Editing Tektronix 4010 (section)

==Display capabilities and limitations==
[[File:Computervision piping.agr.jpg|thumb|[[Computervision]] [[computer aided design|CAD]] system, circa 1979, using Tektronix 19" storage tubes. A slight background glow over the entire screen is characteristic of the storage tube display technology.]]
A display using this technique was instantly recognizable by the bright flash of the high-energy write gun beam as it rapidly moved about the display, "painting" the screen with complex lines and patterns. The stored image glowed brighter than the characteristic faint background glow of the entire display screen. The display was always monochrome, in three different brightnesses of "CRT green".<ref name=tek3>{{cite journal |title= Tektronix: Three Kinds of Storage |journal=Tekscope |date=July 1972 |url=http://www.radiomuseum.org/forum/tektronix_three_kinds_of_storage.html}}</ref>

The storage tube technology was vulnerable to [[screen burn-in]], as the continuous flow of electrons illuminating a stored image gradually degraded the light-emitting phosphors over a long period of time. To reduce the rate of display degradation, the hardware was designed to blank the electron beams after a period of inactivity on the screen. Software [[screensaver]] programs were not useful in protecting storage tube display screens from burn-in of images.

Also, complex and precise images would gradually become more diffused and blurry, as the stored charges on the phosphors slowly migrated and diffused away from their original locations. The only way to correct this gradual blurring was to erase and redraw the entire screen.

Because the display tube itself stored the image, there was no need for any sort of auxiliary graphics memory, greatly lowering the cost of the terminal. The 4010 cost $3,950, almost two [[order of magnitude|orders of magnitude]] less expensive than IBM's competing graphics displays.{{sfn|Tektronix|1973|p=276}} This made very complex and detailed computer graphics practical for a much wider audience. The Tektronix approach also had the advantage that there was no limit to the number of vectors that can be displayed; one could simply keep adding them to a complex image, whereas a solution like the IBM terminal had a limited number of vectors it could refresh on its display. Early CAD systems made by companies such as [[Computervision]] took full advantage of the graphic storage capability, and were able to display arbitrarily complex designs without annoying flicker.<ref>{{cite journal |first=Rosemary |last=Hamilton |title=One company's story |journal=Computerworld |date=30 December 1985 |url=https://books.google.com/books?id=QKWy2rYxAQQC&pg=PA47}}</ref>

The main disadvantage of storage tubes was that once an image was stored, it could only be removed by erasing the entire image. This made such screens unsuitable for working with scrolling text, animation, or any other display where portions of the image were continuously changing. Some early CAD workstations employed both a [[video terminal]] to display frequently changing text, and a Tektronix display showing complex graphics images.

Tektronix introduced the ''write through'' concept for non-stored vectors, but with the terminal itself lacking any memory, the data had to be continually refreshed from the host computer. The communications speed of the connection between the terminal and host limited the number of refreshed objects that could be supported, and was often in the range of a few dozen graphic elements. Another disadvantage is that it required a brief interval for the image to "store" on the display screen, which limited the maximum speed with which the image could be drawn. Tektronix referred to this as the ''stored writing speed'', and measured it in terms of vector-inches-per-second, with ratings between 1500 and 4000 being typical for their displays.<ref name=tek3/>