使用javascript将画布添加到页面

Question

我正在尝试使用Javascript将画布添加到一个原本没有的画面。 我正在尝试执行以下操作：

var canv=document.createElement("canvas");
canv.setAttribute("id", "canvasID");
alert(canv.id);
var c=document.getElementById("canvasID");
alert(c.id);

问题是第一个警报（canv.id）导致canvasID，而第二个警报未定义，因为c为null。

谁能告诉我我做错了什么？

PS：代码设计为在Greasemonkey下运行，因此在HTML本身中添加画布及其ID不是一个可行的选择。

Answer 1

使用Node.appendChild( child )类的东西将它添加到DOM：

var canv = document.createElement('canvas');
canv.id = 'someId';

document.body.appendChild(canv); // adds the canvas to the body element
document.getElementById('someBox').appendChild(canv); // adds the canvas to #someBox

---

或者你可以使用element.innerHTML ：

document.body.innerHTML += '<canvas id="someId"></canvas>'; // the += means we add this to the inner HTML of body
document.getElementById('someBox').innerHTML = '<canvas id="someId"></canvas>'; // replaces the inner HTML of #someBox to a canvas

Answer 2

    var canvas = document.getElementById('canvas'); //finds Original Canvas
    img = document.createElement('img'); 
    img.src = 'images/a.jpg'; //stores image src

    var canv = document.createElement('canvas'); // creates new canvas element
    canv.id = 'canvasdummy'; // gives canvas id
    canv.height = canvas.height; //get original canvas height
    canv.width = canvas.width; // get original canvas width
    document.body.appendChild(canv); // adds the canvas to the body element

    var canvas1 = document.getElementById('canvasdummy'); //find new canvas we created
    var context = canvas1.getContext('2d');

    context.drawImage(img, 0, 0, canvas.width, canvas.height); //draws background image
    context.drawImage(canvas, 0, 0); //draws original canvas on top of background
    cscreen = canvas1.toDataURL(); //generates PNG of newly created canvas
    document.body.removeChild(canv); // removes new canvas

我一直使用这个并且工作得很好......

Answer 3

var canv=document.createElement("canvas");
canv.setAttribute("id", "canvasID");
document.body.appendChild(canv);

没有像第三行那样的东西，你的新画布实际上从未插入到页面中。