[英]How do I extract a portion of an image in canvas and use it as background image for a div?
这是我的代码的样子:
document.addEventListener('DOMContentLoaded', function () {
var canvas = document.querySelector('canvas');
var ctx = canvas.getContext("2d");
var imageData = ctx.getImageData(0, 0, 300, 300);
var tile = {
'id': 1,
'data': imageData,
'dataUrl': imageData.toDataUrl()
};
var div = document.createElement('div');
div.classList.add('tile');
grid.appendChild(div);
div.style.backgroundImage = ('url(' + tile.dataUrl + ')');
});
我正在尝试从(0,0)高度和宽度为300px的画布上提取图像的一部分,并将该imageData对象转换为dataUrl以用作div的背景。
我收到一个错误:imageData.toDataUrl()不是一个函数。 我怎样才能做到这一点?
提前致谢!
toDataURL
是一个HTMLCanvasElement
方法,您必须从canvas元素本身调用它。
在将大小更改为想要的大小后,您可以将生成的imageData绘制回画布,但最简单的解决方案是使用第二个离屏画布,您将在其中绘制第一个画布,这要归功于context.drawImage
方法:
// The crop function var crop = function(canvas, offsetX, offsetY, width, height, callback) { // create an in-memory canvas var buffer = document.createElement('canvas'); var b_ctx = buffer.getContext('2d'); // set its width/height to the required ones buffer.width = width; buffer.height = height; // draw the main canvas on our buffer one // drawImage(source, source_X, source_Y, source_Width, source_Height, // dest_X, dest_Y, dest_Width, dest_Height) b_ctx.drawImage(canvas, offsetX, offsetY, width, height, 0, 0, buffer.width, buffer.height); // now call the callback with the dataURL of our buffer canvas callback(buffer.toDataURL()); }; // #main canvas Part var canvas = document.getElementById('main'); var img = new Image(); img.crossOrigin = "Anonymous"; img.onload = function() { canvas.width = this.width; canvas.height = this.height; canvas.getContext('2d').drawImage(this, 0, 0); // set a little timeout before calling our cropping thing setTimeout(function() { crop(canvas, 100, 70, 70, 70, callback) }, 1000); }; img.src = "https://dl.dropboxusercontent.com/s/1alt1303g9zpemd/UFBxY.png"; // what to do with the dataURL of our cropped image var callback = function(dataURL) { document.body.style.backgroundImage = 'url(' + dataURL + ')'; }
<canvas id="main" width="284" width="383"></canvas>
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