Page 166 - Hacking Roomba

Chapter 7 — Making RoombaView 147

The Processing commands translate() and rotate() act to move and rotate all subsequent
drawing that is done. For simple sketches, using those commands by themselves is fine. When
drawing multiple things that use translate() and rotate(), you need a way to save and
restore the current drawing position. The pushMatrix() and popMatrix() commands save
and store, respectively, the current drawing position and orientation.
For example, the draw() method in Listing 7-1 could be only slightly re-written so that it’s a
blue diamond that follows the mouse instead:
void draw() {
background(51);
fill(0,0,255);
rectMode(CENTER);
pushMatrix();
translate( mouseX, mousey );
rotate(PI/4);
rect(0,0, width/5, height/5);
popMatrix();
}

Notice that a square is still being drawn, but what turns it into a diamond is the rotate(PI/4)
method, which rotates all subsequent drawing by 45 degrees. (In Processing rotation is measured
in radians. 2 × PI radians == 360 degrees, thus PI/4 radians = 45 degrees. PI is π= ~3.14.) This
is the exact technique used to rotate and move the Roomba icon without needing to parameterize
all the Roomba drawing code.

Computing Position
The three values needed to specify the location of the virtual Roomba icon are rx, ry, and
rangle. These are the three arguments to the drawRoombaStatus() method in Listing 7-5.
From the current location and new distance and angle data from the Roomba, the task is to
compute the next on-screen location. Listing 7-6 is the algorithm used every time the sensors
are read to determine the new location.
The first thing it does once it gets the new sensor data is to determine the new rangle by
subtracting it from the current angle. It’s a subtraction because Roomba measures angles
counter-clockwise, while Processing measures angles clockwise. Given the new angle, the x
and y components (dx,dy) of the updated position vector can be computed. With a new dx
and dy, the values of rx and ry can be calculated by adding dx,dy to rx,ry. The sign of dy is
flipped because on-screen graphics are flipped top-to-bottom from normal coordinate systems.
Both dx and dy are scaled by experimentally determined scalex and scaley values. This
scaling effectively converts the sensor data in units of millimeters to units of pixels. Finally, if
rx or ry extend beyond the boundaries of the screen, they are wrapped around, just like in
Asteroids.
Experiment with changing scalex and scaley to see how it affects the speed of the on-screen
version of Roomba.