source: arduino-1-6-7/trunk/fuentes/arduino-ide-amd64/libraries/TFT/examples/Arduino/TFTPong/TFTPong.ino @ 46

/*

 TFT Pong

 This example for the Arduino screen reads the values
 of 2 potentiometers to move a rectangular platform
 on the x and y axes. The platform can intersect
 with a ball causing it to bounce.

 This example code is in the public domain.

 Created by Tom Igoe December 2012
 Modified 15 April 2013 by Scott Fitzgerald

 http://www.arduino.cc/en/Tutorial/TFTPong

 */

#include <TFT.h>  // Arduino LCD library
#include <SPI.h>

// pin definition for the Uno
#define cs   10
#define dc   9
#define rst  8

// pin definition for the Leonardo
// #define cs   7
// #define dc   0
// #define rst  1

TFT TFTscreen = TFT(cs, dc, rst);

// variables for the position of the ball and paddle
int paddleX = 0;
int paddleY = 0;
int oldPaddleX, oldPaddleY;
int ballDirectionX = 1;
int ballDirectionY = 1;

int ballSpeed = 10; // lower numbers are faster

int ballX, ballY, oldBallX, oldBallY;

void setup() {
  // initialize the display
  TFTscreen.begin();
  // black background
  TFTscreen.background(0, 0, 0);
}

void loop() {

  // save the width and height of the screen
  int myWidth = TFTscreen.width();
  int myHeight = TFTscreen.height();

  // map the paddle's location to the position of the potentiometers
  paddleX = map(analogRead(A0), 512, -512, 0, myWidth) - 20 / 2;
  paddleY = map(analogRead(A1), 512, -512, 0, myHeight) - 5 / 2;

  // set the fill color to black and erase the previous
  // position of the paddle if different from present
  TFTscreen.fill(0, 0, 0);

  if (oldPaddleX != paddleX || oldPaddleY != paddleY) {
    TFTscreen.rect(oldPaddleX, oldPaddleY, 20, 5);
  }

  // draw the paddle on screen, save the current position
  // as the previous.
  TFTscreen.fill(255, 255, 255);

  TFTscreen.rect(paddleX, paddleY, 20, 5);
  oldPaddleX = paddleX;
  oldPaddleY = paddleY;

  // update the ball's position and draw it on screen
  if (millis() % ballSpeed < 2) {
    moveBall();
  }
}

// this function determines the ball's position on screen
void moveBall() {
  // if the ball goes offscreen, reverse the direction:
  if (ballX > TFTscreen.width() || ballX < 0) {
    ballDirectionX = -ballDirectionX;
  }

  if (ballY > TFTscreen.height() || ballY < 0) {
    ballDirectionY = -ballDirectionY;
  }

  // check if the ball and the paddle occupy the same space on screen
  if (inPaddle(ballX, ballY, paddleX, paddleY, 20, 5)) {
    ballDirectionX = -ballDirectionX;
    ballDirectionY = -ballDirectionY;
  }

  // update the ball's position
  ballX += ballDirectionX;
  ballY += ballDirectionY;

  // erase the ball's previous position
  TFTscreen.fill(0, 0, 0);

  if (oldBallX != ballX || oldBallY != ballY) {
    TFTscreen.rect(oldBallX, oldBallY, 5, 5);
  }


  // draw the ball's current position
  TFTscreen.fill(255, 255, 255);
  TFTscreen.rect(ballX, ballY, 5, 5);

  oldBallX = ballX;
  oldBallY = ballY;

}

// this function checks the position of the ball
// to see if it intersects with the paddle
boolean inPaddle(int x, int y, int rectX, int rectY, int rectWidth, int rectHeight) {
  boolean result = false;

  if ((x >= rectX && x <= (rectX + rectWidth)) &&
      (y >= rectY && y <= (rectY + rectHeight))) {
    result = true;
  }

  return result;
}