图片合成技术

.clip {
    position: absolute;
    clip: rect(0 0 0 0);
}
HTML代码：
<input type="file" id="uploadFile" class="clip" accept="image/*">
<label class="ui-button ui-button-primary" for="uploadFile">选择图片</label>
<img id="imgCover" src="./watermark.png" class="clip">
<p id="imgUploadX"></p>
JS代码：
var eleUploadFile = document.getElementById('uploadFile');
var eleImgCover = document.getElementById('imgCover');
var eleImgUploadX = document.getElementById('imgUploadX');

if (history.pushState) {
    eleUploadFile.addEventListener('change', function (event) {
        var reader = new FileReader();
        var file = event.target.files[0] || event.dataTransfer.files[0];
        reader.onload = function(e) {
          var base64 = e.target.result;
          if (base64.length > 1024 * 50) {
              alert('图片尺寸请小于50K');
              return;
          } else {
              // 使用canvas合成图片，并base64化
              imgTogether(base64, function (url) {
                  // 尺寸
                  var size = 180 / (window.devicePixelRatio || 1);
                  // 预览
                  eleImgUploadX.innerHTML = '<img src="'+ url +'" width="'+ size +'" height="'+ size +'">';
              });
          }
        };
        reader.readAsDataURL(file);
    });
    
    // canvas图片合成
    var imgTogether = function (url, callback) {
        var canvas = document.createElement('canvas');
        var size = 180;
        canvas.width = size;
        canvas.height = size;

        var context = canvas.getContext('2d');

        // 这是上传图像
        var imgUpload = new Image();
        imgUpload.onload = function () {
            // 绘制
            context.drawImage(imgUpload, 0, 0, size, size, 0,0, size, size);
            // 再次绘制
            context.drawImage(eleImgCover, 0, 0, size, size, 0,0, size, size);
            // 回调
            callback(canvas.toDataURL('image/png'));
        };
        imgUpload.src = url;
    };
} else if (eleImgUploadX) {
    eleImgUploadX.className = 'remind';
    eleImgUploadX.innerHTML = '本演示IE10+下才有效果';
}

　　demo引用地址http://www.zhangxinxu.com/study/201705/js-canvas-image-watermark-synthesis.html