A laser printer needs to have all the information about a page in its memory before it can start printing. How an image is communicated from the PC’s memory to a laser printer depends on the type of printer being used. The crudest arrangement is the transfer of a bitmap image. In this case there is not much the computer can do to improve on the quality, so sending a dot for a dot is all it can do.
However, if the system knows more about the image than it can display on the screen there are better ways to communicate the data. A standard A4 sheet is 8.5in across and 11in deep. At 300dpi, that is more than eight million dots compared with the eight hundred thousand pixels on a 1024 by 768 screen. There is obviously scope for a much sharper image on paper – even more so at 600dpi, where a page can have 33 million dots.
The major way quality can be improved is by sending a page description consisting of outline/vector information and allowing the printer to make the best possible use of it. If the printer is told to draw a line from one point to another, it can use the basic geometric principle that a line has length but not width, and draw that line one dot wide. The same holds for curves, which can be as fine as the resolution of the printer allows. The idea is that one single page description may be sent to any suitable device, which would subsequently print it to the best of its ability – hence the much-touted term, device independent.
Text characters are made up of lines and curves so can be handled in the same way, but a better solution is to use a pre-described font shape, such as TrueType or Type-1 formats. Along with precise placement, the page description language (PDL) may take a font shape and scale it, rotate it, or generally manipulate it to its heart’s content. There’s the added advantage of only requiring one file per font as opposed to one file for each point size. Having predefined outlines for fonts allows the computer to send a tiny amount of information – one byte per character – and produce text in any of many different font styles and many different font sizes.