html, body {
  margin: 0;
  padding: 0;
  width: 100%;
  height: 100%;
  overflow: hidden; /* スクロールなし */
  background-color: #000;
}

.slider {
  position: relative;
  width: 100%;
  height: 100%;
}

.slide {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  opacity: 0;
  transition: opacity 1.5s ease-in-out;
  width: auto;
  height: 100vh; /* 縦100% */
}

.slide img {
  height: 100vh;
  width: auto; /* 縦横比維持 */
  display: block;
}

.active {
  opacity: 1;
}