GRASS programming language

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 whole trees popping into the image at various times. After graduation he 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 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 blitter commands in order to simulate sprites, 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, Windows, SGI platform using OpenGL, HP-UX, AIX, 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 functions and returned values, similar to the C programming language. 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 (1 means "success").

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. In addition the command remains a string, and can be manipulated at runtime with standard string operations.

Most BASIC interpreters 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.

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 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. The system deliberately used the display from the Astrocade hardware as its own coordinate system, but the Astrocade could display only 1/2 the resolution. 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, as the Astrocade 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 Z-Box included the Zilog supplied FPU for added performance.

Zgrass included three priorities (called levels) and allowed macros to be run normally, or in foreground or background levels. This added a simple form of 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. Its not clear why this difference in syntax existed TPUT seems like a better solution.

With programs constructed from random 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)<159,SKIP -2]


This example shows several of the novel features of Zgrass. This text creates a new macro called "SINCURVE" that can be called simply by typing it into the command prompt, or from other macros or programs. SINCURVE uses two local variables, x and angle, as well as a global variable, OFFSET.

The PROMPT/INPUT is a modification of the original BASIC version, which will not ask for the input if the user types it into the command line when calling the macro. In this case typing SINCURVE will result in the prompt appearing and the program waiting for input, whereas typing SINCURVE 30 will skip the prompt and OFFSET will automatically be assigned 30. This allows a single macro to be used both interactively and within a program as a function.

POINT is an example of one of the many graphics commands included in the Zgrass language. POINTrequires an X and Y location, as well as a color. In this example the user supplied OFFSET moves the x position of the curve on the screen, while the Y position is supplied by the trig function, suitably enlarged for display (in this case, 80 times). The color is supplied in the last input, and in this case is 3. The Z Box used color registers, so 3 did not imply a particular color, but a color selected from the current palette.

The IF is likewise interesting. It places an increment, (x=x+1), in front of the test, a feature not normally available in BASIC. In this case the IF is told to call SKIP -2 if true, which will move back two lines and can be used in place of a GOTO.

External links:

Bally Alley Miscellaneous Documentation (http://www.classicgaming.com/ballyalley/misc_docs/misc_docs)

- includes PDF versions of scans of the UV-1 Zgrass manual