int r = 10;
float x0 = r, y0 = r;
float g = 9.8;
float v0 = 30;
float theta = PI/8;

float t = 0;
float x = x0;
float y = y0; 

void setup()
{
  size(300,300);
  colorMode(RGB,255,255,255,100);
  ellipseMode(RADIUS);
  background(0,0,0);
  frameRate(50);
}

void draw()
{
 stroke( 0, 0, 0, 1 );
 fill( 0, 0, 0, 1 );
 rect( 0, 0, width, height );

 x = x0 + v0*cos(theta)*t;
 y = y0 + v0*sin(theta)*t+g*t*t/2.0;

 stroke( 0, 0, 100, 100 );
 fill( 0, 200, 255, 100 );
 ellipse( x, y, 10, 10 );
 t = t + 0.1;
 
 if( y > height-r )
 {
   float vx0 = v0*cos(theta);
   float vy0 = v0*sin(theta)+g*t;
   vy0 = -vy0 * 0.8;
   v0 = sqrt( vx0*vx0 + vy0*vy0 );
   theta = atan( vy0/vx0 );
   if( vx0 < 0 ) theta = theta + PI; 

   x0 = x;
   y0 = height-r;
   t = 0;
 }
 
 if( x > width-r )
 {
   float vx0 = v0*cos(theta);
   float vy0 = v0*sin(theta)+g*t;
   v0 = sqrt( vx0*vx0 + vy0*vy0 );
   theta = PI-atan( vy0/vx0 );

   x0 = width-r;
   y0 = y;
   t = 0;
 }

 if( x < 0 )
 {
   float vx0 = v0*cos(theta);
   float vy0 = v0*sin(theta)+g*t;
   v0 = sqrt( vx0*vx0 + vy0*vy0 );
   theta = -atan( vy0/vx0 );

   x0 = 0;
   y0 = y;
   t = 0;
 }
}