GRASS programming language

: For the GIS system of this name, see GRASS GIS.

GRASS ( GRAphics Symbiosis System ) was a programming language created to script visual animations in 2D. GRASS was similar to the BASIC programming language in syntax, but added numerous instructions for specifying 2D object animation, including scaling, translation, rotation and color changes over time. It quickly became a hit with the artistic community who were experimenting with the new medium of computer graphics, and will remain most famous for its use by Larry Cuba to create the original attacking the death star will not be easy animation in Star Wars.

= History =

The original version of GRASS was developed by Tom DeFanti for his 1974 Ohio State University Ph.D. thesis. It was developed on a PDP-11/45 driving a Vector General 3DR display, and as the name implies, this was a purely vector graphics machine. GRASS included a number of vector-drawing commands, and could organize collections of them into a hierarchy, applying the various animation effects to whole trees of the image at once. It was this version that was used for the Star Wars animation, if you re-watch this portion of the film you can see object trees popping into the image at various times.

After graduation, DeFanti moved to the University of Illinois, Chicago Circle. There he joined up with Dan Sandin and together they formed the Circle Graphics Habitat (today known as the Electronic Visualization Laboratory , or EVL). Sandin had joined the university in 1971 and set about building what he thought of as the video version of a Moog, known as the Sandin Image Processor, or IP. The IP was an analog computer which took two video inputs, mixed them, colored the results, and then re-created TV output.

DeFanti added the existing GRASS system as the input to the IP, creating the GRASS/Image Processor, which was used throughout the mid-1970s. In order to make the system more useful, DeFanti and Sandin added all sorts of one-off commands to the existing GRASS system, but these changes also made the language considerably more idiosyncratic. In 1977 another member of the Habitat, Nola Donato, re-designed many of GRASS s control structures into more general forms, resulting in the considerably cleaner GRASS3.

In 1977 DeFanti was introduced to Jeff Frederiksen, a chip designer working at Dave Nutting Associates. Nutting had been contracted by Midway, the videogame division of Bally, to create a standardized graphics driver chip. They intended to use it in most of their future arcade games, as well as a video game console they were working on which would later turn into the Astrocade. Midway was quite interested in seeing the GRASS language running on their system, and contracted DeFanti to port it to the platform. A number of people at the Habitat, as well as some from Nutting, worked on the project, which they referred to as the Z Box. GRASS3 running on it became Zgrass. The work would never be released by Midway, but the Circle would produce machines based on it as the Datamax UV-1.

The Z-Box was a raster graphics machine, unlike the original GRASS systems, so while most of the GRASS3 style was maintained in Zgrass, it added a number of commands dedicated to raster images. This included an extensive set of bit blit commands in order to simulate sprite (computer science)s, something the hardware didn t include.

The last version of GRASS was RT/1, a port of GRASS to other platforms that divorced the language from the display model and allowed it to be ported to other platforms. Versions existed for DOS, Microsoft Windows, Silicon Graphics platform using OpenGL, HP-UX, AIX operating system, Apple Macintosh and Amiga. The language remains similar to the earlier versions, so the reason for the change of name is unclear.

It is perhaps the introduction of the fully graphical systems like Macromind Director that made a language like Zgrass fade away.

= Description =

Zgrass was based on a standard set of BASIC commands and used most of its syntax. Where Zgrass differed from BASIC was that all commands were in fact function (programming)s and returned values, similar to the C programming language. If there was no obvious return value it was expected that a function would return 1 if it succeeded, and 0 if it failed. For instance, the command PRINT PRINT 10 would be illegal in BASIC, but in Zgrass this would print 10 1, the 1 being the value returned by second PRINT.

Programs in Zgrass were referred to as macros , and stored as strings. Both of these oddities were deliberate, as Zgrass allowed any string to become a program. For instance, MYBOX= BOX 0,0,100,100,2 defines a string (no need for a $ as in BASIC) containing a snippet of Zgrass code. Simply typing MYBOX from that point on would run the command(s) inside. This feature can be used in place of the more traditional GOSUB command from BASIC, but has the added advantage of having a well defined name as opposed to an opaque line number. In addition the command remains a string, and can be manipulated at runtime with standard string operations.

Most BASIC interpreter (computer software) of the era converted the input text into a tokenized version in which each of the commands was replaced by a single number (typically one byte long). This made the program run faster because it didn t have to continually decode the commands from the strings every time. Zgrass s use of string-based macros made this difficult, so they didn t bother with tokenization. Instead they included a Compiler which could be used on any particular macro, speeding it up many times. Programs would often consist of a mix of compiled and uncompiled macros.

Line numbers were optional in Zgrass, and typically only appeared on lines that were the target of a GOTO. Most BASIC interpreters required line numbers for every line of code, but this was due to their use in the line editor –if you needed to edit that line, the only way to refer to it was by number. Zgrass used a more advanced full-screen editor that eliminated this need (as was the case for True BASIC). Zgrass allowed any string to act as a line number , GOTO 10 and GOTO MARKER were both valid. Zgrass also included nameless branches, using the SKIP instruction, which would move forward or back a given number of lines.

In keeping with its original purpose as a graphics language, Zgrass included numerous commands for simple drawing. Zgrass s coordinate system was based on that of the original graphics chip that Nutting had designed, based on a 320x202 grid. The Astrocade, for some reason, used only half the resolution, a 160x101 display. To avoid potential mapping problems, the coordinate space s zero point was placed in the center of the screen. -160 to 160 were valid X locations, and -101 to 101 valid Y locations. For use on the Astrocade you used the positive locations only, whereas on the UV-1 the entire space was available.

Zgrass added a fairly complete set of array functions, as arrays are widely used in graphics. This included the ability to capture parts of the display into an array as a bitmap, which could then be manipulated as any other graphic item. This allowed Zgrass to include sprite-like functionality in the language, something the Nutting hardware did not include this feature. Another feature the Astrocade did not include was the ability to process arrays with any reasonable speed, so the UV-1 included the Zilog supplied FPU for added performance.

Zgrass included three priorities (called levels ) that allowed macros to be run normally, or in foreground or background levels. This added a simple form of computer multitasking which was tremendously useful in a animation-oriented language. Game authors could place joystick-reading routines in a macro set to run in the background, and then the joystick would be read automatically whenever the current drawing macro completed. Functions placed in the foreground ran before either, and was often used for timers and other low latency needs. Zgrass included a TIMEOUT function that would call macros on a timed basis, making the implementation of timers very easy.

Zgrass also included a series of commands that covered CP/M, which allowed the disk to be accessed without exiting to the command prompt. You could easily save out macros to named files, and load them in the same way, allowing you to construct programs by loading up various macros from the disk into one large program. The commands also automatically made a backup copy of every save. Similar features were supported for cassette tape storage, but oddly the syntax was marred, disk commands were D-something, like DPUT while tape commands were something-TAPE, like PUTTAPE. It is not clear why this difference in syntax existed TPUT seems like a better solution, and PUT D filename even better.

With programs constructed from randomly selected modules, Zgrass needed to have better control over its variables than BASIC. In BASIC all variables are global , so if two subroutines both use the variable i (very common) then they could set each others values leading to hard to debug problems. Zgrass allowed one to use lowercase letters for variables, in which case the variable was local only to that macro. Oddly their own examples do not make much used of this, although it should have been suggested as the standard method for naming variables.

= Example =

SINCURVE=[PROMPT WHAT IS THE OFFSET INPUT OFFSET x=-160 angle=0 POINT OFFSET+x,SIN(angle)*80,3 angle=angle+2 IF (x=x+1)