.primary{
  position: absolute;
  /*opacity: 1;*/
  z-index: 2;
  /*transition: opacity 1s linear;*/
}
.secondary{
  position: absolute;
  /*opacity: 0;*/
  transform: rotateY(180deg);
  /*transition: opacity 1s linear;*/
}
/*.primary:hover{
  opacity: 0;
}
.secondary:hover{
  opacity: 1;
}*/
.portfolio{
    position:relative;
    perspective:1000;
}
.portfolio-item{
    width:200px;
    height:291px;
    position:relative;
    float:left;
    margin-left:5px;
    transition: all 2s ease;
    transform-style: preserve-3d;
}
.portfolio-item:hover{
    transform: rotateY(180deg);
}