There are 3 bugs!

• zoom needs to be a fractional data-type
• you need background() to clear the screen to draw a new scaled rect
• you need translate() to position rect in the center of screen while scaling

This is just great!
These two solutions work like charm! Thank you, Mr. GoToLoop.

Just when I change these lines of code:

1. final static float inc = .05;

2. final static short sz  = 30;

to these

1. float inc = .05;
2. short sz  = 30;
   

it runs just the same. I learnt that the reserved word "final" is just like "const" in C. It's a constant, thus the value can't be modified. Am I right?

To avoid confusion for a newbie like me, I'm doing a little tidying up for IF statements, like this:

1. if (mousePressed)
   
2.   {
   
3.     if (mouseButton == LEFT)  
   
4.     {
   
5.       zoom += inc;
   
6.     }
   
7.     else
   
8.     if (mouseButton == RIGHT) 
   
9.     {
   
10.       zoom -= inc;
    
11.     }
    
12.   }
    

This may take longer lines, but to me it's really easier to understand

Just one more question though:

What these two statements do anyway?

1. translate(width>>1, height>>1);

2. rect(width>>1, height>>1, zoom, zoom);

I know that "translate" is for changing the coordinate system and these two ">>" are shift-right operators (I remembered back then in C). But I mean, what those two lines actually do to the sketch?

" what those two lines actually do to the sketch?"
>> 1 is just a fancy way to write / 2...
Personally, if performance is the target, I prefer to make two constants to hold half-width and half-height, and it is more readable...

Personally, if performance is the target, I prefer to make two constants to hold half-width and half-height, and it is more readable...

Me too! Problem is that both width & height variables have their value known only when code reaches setup()!
Thus, it precludes creating global field variable types from it!
And creating a local variable just to be used once is not worth it in most cases, methinks!

And I was almost sure it'd be faster than a regular division operation.
Now I'm very 100% sure! Thx! 

It runs just the same. I learnt that the reserved word "final" is just like "const" in C. It's a constant, thus the value can't be modified. Am I right?

It's similar, but C's const is more "final" than Java's final is!  
And #define is even more "final" than all of them!

" Java's final is basically equivalent to a const pointer in C++. It does not provide the other features of const."

Nevertheless, when you tell the compiler that your variable won't change its value anymore, by declaring it final,
it means that the compiler is free to use any optimizations in its arsenal upon it.
The most important being inlining!

Well, I like to use bitshift fancies and having less curly brackets as possible.
But do it in the way you feel more comfortable to understand it!  

Thank you for all those wonderful feedback!