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Visibility Turtle Graphics Package Introduction

The turtle is driven by a set of logical instructions such as:

turn: 45
move: 20
penUp

and so on...

These all use real world measurements, so turn: 45 tells the turtle to turn 45 degrees. If the turtle was currently facing 132 degrees then it will turn to 177 as a new heading. There is a corresponding turnTo: aHeading command which will give the turtle an absolute heading to face, thus turnTo: 45 with cause the turtle in the previous example to turn from 132 degrees to 45 degrees; i.e the equivalent of turn: -87. The move command operates in a similar manner except that the number provided can be supplied in pixels, inches or millimeters, so if the movement setting is millimeters, then move: 20 with instruct the turtle to move 20 mm along the current heading. There is a corresponding moveTo: aPoint command whereby moveTo: 20 @ 50 will instruct the turtle to move to a location 20mm across the page from the top left hand corner and 50 mm down. The turtle will maintain its previous heading, so if it was facing 45 degrees before this, then it will still be doing so.

Not only moving

To add to the basic movement commands, we have created a whole set of functions that make it easy to define a page. The major Visibility objects have all been wrappered so there are commands such as:

drawBox;
drawEllipse;
drawCircle;
and so on. To make these commands as simple as possible, there are a whole series of defaults and current settings that enable you to carry over the basics of these objects from one to another. Thus, there are setting such as currentBoxExtent, currentFill and so on, which are used with each of these commands. Of course, there are more extensive commands which give you progressively more control over each individual object and slowly override the defaults. An example of this is 

drawEllipse: extent angles: angles orientation: orientation fill: fill

"Draw an ellipse using extent as the bounding box, angles ( a point ) as the two angles and the remaining current circle defaults except orientation and fill.
Currently, the only orientation implemented is top left corner justified."

There are a set of defaults that are built into the system that can be overridden by setting the current setting. Thus, there is a defaultTextJustification which is set to ##l (left justification). If you do not set currentTextJustification, then this is what you get. You can set currentTextJustification as you wish. To revert to the system default, you just issue the resetTextJustification command.

What can you draw

Boxes, 3D boxes, boxes with shadows, Ellipses, circles, lines (dashed and dotted, with our without arrow heads), n sided polygons, images and text

How does it all go together

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