coloured light bulbs without discerning any pattern within them such as concentric circles. However, if the observer were to stand a hundred metres away, to his naked eye, the bulbs will have coalesced into a smooth display of different coloured circles on a white background and he will scarcely be aware that the image that he is seeing consists in fact of a grid of individual light bulbs. A Raster image formed using a matrix of pixels works in much the same way to display an image on the computer screen (or monitor).

The term "Raster" is by no means an innovation of computer graphics. It has in the past been used widely, in the context of the television imaging technique known as "raster scanning" which forms the basis of television pictures. (for the benefit of those interested in the etymology of the word, the term "Raster" is said to have been derived from a Latin root meaning "Rake").

Raster graphics are typically used for photographic images, or images of drawings and paintings in which there is a large variation of colour and contrast.

Quality of a Raster Image - "dots per inch" and "lines per inch"

Whilst the pixel-grid described above forms the basis of a Raster graphic, its quality is determined chiefly by two measures of pixel density. These are the "Dots per Inch" (dpi) and "Lines per Inch" (lpi).

The dpi measure refers to the number of pixels (a dot being synonymous with a pixel in this context) contained in each inch of the computer screen that displays the image (usually the computer monitor). It should be appreciated that in modern computers, the display screen is not pre-designed with a fixed number of pixels or dots per inch. The dpi value can be changed dynamically using the computer's software. Some Windows XP laptops, for example, provide an option of using either 96 dpi or 120 dpi. As may be expected, the higher the dpi value, the higher the resolution of the image and correspondingly greater its quality.

The lpi measure pertains to the quality of the printed image of a Raster graphic rather than its screen display. As with dpi, the higher the lpi value (or lpi "frequency" as it is often referred to) the better the quality of the printed image. There is however, a relationship between dpi and lpi that needs to be taken into account for high quality printing of Raster graphics. The formal mathematical expression for this relationship need not concern us here but as a general rule, for best results, the dpi value of the on-screen image should be set to be twice that of the printer's lpi frequency. Thus if the printer is operating at 200 lpi, the dpi value should be set at 400 dpi.

Raster Graphic file formats

There is a large variety of file formats used for the storage of bit data pertaining to a Rster graphic. The more commonly used file formats include, JPEG, PNG, TIFF and GIFF . All of these formats have commonalities as well as differentiators.

Perhaps the most significant differentiator of the JEPG format lies in the technique that it uses to achieve data compression. JEPG format holds the bit information pertaining to an image in a highly compressed mode making the image file very compact and thereby improving immensely its file transfer speed. The latter is particularly useful for downloading and uploading image data to the Internet. The drawback of JPEG format’s high compression is that it tends to loose some data in decompression, which in turn leads to the loss of some quality in the image subsequently displayed.