Picture description instructions, PDI, for the Telidon videotex system / by H.G. Bown, C.D. O'Brien, W. Sawchuk and J.R. Storey.: Co24-3/8-699-1979E-PDF

"The Telidon Videotex system is a method by which information can be accessed from central data bases by the general public. By the use of a domestic home television receiver augmented by a micro-computer controlled interface device a user can access pages of graphical and textual information over a public common carrier communications facility such as the telephone network, a cable television line or the data may be even encoded into unused space in a broadcast television signal. In order to transmit information to a Telidon terminal, at minimum bandwidth, and in a manner independent of the type of communications channel, a coding scheme was devised which encodes a picture into the geometric drawing elements which compose it. These "Picture Description Instructions" are an alpha-geometric coding model and are based on the primitives of POINT, LINE, ARC, RECTANGLE, POLYGON, point by point BIT encoded images and TEXT encoded as ASCII characters. This document provides a detailed specification of this code as well as a description of the principles which make it independent of communications channel and display hardware"--Abstract, page 1.

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