01make-symmetry-image.css

.file-input {
  display: block;
  margin: 16px 0;
}
#message-box {
  padding-left: 12px;
  color: red;
}
#preview-image {
  width: 100px;
}
/*  */
#slide-container {
  box-sizing: border-box;
  height: 85vh;
  position: relative;
  border: 1px solid #9e9e9e;
  user-select: none;
}
.slide-unit {
  display: flex;
  align-items: center;
  width: 100%;
  height: 100%;
  position: absolute;
  top: 0;
  left: 0;
  overflow: hidden;
  background-color: #fff;
}
#slideA {
  width: 50%;
  z-index: 2;
}
#slideB {
  z-index: 1;
}
#slidebar {
  box-sizing: border-box;
  width: 4px;
  height: 100%;
  position: absolute;
  z-index: 3;
  border: #fff solid;
  border-width: 0 1px;
  background-color: #616161;
  cursor: col-resize;
}
.slide-btn {
  position: absolute;
  bottom: 4px;
  z-index: 5;
  cursor: pointer;
  user-select: none;
}
#slide-left-btn {
  left: 4px;
}
#slide-right-btn {
  right: 4px;
}

/*  */
.result {
  display: flex;
  flex-direction: column;
  row-gap: 20px;
}
.result__row {
  display: flex;
  flex-direction: column;
  row-gap: 8px;
  align-items: flex-start;
}
.canvas {
  object-fit: contain;
  width: 100%;
  max-width: 500px;
  height: 300px;
}

02make-symmetry-image.html

<h2>概要</h2>
<p>1枚の画像から、間に鏡を置いたような左右対称の画像を作成するツール。</p>

<h2>本体</h2>
<h3>画像読み込み</h3>
<input type="file" id="file-input" />
<div id="message-box"></div>
<div class="preview-box">
  <img src="" alt="" id="preview-image" />
</div>

<h3>分割点設定</h3>
<div id="slide-container">
  <div id="draft-image-container" class="slide-unit">
    <img src="" alt="" id="draft-image" draggable="false" />
  </div>
  <div id="slidebar"></div>
  <button id="slide-left-btn" class="slide-btn">←</button>
  <button id="slide-right-btn" class="slide-btn">→</button>
</div>

<h3>結果</h3>
<div class="result">
  <div class="result__row">
    <canvas id="canvas1" class="canvas"></canvas>
    <button id="dl-btn1">保存</button>
  </div>
  <div class="result__row">
    <canvas id="canvas2" class="canvas"></canvas>
    <button id="dl-btn2">保存</button>
  </div>
</div>

03make-symmetry-image.js

'use strict';
(() => {
  // @ts-ignore
  const nowloading = new nowLoading();
  nowloading.start();
  //画像読み込み
  const fileInput = document.getElementById('file-input');
  const messageBox = document.getElementById('message-box');
  const previewImage = document.getElementById('preview-image');
  fileInput.addEventListener('change', (e) => {
    checkFile(fileInput.files);
  });
  //スライド比較
  const slideContainer = document.getElementById('slide-container');
  const draftImageContainer = document.getElementById(
    'draft-image-container'
  );
  const draftImage = document.getElementById('draft-image');
  const slidebar = document.getElementById('slidebar');
  let IsSlidebarMoving = false;
  let slidebarX;
  draftImage.onload = () => {
    resizeImage(draftImage);
  };
  slidebar.style.transform = `translateX(${
    slideContainer.clientWidth / 2
  }px)`;
  slidebar.addEventListener('pointerdown', slidebarMoveStart);
  slideContainer.addEventListener('pointermove', slidebarMove);
  slideContainer.addEventListener('pointerup', slidebarMoveEnd);
  slideContainer.addEventListener('pointerleave', slidebarMoveEnd);
  const slideLeftBtn = document.getElementById('slide-left-btn');
  const slideRightBtn = document.getElementById('slide-right-btn');
  slideLeftBtn.addEventListener('pointerdown', () => {
    slideStart(-1);
  });
  slideLeftBtn.addEventListener('pointerup', () => {
    slideEnd();
  });
  slideLeftBtn.addEventListener('pointerleave', () => {
    slideEnd();
  });
  slideRightBtn.addEventListener('pointerdown', () => {
    slideStart(1);
  });
  slideRightBtn.addEventListener('pointerup', () => {
    slideEnd();
  });
  slideRightBtn.addEventListener('pointerleave', () => {
    slideEnd();
  });
  const canvas1 = document.getElementById('canvas1');
  const ctx1 = canvas1.getContext('2d');
  const canvas2 = document.getElementById('canvas2');
  const ctx2 = canvas2.getContext('2d');
  const dlBtn1 = document.getElementById('dl-btn1');
  const dlBtn2 = document.getElementById('dl-btn2');
  dlBtn1.addEventListener('click', () => {
    downloadImage(0);
  });
  dlBtn2.addEventListener('click', () => {
    downloadImage(1);
  });
  //サンプル画像読み込み
  draftImage.src = '../img/sample01.jpg';
  nowloading.stop();
  //関数
  function checkFile(files) {
    messageBox.textContent = '';
    if (files === null) return;
    if (files.length == 0) {
      return;
    }
    previewImage.src = '';
    if (files[0].type.indexOf('image') == -1) {
      fileInput.value = '';
      messageBox.textContent = '画像ファイルを選択してください。';
      return;
    }
    const reader = new FileReader();
    reader.onload = (ev) => {
      const src = reader.result;
      previewImage.src = src;
      draftImage.src = src;
    };
    reader.readAsDataURL(files[0]);
  }
  function resizeImage(img) {
    const outerW = slideContainer.clientWidth;
    const outerH = slideContainer.clientHeight;
    const imgW = img.naturalWidth;
    const imgH = img.naturalHeight;
    const outerRate = outerH / outerW;
    const imgRate = imgH / imgW;
    if (outerRate >= imgRate) {
      //width合わせ
      const new_h = (outerW / imgW) * imgH;
      img.style.width = outerW + 'px';
      img.style.height = new_h + 'px';
      img.style.transform = '';
    } else {
      //height合わせ
      const new_w = (outerH / imgH) * imgW;
      img.style.width = new_w + 'px';
      img.style.height = outerH + 'px';
      img.style.transform = `translateX(${(outerW - new_w) / 2}px)`;
    }
    slidebarX = outerW / 2;
    slidebar.style.transform = `translateX(${slidebarX}px)`;
    drawImage();
  }
  //
  function slidebarMoveStart() {
    IsSlidebarMoving = true;
  }
  function slidebarMove(e) {
    if (!IsSlidebarMoving) return;
    const d =
      e.pageX -
      (window.scrollX + slideContainer.getBoundingClientRect().left);
    const x = (() => {
      if (d < 0) {
        return 0;
      } else if (d > slideContainer.clientWidth) {
        return slideContainer.clientWidth;
      } else {
        return d;
      }
    })();
    slidebarX = x;
    slidebar.style.transform = `translateX(${slidebarX}px)`;
    drawImage();
  }
  function slidebarMoveEnd() {
    IsSlidebarMoving = false;
  }
  let slidetimer;
  function slideStart(diff) {
    slidetimer = setInterval(() => {
      const d = slidebarX + diff;
      const x = (() => {
        if (d < 0) {
          return 0;
        } else if (d > slideContainer.clientWidth) {
          return slideContainer.clientWidth;
        } else {
          return d;
        }
      })();
      slidebarX = x;
      slidebar.style.transform = `translateX(${slidebarX}px)`;
      drawImage();
    }, 10);
  }
  function slideEnd() {
    clearInterval(slidetimer);
  }
  function drawImage() {
    const actualX =
      (draftImageContainer.clientWidth - draftImage.clientWidth) / 2;
    const actualRate = draftImage.naturalWidth / draftImage.clientWidth;
    const leftImageWidth = Math.round(
      (slidebarX - actualX) * actualRate
    );
    canvas1.width = leftImageWidth * 2;
    canvas1.height = draftImage.naturalHeight;
    ctx1.clearRect(0, 0, canvas1.width, canvas1.height);
    ctx1.save();
    ctx1.drawImage(
      draftImage,
      0,
      0,
      leftImageWidth,
      draftImage.naturalHeight,
      0,
      0,
      leftImageWidth,
      canvas1.height
    );
    ctx1.scale(-1, 1);
    ctx1.drawImage(
      draftImage,
      0,
      0,
      leftImageWidth,
      draftImage.naturalHeight,
      -leftImageWidth * 2,
      0,
      leftImageWidth,
      canvas1.height
    );
    ctx1.restore();
    //
    const rightImageWidth = draftImage.naturalWidth - leftImageWidth;
    canvas2.width = rightImageWidth * 2;
    canvas2.height = draftImage.naturalHeight;
    ctx2.clearRect(0, 0, canvas2.width, canvas2.height);
    ctx2.save();
    ctx2.drawImage(
      draftImage,
      leftImageWidth,
      0,
      rightImageWidth,
      draftImage.naturalHeight,
      rightImageWidth,
      0,
      rightImageWidth,
      canvas2.height
    );
    ctx2.scale(-1, 1);
    ctx2.drawImage(
      draftImage,
      leftImageWidth,
      0,
      rightImageWidth,
      draftImage.naturalHeight,
      -rightImageWidth,
      0,
      rightImageWidth,
      canvas2.height
    );
    ctx2.restore();
  }
  function downloadImage(canvasId) {
    nowloading.start();
    const canvases = [canvas1, canvas2];
    canvases[canvasId].toBlob((blob) => {
      if (blob === null) {
        nowloading.stop();
        return;
      }
      const alink = document.createElement('a');
      alink.download = 'download.png';
      alink.href = URL.createObjectURL(blob);
      alink.click();
      URL.revokeObjectURL(alink.href);
      nowloading.stop();
    }, 'image/png');
  }
})();