8/18/2023 0 Comments Sketchpad 1962![]() ![]() In other words the actual drawing area could be much larger than the screen. Sketchpad is also capable of zooming and scrolling the picture. A bank of toggle switches to the left is used to turn functions on or off. The box with the buttons holds the commands. The presence of the console indicates that all such elements are still external to the screen. Sketchpad does not display any user interface controls like menus and scrollbars. It is clear right now, that Sketchpad can be taken as the ancestor of CAD application programs par excellence. This is the earliest application of object-oriented concepts for a computer graphics editor. If the image of the bolt is modified, all instances change as well. One master graphic might be a bolt that is used at many places, in different angles in a building plan. Once a picture is stored on magnetic tape of the TX-2 it can be used as a master graphic for new instances. If the user moves one point the algorithms of Sketchpad try to adapt the lines according to the conditions. ![]() Many of such constraints can be applied to a drawing. Constraint P for example defines the relation between four points: « Line from first to second would be parallel or perpendicular to line from third to fourth». “Horv” for example constrains a line to be horizontal or vertical. Other command buttons are used to apply constraints to the drawing. The current position is taken as final and the drawing of the line or circle is completed. Pressing a button labeled “Stop” has the same effect as a termination flick of the light pen. Now, choosing a point on the circle (which fixes the radius) we press the button “draw” again, this time getting a circle arc, whose angular length only is controlled by the light pen position we place the light pen where the center is to be and press the button “circle center,” leaving behind a center point. If we point the light pen at the display system and press a button called “draw,” the computer will construct a straight line segment which stretches like a rubber band from the initial to the present location of the pen A sudden flick of the pen terminates drawing. Sutherland explains how to draw a line with Sketchpad , A box holds about 40 buttons that correspond to operations like DRAW, MOVE and DELETE. The second hand is used to press command buttons. A termination flick event is created when the pen is abruptly moved away from screen. The position is tracked whilst the light pen is moving. A light pen is attached to directly point to the screen. The central device is a 7 by 7 inch scope with a 1024 by 1024 raster. The console to operate the Sketchpad system is shown in Fig. The four black knobs below the screen control position and scale of the picture. Sketchpad is operated with a light pen and a command button box (under left hand). Ivan Sutherland’s Sketchpad console, 1962. ![]()
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