the beam scanned the line, the phosphor was pushed well beyond the secondary emission threshold. If the beam was located over a blank area, a certain number of electrons would be released, but if it was over a lit area, the number would be increased by the number of electrons previously stuck to that area. In the Williams tube, these values were read by measuring the capacitance of a metal plate just in front of the display side of the tube. Electrons leaving the front of the CRT hit the plate and changed its charge. As the reading process also erased any stored values, the signal had to be regenerated through associated circuitry. A CRT with two electron guns, one for reading and one for writing, made this process trivial. Imaging systems The earliest computer graphics systems, like those of the TX-2 and DEC PDP-1, required the entire attention of the computer to maintain. A list of vectors stored in main memory was periodically read out to the display to refresh it before the image faded. This generally occurred frequently enough that there was little time to do anything else, and interactive systems like Spacewar! were tour-de-force programming efforts. For practical use, graphical displays were developed that contained their own memory and an associated very simple computer which offloaded the refreshing task from the mainframe. This was not inexpensive; the IBM 2250 graphics terminal used with the IBM S/360 cost $280,000 in 1970. A storage tube could replace most or all of the localized hardware by storing the vectors directly within the display, instead of an associated local computer. Commands that previously caused the terminal to erase its memory and thus clear the display could be emulated by scanning the entire screen at an energy above . In most systems, this caused the entire screen to quickly "flash" before clearing to a blank state. The two main advantages were: Very low bandwidth needs compared to vector graphics displays, thus allowing much longer cable distances between computer and terminal No need for display-local RAM (as in modern terminals), which was prohibitively expensive at the time. Generally speaking, storage tubes could be divided into two categories. In the more common category, they were only capable of storing "binary" images; any given point on the screen was either illuminated or dark. The Tektronix Direct-View Bistable Storage Tube was perhaps the best example in