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Interactive Graphics and Image Display

IP.com Disclosure Number: IPCOM000087194D
Original Publication Date: 1976-Dec-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 3 page(s) / 46K

Publishing Venue

IBM

Related People

Greasley, NM: AUTHOR [+2]

Abstract

Early graphic display devices were based on a cathode ray tube (CRT) with the electron beam being directed across the screen in a series of straight lines. The quality of the visible image is affected by the writing speed of the electron beam (which is a function of the deflection circuits) and the persistence characteristics of the tube phosphor. Too short a persistence limits the number of lines that may be displayed without flicker, while too long a persistence means after-glow images of lines no longer having any importance. Flicker can be overcome as long as the screen image is refreshed at least 40 times a second; normal television techniques make use of high refresh rates to prevent flicker.

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Interactive Graphics and Image Display

Early graphic display devices were based on a cathode ray tube (CRT) with the electron beam being directed across the screen in a series of straight lines. The quality of the visible image is affected by the writing speed of the electron beam (which is a function of the deflection circuits) and the persistence characteristics of the tube phosphor. Too short a persistence limits the number of lines that may be displayed without flicker, while too long a persistence means after-glow images of lines no longer having any importance. Flicker can be overcome as long as the screen image is refreshed at least 40 times a second; normal television techniques make use of high refresh rates to prevent flicker.

There are two convenient methods of drawing lines or vectors on a CRT using raster techniques. In a first method, the raster scan is divided into a finite number of bits or picture elements (pels) with every pel having a corresponding bit in a store; thus each raster line has its corresponding store of bits. The advantage of this method is that every addressable pel can be activated. The disadvantage is that to be interactive with a light pen, the complete bit pattern has to be generated to attempt a match. Any slight error in positioning the light pen requires another pass through the display list using a circle of confusion technique to identify the required line or vector. In a second method, the bit pattern for each raster line is generated in a raster line assembly store immediately before that raster line is to be displayed. The advantage of this method is that there is fast light pen interaction with a 75% reduction in memory size. The disadvantage is that the number of vectors that can be displayed at any one time is limited by the speed of the circuitry loading the raster line assembly store. If the two methods are combined as described below, it is possible to make use of the advantages of each method so that a virtually unlimited number of vectors can be displayed with fast light-pen interaction.

A raster type of display is also ideal for displaying images. Each pel of the image display can be stored electronically, but in practice this would entail a large amount of storage. However, the image data can be stored in compressed format using some compression algorithm which can reduce storage requirements by a factor of 6 to 8. To display an image, the compressed image is decompressed into its full form.

Referring now to the drawing, display image data is encoded by field or vector in compressed f...