(6) A moving paddle

Remember that link I gave you to the breakout game? Well, now we have a bouncing ball. What we need is a paddle that we can move to the left and right on the screen. This is simply added by using the "rect()" command. This command, like the ellipse() takes four inputs:

 rect(x position, y position, width, height)

But what we have not pointed out is what the x,y position is referencing to. For a ellipse the default is to the middle/center of the ellipse, but for a rectangle it is the upper left corner. One can change this reference for these types of objects with a command:

  rectMode(CENTER);

This will now make the x,y reference the center of the rectangle. It makes it a little easier to deal with the position of the obejects, I think.

to detect key presses, we can use some built in variables in the draw() routing. They are

• keyPressed - set to 1 (TRUE) when any key is pressed
• keyCode - Set to special keys that is not a sign, like arrow left (LEFT)
• key - Set to the key pressed, like 'A' or 'a' (note upper/lower case is not same)

This can now be put together to this program:

// Run once at start

void setup() {
  // 500 pixels wide, 400 pixel height
  // This call will set system variables (width, height)
  size(500, 400);
  rectMode(CENTER);
}

// Variables to keep track of paddle
int x_paddle = 250, y_paddle = 370;
int paddle_width_half = 40;

// Called every re-draw, defaul 30 times per second
void draw() {

  // Check if keys are pressed
  if (keyPressed) {
    if (keyCode == RIGHT || key == 'd') {
      // Move paddle right
      x_paddle = x_paddle + 8;
    } else if (keyCode == LEFT || key == 'a') {
      // Move paddle left
      x_paddle = x_paddle - 8;
    }
  }

  // Clear screen to black
  background(0);
  // Set fill color to white
  fill(255);
  
  // draw paddle
  rect(x_paddle, y_paddle, paddle_width_half*2+1, 11);
}

Enter it, or copy paste, and run:

Try moving it with the left/right arrow. Personally I think we need to add so that one cannot go outside the screen. Adding this after the checking of keys will do that.

  if (x_paddle>width) x_paddle = width;
  if (x_paddle<0) x_paddle = 0;