.slider {
  height: 300px;
  display: block;
  perspective: 2500px;
  position: relative;
  width: 100%;
}
.slider img {
  display: inline-block;
  width: 470px;
  position: absolute;
  backface-visibility: hidden;
  transition: .4s all;
  -webkit-filter: grayscale(70%);
  filter: grayscale(70%);
}
.slider img.activegallery {
  z-index: 4;
  -webkit-filter: none;
  filter: none;
}