title: Invert z-axis, when projecting in perspective
body: <p>I am creating a rhythm game. But I am not using metronome to calculate¬ the positions of the notes on the screen (game interface).  What I have is a file with notes map that is very similar to a subtitles file.</p>
  
  <p>The problem is that when I project the notes in perspective, the  notes are not shown correctly. It seems that I have inverted the Z axis and I can not change it. I would like to know how to change the axis correctly because. Or if someone can help me with other solution, I would appreciate. I tried different things but I am not capable to make appear the notes correctly.</p>
  
  <p><a href="https://jsfiddle.net/triskadek/uev2zabc/" rel="nofollow noreferrer">Here is the fiddle</a>, and the function where I execute the calculation of the perspective.</p>
  
  <pre><code>function updateNotes() {
      currentPosition = (sound.seek() * 1000);
  
      notes.forEach(function(note, index) {
      var notePosition = (currentPosition * noteSpeed) - ((note.position * noteSpeed) - deadLine);
  
      if (notePosition &gt; offScreenY &amp;&amp; notePosition &lt; height) {
      var ball = new Ball3d(5, '#000000');
      var scale = fov / (fov + notePosition);
      console.log(notePosition);
      ball.x = halfWidth;
      ball.y = halfHeight + notePosition * scale;
      ball.scaleX = ball.scaleY = scale;
      ball.draw(context);
      balls.push(ball);
    } else {
      // elimino la nota
      balls.splice(index, 1);
    }
    });
  }
  </code></pre>
  
  <p>Thanks in advance.</p>