Sketch Pad

-Ivan E. Sutherland

COMMENTS ON OTHER PEOPLE'S BLOGS

1. Comment on Nabeel's blog

2. Comment on Daniel's Blog

3. The Comment on my own blog!!


SUMMARY

The paper contains the description of sketch pad, a tool that could be used for simplifying creation, manipulation and storage of drawings. It begins with an example of drawing a regular hexagon and associated patterns to give readers an idea of what Sketch pad can accomplish. The author uses this example to illustrate underlying concepts like sub picture, constraints and definition copying. The sub picture is an instance of a master figure which can be used repeatedly. This saves a lot of design time in creation of highly repetitive drawings. Enforcing a constraint implies establishing a relationship between different parts of a picture. Thus if one component among all the components involved in a constraint relationship is changed (translated, rotated, deleted etc), other components will automatically change in order to satisfy the constraints. All such elements are stored as the n-component ring structure (which actuates the propagation of constraints). One of the remarkable things about Sketch pad is the hierarchical storage of drawing elements. This ensures that changes made to a basic element will automatically propagate to higher levels. Moreover, this allows usage of general functions which act on higher level drawing objects by acting recursively on the lower levels. The concept of these general functions is akin to the modern day OOPS concept of abstraction. The author then describes the light pen tracking, pointing and display generation. For pointing to a picture element the system only selects the spots which are within a threshold distance from the center of the position of the light pen. For display generation, the coordinates of the spots are stored in a file with 36 bits allotted to each display spots, out of which 20 bits contain coordinate information and remaining 16 bits contain the address of the n-component element to which the spot belongs. This 16 bit tag allows the system to know which picture element is being aimed at. Another very useful feature of sketch pad is that it enables user to draw intricate details with high accuracy using the magnification feature. In the end, several practical applications of sketch pad are suggested like analysis of force distribution in bridge structures; in artistic drawings and animation; in electrical circuit diagram. The author suggests that future efforts could be directed towards 3D drawing and capability of defining transformation functions on drawing objects.

DISCUSSION

Sutherland’s paper must have been a groundbreaking idea when it was published. The fact that despite the vast difference in technology - both hardware and software, between now and back in 1960s, there is not much difference between present day design tools and Sutherlands sketchpad is what is remarkable. Having said that, as far as the recognition part of Sketch recognition (SR) is concerned, the sketch pad doesn’t do much, nothing actually. My interpretation of recognition is inputting noisy data say a set of pixels (with a high cardinality), and getting in return pure information, a set which consists only of classifying features. The Sketch pad doesn’t do that. When we move the light pen and turn the knobs, in effect we are providing it with pure information (feeding it with coordinates, the critical defining ones and telling it what shape we are trying to draw) and all it does is manipulate that information in various ways (which I must admit it does brilliantly). Now I am not claiming this to be a fault with the system. I am just suggesting that sketch pad might not be relevant to recognition. Therefore, in the context of SR, I would not think of improving this system. Sketch pad indeed could be made better by using SR algorithms in it. But sketchpad itself does not provide any knowledge base for the development of SR per se.