Sketchpad was designed to be extensible, with provision for adding both new graphical element types, and new constraint types.
It supported rubberbanding when drawing or editing entities (so the entities would stretch as you moved the cursor.) It supported dynamic move, rotate, and scale of entities (meaning that they moved, rotated, and scaled as you moved the cursor.) It not only supported zoom and pan (dynamically, of course), but did so transparently-even when you were in the midst of another drawing or editing operation. Visually, Sketchpad was surprisingly interactive. It was even possible to use constraints to do structural analysis of lattice trusses, such as might be found on cantilever and arch bridges. With the constraint system, it was possible to loosely sketch a shape, then add geometric and topological relationships to modify it into the exact shape you needed. Sketchpad even allowed the visual display of constraints on screen, using icons (symbols) to represent each type. These native (or “atomic”) constraints could be combined, to create more complex relationships. Sketchpad included 17 different types of constraints, including vertical, horizontal, perpendicular, coincident, parallel, aligned, equal size, and more. If, while editing, you moved one line, the other line would move with it. And it would remember that the two lines were connected. For example, if you started to draw a line, and brought the cursor close to the endpoint of another line, it would snap to that endpoint. It not only supported explicit constraints, added to entities after they were drawn, it supported implicit constraints, created as entities were drawn. One thing that made Sketchpad really stand out was its constraint management subsystem. This facility was used to create alphanumeric character glyphs, and electrical schematic symbols. The program supported points, line segments, and arcs as basic elements, but allowed these to be saved into master drawings, which could be copied or instanced. Using the light pen and input buttons, you could draw directly on the screen, using a crosshair cursor. Unlike earlier computer applications, which were batch oriented, Sketchpad was interactive. Ivan Sutherland on MIT Lincoln Labs’ TX-2 computer.