var imageCount;
var firstVisibleImage = 0;
var imageWidth = 121;

var elemStyle;
var increment = 5;
var interval = 5;  // milisegundos
var intervalId = null;
var pos1;
var pos2;

function initCarrusel(count) {
  imageCount = count;
  elemStyle = document.getElementById('dynamic').style;
  elemStyle.width = ((imageCount+4)*imageWidth) + 'px';
}
function rotateRight() {
  if (intervalId != null)
    return;
  if (firstVisibleImage == 0)
    firstVisibleImage = imageCount;  // esta es la repetición de la 1a imagen
  pos1 = -firstVisibleImage*imageWidth;
  pos2 = pos1 + imageWidth;
  pos1 += imageWidth % increment;
  firstVisibleImage--;
  intervalId = setInterval(doRotationRight, interval);
}
function rotateLeft() {
  if (intervalId != null)
    return;
  pos1 = -firstVisibleImage*imageWidth;
  pos2 = pos1 - imageWidth;
  pos1 -= imageWidth % increment;
  firstVisibleImage = (firstVisibleImage + 1) % imageCount;
  intervalId = setInterval(doRotationLeft, interval);
}
function doRotationRight(){elemStyle.left=pos1+'px';if(pos1<pos2)pos1+=increment;else{clearInterval(intervalId);intervalId=null}}
function doRotationLeft(){elemStyle.left=pos1+'px';if(pos1>pos2)pos1-=increment;else{clearInterval(intervalId);intervalId=null}}