Python Tutorial 3: Arrows and Lines (and Poly Lines)

In addition to the circle and rectangle objects that can be used to represent real world objects, there are other objects that can be used for different visualizations.

Visualizing Vectors

The arrow class represents a graphical arrow and is commonly used to illustrate vectors, but can be used for representing anything in your simulations.

# Attach an arrow to a circle
c1 = circle(pos=vec(0,0), radius=5, color="green")
a1 = arrow(pos=c1.pos, size=vec(10,10), color="purple")

Arrow without components

The code above creates an arrow object and places the "tail" of the arrow at the position of the circle. The arrow's "tip" then extends in the x direction 10 units, and extends in the y direction 10 units.

The second attribute, the size, should be seen as the actual size of the vector, not the end point of the vector. So for example, if you want to represent the vector (10, 10) but starting at the point (0, 10), notice that the actual tip of the vector extends to the point in space of (10, 20).

• pos — The initial position of your arrow tail as a vector with x,y,z coordinates.
• size — The size of the arrow given as a vector with x,y,z coordinates.
• color — The arrow will be drawn in this color. Use HTML colors e.g. "#ff3300", "blue".
• stroke — The arrow thickness in pixels.
• style — An arrow can have a "dash" style, or a "dot" style, or "none" which is the default.
• opacity — The arrow will be drawn with an opacity between 1 and 0, representing 100% opaque to 100% transparent.
• show_components — This flag tells Tychos to attach additional x and y component arrows.

• visible — The arrow can be hidden from view by setting this flag to False.
• motion_map — This flag tells Tychos to attach a series of strobe images called a motion map.

You can also visualize the vector components by designating the attribute show_components as True.

a1.show_components = True

This will display the component vectors for the diagonal vector.

Arrow with components

Lines (and poly_lines) to Represent Other Geometry

Lines can be used to represent things like ropes, strings, wall boundaries, ramps, or really anything that you can imagine that could be idealized as a straight line.

Lines are quite similar to arrows except that they are defined by two positions rather than a position and a size:

# A line representing a string
string = line(pos=vec(0,0), pos2=vec(0,10), color="black", stroke=2)

This creates a line with a thickness of 2 pixels, that extends from the point (0,0) to (0, 10). These are other attributes of the line object:

• pos — coordinates for the starting point of the line as vector
• pos2 — coordinates of the termination point of the line as a vector.
• color — HTML color value for your line, e.g. "red" or "#ff0000".

• stroke — Stroke value that determines the visual thickness of theline . This is measured in pixels.
• style — Sets the line as either solid (default = "none") or "dash" for a dashed line, or "dot" for a dotted line.
• opacity — The line will be drawn with an opacity between 1 and 0, representing 100% opaque to 100% transparent.
• visible — The line can be hidden from view by setting this flag to False.
• motion_map — This flag tells Tychos to attach a series of strobe images called a motion map.

There is one other object that can also be helpful in creating even more complex geometry in your simulations. These are called poly_line() objects. They are a collection of line objects defined by connecting points.

poly_line() objects are a bit more complicated, and a bit beyond this introductory tutorial, but here are some links to get you started if you want to learn how to use them:

Tychos Python Language Reference: poly_line()

Python Demonstrations: poly_line()