Digital images and animation in PowerPoint.

Abstract

Computer-generated images fall into two categories, bitmap or raster images and vector graphics. It is important to understand the difference between the two because they behave differently in PowerPoint. Bitmap or raster images are made up of individual small squares or pixels. Every image has its own unique set of pixels containing all the image’s information. Each pixel is given a specific colour value and location in the image. The technique used by Pointillist painters, such as Georges Seurat, is the closest analogy – he used thousands of dots of paint which the human eye then blends together to create the picture. Because they can represent subtle shades and colours, bitmap images are the most common electronic medium for photographs or continuous-tone illustrations. The size of bitmap images is defined in pixels – sometimes wrongly referred to as dots per inch (dpi), which is from printing parlance where screens are measured in dots per inch. Bitmaps are resolution-dependent, which means that the number of pixels determines the sharpness and amount of detail in the image for any given size – the higher the number of pixels, the higher the quality of the image. A high resolution image has lots of pixels (information or data) in each square inch. A low resolution allows only a few pixels for that square inch. The resolution of the image therefore relates directly to the quality of the image. Bitmap images have to be scaled carefully to avoid losing detail. An image has its own specific pixel information when it is created. You can take pixels away but you cannot add ‘new’ pixels – the software can only copy ones that are already there or make a guess as to how new ‘intermediate’ pixels should look to fill a gap you have created (this is what is meant by ‘interpolation’). Vector graphics consist of lines and curves defined by mathematical objects called vectors. Vectors describe graphics according to their geometric characteristics and therefore can be moved or resized without losing quality. A vector image remains crisp and sharp on any output device and will print at the normal resolution of that device. Vector programs are the best choice for type and graphics such as charts, graphs and logos, which need to be very flexible in terms of scale.