javascript - 在 Canvas 中绘制裁剪后的图像

Question

如何获取图像的正确位置并将我在镜头中看到的内容绘制到 Canvas 中？ 这是我的演示示例。

var isDown,
  lens,
  img,
  canvas,
  cx,
  cy,
  ctx,
  image;

img = document.getElementById('img');
lens = document.getElementById('lens');
canvas = document.getElementById('canvas');
cx = canvas.offsetWidth / lens.offsetWidth;
cy = canvas.offsetHeight / lens.offsetHeight;

ctx = canvas.getContext('2d');
image = new Image();
image.src = img.src;

lens.addEventListener("mousemove", (e) => {
  moveLens(e);
});

img.addEventListener("mousemove", (e) => {
  moveLens(e);
});


function moveLens(e) {
  var pos, x, y;
  e.preventDefault();
  pos = getCursorPos(e);
  x = pos.x - (lens.offsetWidth / 2);
  y = pos.y - (lens.offsetHeight / 2);
  if (x > img.width - lens.offsetWidth) {
    x = img.width - lens.offsetWidth;
  }
  if (x < 0) {
    x = 0;
  }
  if (y > img.height - lens.offsetHeight) {
    y = img.height - lens.offsetHeight;
  }
  if (y < 0) {
    y = 0;
  }
  lens.style.left = x + "px";
  lens.style.top = y + "px";
  draw(x, y);
}

function getCursorPos(e) {
  var a, x = 0,
    y = 0;
  e = e || window.event;
  a = img.getBoundingClientRect();
  x = e.pageX - a.left;
  y = e.pageY - a.top;
  x = x - window.pageXOffset;
  y = y - window.pageYOffset;
  return {
    x: x,
    y: y
  };
}

function draw(x, y) {
  ctx.drawImage(image, x, y, canvas.width, canvas.height, 0, 0, canvas.width, canvas.height);
}

#container {
  position: relative;
  width: 300px;
  display:inline-block;
}

#container>img {
  max-width: 100%;
}

#lens {
  position: absolute;
  border: 1px solid #d4d4d4;
  width: 80px;
  height: 80px;
}

<div id="container">
  <div id="lens"></div>
  <img src="https://cdn.pixabay.com/photo/2016/06/18/17/42/image-1465348_960_720.jpg" id="img">
</div>
<canvas width="200" height="200" id="canvas"></canvas>

Answer 1

ctx.drawImage(图像, sx, sy, sWidth, sHeight, dx, dy, dWidth, dHeight)

sx,sy,sWidth,sHeight是根据源码image<测量的。假设我们有一张HIGH 分辨率的图像，例如 2000 * 1000，那么 sWidth 和 sHeight 应该按比例放大到该分辨率.在您的情况下，sWidth 应为 80/300 * 2000。

我已经对您的代码进行了这些调整。

function draw(x, y) {
  ctx.clearRect(0, 0, canvas.width, canvas.height); // you might want to draw on a clean canvas each time
  let scaleX = x / img.offsetWidth * image.width;
  let scaleY = y / img.offsetHeight * image.height;
  let scaleWidth = lens.offsetWidth / img.width * image.width;
  let scaleHeight = lens.offsetHeight / img.height * image.height;
  ctx.drawImage(image, scaleX, scaleY, scaleWidth, scaleHeight, 0, 0, canvas.width, canvas.height);
}