如何在画布后面与HTML交互？

Question

情景：
我有一个网站，目前在本地托管，我希望能够在网站顶部的画布上绘图，如下图所示。但是，我不仅要允许用户在网站上绘图，还要在用户与该页面的其余部分进行交互时将图形保留在该特定网页上。例如。填写表格或与下拉菜单交互等。 NB： HTML Canvas涵盖整个页面（不仅仅是视口）。此外，还有一个用于区分绘图和与HTML交互的按钮



具体问题： 如果存在画布，如何与网页上的普通HTML元素进行交互。我知道画布将在页面之间重置，这很好 - 我不是要解决这个问题。我只是想解决一个页面。

尝试的方法
我尝试过使用透明度，zIndex和其他css属性。

替代解决方案
1.我知道这可以使用SVG解决，只需将它们覆盖在整个网页上的透明DIV中即可。但是如果可能的话，我宁愿不使用这种方法并坚持使用画布寻找解决方案。

2.目前HTML Canvas覆盖整个页面（不仅仅是视口），所以我想我可以根据页面上的绘制量来动态调整大小。但是，我不妨只使用SVG。

思考？我对libs / snippets一无所知。

Answer 1

解决此问题的一种方法是：

• 使用CSS
将默认点事件设置为无
• 为要用于点击等的元素启用指针事件，包括事件捕手（背景，正文或元素）
• 根据您的设计需要使用正文或元素，作为上述消费事件的事件捕捉者。
• 将画布放在指针事件设置为无
的所有内容之上

任何可点击元素（或您想要用于事件的元素）所消耗的点击现在都会在后面结束。只需将坐标从此背衬元素转移到画布即可绘制。

演示

演示不是完美的（即，不会考虑原生触摸事件/手势，它会画点而不是线条等）但它应该足以显示基本原理并让你前进。

我们只是在这里使用body来捕捉任何没有被消耗的事件。向下滚动以绘制并单击文本中的第二个链接。

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var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var isDown = false;

setup();
window.onresize = setup;
window.onmousedown = function(e) {
  isDown = true;
  draw(e);
};
window.onmousemove = function(e) {if (isDown) draw(e);};
window.onmouseup = function() {isDown = false};

function setup() {
  canvas.width = window.innerWidth;
  canvas.height = parseInt(getComputedStyle(document.body).getPropertyValue("height"), 10);
  ctx.fillStyle = "rgba(0,160,255,0.4)";
}

function draw(e) {
  var rect = canvas.getBoundingClientRect();
  ctx.beginPath();
  ctx.arc(e.clientX - rect.left, e.clientY - rect.top, 15, 0, 6.28);
  ctx.fill();
}

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* { /* optimize by being more specific, here just for demo */
  pointer-events:none;
  }
a {
  color:#00c;
  pointer-events:auto;
  cursor:pointer;
  text-decoration:underline;
  }
html, body {
  height:3500px;
  width:100%;
  }
body {
  pointer-events:auto;
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
  }
#poster {
  width:80%;
  margin:50px auto 0 auto;
  background:rgba(0,0,0,0.2);
  padding:20px;
  box-sizing:border-box;
  z-index:1;
  }
#canvas {
  position:absolute;
  left:0;
  top:0;
  z-index:1000;
  }

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<h1>Demonstration</h1>

<div id="poster">
  <h2>Drawable page</h2>
  <a onclick="alert('Was clicked');return false">Click me</a>
</div>

<h3>The standard Lorem Ipsum passage, used since the 1500s</h3><p>"Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."</p><h3>Section 1.10.32 of "de Finibus Bonorum et Malorum", written by Cicero in 45 BC</h3><p>"Sed ut perspiciatis unde omnis iste natus error sit voluptatem accusantium doloremque laudantium, totam rem aperiam, eaque ipsa quae ab illo inventore veritatis et quasi architecto beatae vitae dicta sunt explicabo. Nemo enim ipsam voluptatem quia voluptas sit aspernatur aut odit aut fugit, sed quia consequuntur magni dolores eos qui ratione voluptatem sequi nesciunt. Neque porro quisquam est, qui dolorem ipsum quia dolor sit amet, consectetur, adipisci velit, sed quia non numquam eius modi tempora incidunt ut labore et dolore magnam aliquam quaerat voluptatem. Ut enim ad minima veniam, quis nostrum exercitationem ullam corporis suscipit laboriosam, nisi ut aliquid ex ea commodi consequatur? Quis autem vel eum iure reprehenderit qui in ea voluptate velit esse quam nihil molestiae consequatur, vel illum qui dolorem eum fugiat quo voluptas nulla pariatur?"</p>
<h3>1914 translation by H. Rackham</h3>
<p>"But I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system, and expound the actual teachings of the great explorer of the truth, the master-builder of human happiness. No one rejects, dislikes, or avoids pleasure itself, because it is pleasure, but because those who do not know how to pursue pleasure rationally encounter consequences that are extremely painful. Nor again is there anyone <b><a onclick="alert('yeehaw');return false">who loves</a></b> or pursues or desires to obtain pain of itself, because it is pain, but because occasionally circumstances occur in which toil and pain can procure him some great pleasure. To take a trivial example, which of us ever undertakes laborious physical exercise, except to obtain some advantage from it? But who has any right to find fault with a man who chooses to enjoy a pleasure that has no annoying consequences, or one who avoids a pain that produces no resultant pleasure?"</p>
<h3>Section 1.10.33 of "de Finibus Bonorum et Malorum", written by Cicero in 45 BC</h3>
<p>"At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium voluptatum deleniti atque corrupti quos dolores et quas molestias excepturi sint occaecati cupiditate non provident, similique sunt in culpa qui officia deserunt mollitia animi, id est laborum et dolorum fuga. Et harum quidem rerum facilis est et expedita distinctio. Nam libero tempore, cum soluta nobis est eligendi optio cumque nihil impedit quo minus id quod maxime placeat facere possimus, omnis voluptas assumenda est, omnis dolor repellendus. Temporibus autem quibusdam et aut officiis debitis aut rerum necessitatibus saepe eveniet ut et voluptates repudiandae sint et molestiae non recusandae. Itaque earum rerum hic tenetur a sapiente delectus, ut aut reiciendis voluptatibus maiores alias consequatur aut perferendis doloribus asperiores repellat."</p>
<h3>1914 translation by H. Rackham</h3>
<p>"On the other hand, we denounce with righteous indignation and dislike men who are so beguiled and demoralized by the charms of pleasure of the moment, so blinded by desire, that they cannot foresee the pain and trouble that are bound to ensue; and equal blame belongs to those who fail in their duty through weakness of will, which is the same as saying through shrinking from toil and pain. These cases are perfectly simple and easy to distinguish. In a free hour, when our power of choice is untrammelled and when nothing prevents our being able to do what we like best, every pleasure is to be welcomed and every pain avoided. But in certain circumstances and owing to the claims of duty or the obligations of business it will frequently occur that pleasures have to be repudiated and annoyances accepted. The wise man therefore always holds in these matters to this principle of selection: he rejects pleasures to secure other greater pleasures, or else he endures pains to avoid worse pains."</p>

<canvas id="canvas"></canvas>

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