相关疑难解决方法(0)

如何检查画布上是否有数据或某些内容？

我有这个<canvas id="my-canvas"></canvas>元素,我想检查我的画布上是否画了一些东西.

jSignature有画布,它有一个类.如何验证jSignature我是否画了一些东西？

我为click事件添加了一个绑定.

$sigdiv.bind('click', function(e) {
    $("#num_strok").val(parseInt($("#num_strok").val()) + 1);
});

问题是我点击某个角落也会num_strock增加.对于一些拖累它不会增加.

我曾在谷歌尝试过是否有内置isEmpty功能.但我还没有找到任何东西.

我的网站需要以下功能。一切都必须在客户端（javascript 或任何 javascript 库）上完成。

我的本地计算机中有一个图像，将其拖放到浏览器中。在没有向服务器发出任何请求的情况下，JavaScript 必须将此图像转换为 Base64。

我有一个在客户端将图像转换为 base64 的代码，但这需要 HTTP URL。我需要的是，图像需要从本地上传。

function toDataURL(url, callback) {
  var xhr = new XMLHttpRequest();
  xhr.onload = function() {
    var reader = new FileReader();
    reader.onloadend = function() {
      callback(reader.result);
    }
    reader.readAsDataURL(xhr.response);
  };
  xhr.open('GET', url);
  xhr.responseType = 'blob';
  xhr.send();
}

toDataURL('https://www.gravatar.com/avatar/d50c83cc0c6523b4d3f6085295c953e0', function(dataUrl) {
  console.log('RESULT:', dataUrl)
})

我正在使用以下解决方案：如何检查画布是否为空白？但它不起作用。我的空白画布 URL 与新创建的画布 URL 不匹配，我无法弄清楚原因。它们是相同的尺寸，但它们的 .toDataURL 返回不同的东西，所以我无法判断它是否已被绘制。下面是我在空白画布上执行 .toDataURL 和在 .toDataURL 上执行时发生的情况的示例新创建的画布，如您所见，随着您继续前进，它们会有所不同。

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参考这里是功能：

function isCanvasBlank(canvas) {
    var blank = document.createElement('canvas');
    blank.width = canvas.width;
    blank.height = canvas.height;

    return canvas.toDataURL() == blank.toDataURL();
}