H5的video第一帧预览图截取

 var videoElement = $("video")[0];
           videoElement.addEventListener("canplay", function (_event) {
               var canvas = document.createElement("canvas");
               canvas.width = videoElement.videoWidth;
               canvas.height = videoElement.videoHeight;

               canvas.getContext('2d').drawImage(videoElement, 0, 0, canvas.width, canvas.height);

               img.src = canvas.toDataURL("image/png");
               $("video").attr('poster',img.src);

               URL.revokeObjectURL(this.src); // 释放createObjectURL创建的对象