Learning Processing: A Beginner's Guide to Programming Images ...

194 Learning Processing
Let’s say the result is:
x: 9000000 y: – 900000
x: 9000116 y: – 901843
x: 9000184 y: – 902235
x: 9000299 y: – 903720
x: 9000682 y: – 904903
It is pretty obvious that these values are not reasonable pixel coordinates. So something would be off in
the way the object is calculating its ( x , y ) location. However, if the values were perfectly reasonable, then
you would move on. Maybe the color is the problem?
println( " brightness: " + brightness(thing.col) + " alpha: " + alpha(thing.col));
Resulting in:
brightness: 150.0 alpha: 0.0
Well, if the alpha value of the object’s color is zero, that would explain why you can’t see it! We should
take a moment here to remember Tip #3: Simplify. Th is process of printing variable values will be much
more eff ective if we are doing it in a sketch that only deals with the Th ing object. Th is way, we can be sure
that it is not another class which is, say, drawing over the top of the Th ing by accident.
You may have also noticed that the above print statements concatenate actual text with the variables.
(See Chapter 17 for more on concatenation.) It is generally a good idea to do this. Th e following line of
code only prints the value of x , with no explanation.
println(x);
Th is can be confusing to follow in the message window, especially if you are printing diff erent values in
diff erent parts of the code. How do you know what is x and what is y ? If you include your own notes in
println( ) , there can’t be any confusion:
println( " The x value of the thing I’m looking for is: " + x);
In addition, println( ) can be used to indicate whether or not a certain part of the code has been reached.
For example, what if in our “ bouncing ball ” example, the ball never bounces off of the right-hand side of
the window? Th e problem could be (a) you are not properly determining when it hits the edge, or (b) you
are doing the wrong thing when it hits the edge. To know if your code correctly detects when it hits the
edge, you could write:
if (x > width) {
println( " X is greater than width. This code is happening now! " );
xspeed * = – 1;
}
If you run the sketch and never see the message printed, then something is probably fl awed with your
boolean expression.
Admittedly, println( ) is not a perfect debugging tool. It can be hard to track multiple pieces of
information with the message window. It can also slow your sketch down rather signifi cantly (depending
on how much printing you are doing). More advanced development environments usually off er
debugging tools which allow you to track specifi c variables, pause the program, advance line by line in
the code, and so on. Th is is one of the trade-off s we get using Processing . It is much simpler to use, but
it does not have all of the advanced features. Still, in terms of debugging, some sleep, a little common
sense, and println( ) can get you pretty far.