Oakley and Norris in Page Description Languages: development, implementation and standardization have collected the following definitions:
An interface between composition software and the output device.
A program to communicate a description of a document from a composition system to a printing system.
A software tool for composing graphical images on a page (including font rotations, scaling, graphics).
A software solution to the transfer of logical and structural information from host processors to printers with sufficient abstraction to enable device independence.
A printer control language based on the page.
Screen dumps were the first way to transfer an image from the computer screen to paper (1 pixel on the screen = 1 dot on the paper)
The problem is that high-resolution screens have 72-100 dots per inch (dpi), while even low-end laser printers have a resolution of 300 dpi (with phototypesetters up around 1250-2500 dpi)
The first printer drivers (programs that send documents to the printer) consisted of escape codes (How many of you have done this?) They were not what you might call user-friendly. Also, they were line rather than page-oriented, and not device independent.
The first Command Stream image description languages were easier to use, and more device independent, but still line-oriented.
PDL's combine the typographical quality of typesetters and the graphics quality of CAD-plotter systems. Because of the evolving low-cost of printer memory, they can store a full bit-mapped page.
Most PDL's are procedural, rather than declarative, device independent, PDL output can be sent directly to the printer or programmed directly (albeit tediously).
To understand PDL's, it helps to understand laser-type printers
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