next up previous

2.4 I/O     continued...

Most users who generate high quality images will do so on workstations configured with extra hardware for creating and manipulating images. Almost every workstation manufacturer includes in its product line versions of their basic systems that are augmented with extra processors that are dedicated to drawing images. These extra processors work in parallel with the main processor in the workstation. In most cases data generated on a supercomputer is saved in a file and later viewed on a video console attached to a graphics workstation. However there are situations that make use of high bandwidth connections from supercomputers directly to video displays; these are useful when the computer is generating complex data that should be viewed in ``real time.'' For example, a demonstration program from Thinking Machines, Inc. allows a user to move a mouse over the image of a fluid moving through a pipe. When the user pushes the mouse button, the position of the mouse is sent to a parallel processor which simulates the path of particles in a turbulent flow at this position. The results of the calculations are sent directly to the video display, which shows the new positions of the particles in real time. The net effect is as if the user is holding a container of fluid that is being poured into the pipe.

There are many different techniques for drawing images with a computer, but the dominant technology is based on a raster scan. A beam of electrons is directed at a screen that contains a quick-fading phosphor. The beam can be turned on and off very quickly, and it can be bent in two dimensions via magnetic fields. The beam is swept from left to right (from the user's point of view) across the screen. When the beam is on, a small white dot will appear on the screen where the beam is aimed, but when it is off the screen will remain dark. To paint an image on the entire screen, the beam is swept across the top row; when it reaches the right edge, it is turned off, moved back to the left and down one row, and then swept across to the right again. When it reaches the lower right corner, the process repeats again in the upper left corner.