Processing: Creative Coding and Computational Art

PROCESSING: CREATIVE CODING AND COMPUTATIONAL ART
504
Figure 11-11. Ravenous Triangle I sketch
I used the basic unit circle relationships cos(theta) = x and sin(theta) = y to plot the triangle
in the createPredatoryTriangle() function. The variables predCntrX and predCntrY
give the center point of the triangle, and the variable predLen is the length of each of the
triangles’ three vertices from the center point. I’ve used these unit circle relationships to
plot polygons throughout the book, so hopefully this is a review.
To get the triangle to rotate and stay oriented, I used Processing’s atan2(y, x) function.
The Processing documentation says this about the function: “Calculates the angle (in radians)
from a specified point to the coordinate origin as measured from the positive x-axis.”
(See http://processing.org/reference/atan2_.html.)
Don’t worry if this doesn’t help much. The “atan” part of the function name refers to arctangent.
Normally, using trigonometry (e.g., sine, cosine, and tangent), we begin with an
angle and we can calculate the ratio of two sides of a right triangle. The arc versions of the
trig functions are used when we know the ratio of two of the sides and we want to find
the angle between them. In the sketch, deltaY and deltaX (representing the change in y
and the change in x) provide the ratio of two sides of a right angle, which I plugged into
the atan2(deltaY, deltaX) function to get the angle I need, like so:
predRot = atan2(deltaY, deltaX);
I then added predRot (which is already in radians) directly into the two trig equations that
plotted the triangle:
predX[i] = predCntrX+cos(radians(predAng)+predRot)*predLen;
predY[i] = predCntrY+sin(radians(predAng)+predRot)*predLen;