Basic Concepts

The basic concepts covered include:

Program Input, Program Output, Command Format, Display Page Model, Sequence of Commands, and Coordinate Space .

Program Input

The program accepts a series of commands. They are used to set up the program, change the graphic state, or create the various parts of the graphic output.

These commands may be issued by:

• executing commands stored in a text file.
• typing into the stdin command input on the VAX or UNIX.

Depending on the commands issued, the program will also read in the user´s ASCII text data file, overlay files, and contour level files etc.

In this manual ACON commands are usually referred to as "commands" when the command doesn´t return a result, and as "functions" when they do.

Program output

The primary output is a plot of the data. The plot may be created in one of 3 ways:

• Screen - using the screen (and printing the graph on the screen),
• PostScript - as an ASCII text file containing PostScript® (a page description language) source code, which is meant to be sent to a PostScript compatible printer, or viewed with an application like GhostView.
• Illustrator - as an ASCII text file containing Illustrator compatible PostScript in lieu of the standard PostScript interface, permitting interactive editing of the resultant page before printing using Adobe Illustrator (or other compatible application).

Command Format

Command lines in a Script file have the command keyword beginning as the 1st word in the line. The keyword may be upper or lower case, with underline characters in place of blank spaces. Parameters are enclosed in parentheses, and comma delimited. Unrecognized command keywords are ignored.

command_name (parameter,parameter)
Interpolation_Type(1,5.0);

Command lines may be terminated with a ";". This is may be used to allow multiple commands on a single line, and reduce ambiguity.

Some commands are available only as menu items, to manage the display of the windows on the screen. Other commands are not available as menu items.

A command may produce an immediate result; e.g., AXIS draws and labels and axis.

Others alter the graphic environment; e.g., INTERPOLATION_TYPE defines the type of surface interpolation.

Lastly some commands may be used to formulate user-defined functions. Function definitions are indicated by preceding the function name with Function e.g.

Function Hours_To_Seconds(x) { Return(x * 3600); };

Display Page Model

The page (or screen window) is blank surface which all commands producing output, draw using the currently defined colour (or grey level). The "paint" is opaque, in that any command (even using white paint) will obscure previously drawn portions of the page when an area is redrawn. As a result, commands must be specified in the sequence necessary to produce the desired shaded contour plot.

Sequence of Commands

Finding the correct sequence of commands is one of the more confusing problems facing a new ACON user. There are 4 conceptual processes in creating a plot: reading your data, choosing the output device, adjusting graphic attributes, and generating graphics output. There is a correct sequence to logically produce the plot you would like to see.

In general you should:

Read in data from a file (or create it within ACON) before "drawing" the data.

Specify the output device (using an xxx_Output() command) before issuing a "drawing" command. Only one output device is active at a time.

Alter the drawing attributes that control the appearance of a drawing command before issuing the drawing command. For example, you may change the current line width using the Line_Width(...) command before issuing the Axis(...) command. Most graphic attribute commands (e.g. Line_Width() Character_Height(), Font(), etc.) will affect all subsequent drawing commands until they are reissued with different values.

- Yes, you´re right, this is not particularly object-oriented at all.

Issue the drawing commands in the sequence required to create the graphics (paying attention to the opaque painting model).

Close the output device when finished drawing. This is done automatically by issuing a new xxx_Output() command, or by quitting the program using the Quit() command.

Coordinate Space

The definition of where to draw the user data on the page is specified using the "Data_Window" and "Data_Viewport" commands.

Drawing nominally occurs on a page which is 8.5 inches x 11 inches in size. Coordinates are usually addressed using a Normalized Device Coordinate (NDC) space which ranges from 0 - 1. (see Coordinate Conversion).