trajectoryPoints.js

    function line(x1, y1, x2, y2) {
        game.debug.context.beginPath();
        game.debug.context.moveTo(x1, y1);
        game.debug.context.lineTo(x2, y2);
        game.debug.context.stroke();
    }   
   
   
    function getTrajectoryPoint(startX, startY, velocityX, velocityY, n) {

        //velocity and gravity are given per second but we want time step values here
        var t = 1 / 60.0; // seconds per time step (at 60fps)
        
        var stepVelocityX = t * game.physics.box2d.pxm( -velocityX ); // m/s
        var stepVelocityY = t * game.physics.box2d.pxm( -velocityY );
        
        var stepGravityX = t * t * game.physics.box2d.pxm( -game.physics.box2d.gravity.x ); // m/s/s
        var stepGravityY = t * t * game.physics.box2d.pxm( -game.physics.box2d.gravity.y );

        startX = game.physics.box2d.pxm(-startX);
        startY = game.physics.box2d.pxm(-startY);
        
        var tpx = startX + n * stepVelocityX + 0.5 * (n*n+n) * stepGravityX;
        var tpy = startY + n * stepVelocityY + 0.5 * (n*n+n) * stepGravityY;
        
        tpx = game.physics.box2d.mpx(-tpx);
        tpy = game.physics.box2d.mpx(-tpy);
        
        return { x: tpx, y: tpy };
    }
    
    function render() {
        var launchVelocity = getLaunchVelocity();
        game.debug.start();

        var lastPos = null;    

        for (var i = 0; i < 180; i += 6)
        {
            var trajectoryPoint = getTrajectoryPoint(launchX, launchY, launchVelocity.x, launchVelocity.y, i);

            if (lastPos && i % 12 == 0)
            {
                line(lastPos.x, lastPos.y, trajectoryPoint.x, trajectoryPoint.y);
            }

            lastPos = trajectoryPoint;
        }

        game.debug.stop();
        //
    }