从 canvas.toDataURL() 创建图像 bloburl

Question

我将 pdf 输出为 base64 字符串。我想用这种格式为每个页面创建 bloburl。因此，我使用这个 base64 字符串为整个 pdf 创建了 bloburl 。然后，我使用 pdfjs 并在新创建的 canvas 中显示第一页。现在，我尝试将显示的图片作为图像 blob。

创建 blob 时出现错误 (Failed to execute 'atob' on 'Window': The string to be decoded is not correctly encoded.) 执行代码 b64toBlob(pag1.toDataURL('image/png',1.0),'image/png').
然后我试了 b64toBlob(window.btoa(pag1.toDataURL('image/png',1.0)),'image/png') 它显示为该 blob 创建的 url 中的空图像。

var b2 =pag1.toBlob(function(blob) {}, 'image/png') 输出未定义。

<?php
ob_start();
require_once('plugin/tcpdf/tcpdf.php');
$pdf    =new TCPDF();
$pdf->setPrintHeader(false);
$pdf->setPrintFooter(false);
$pdf->AddPage();
$html_p1='Text messaging, or texting, is the act of composing and sending electronic messages, typically consisting of alphabetic and numeric characters, between two or more users of mobile devices, desktops/laptops, or other type of compatible computer. Text messages may be sent over a cellular network, or may also be sent via an Internet connection';
$pdf->writeHTML($html_p1, true, 0, true, 0);
$pdf->AddPage();
$html_p2='A telephone call is a connection over a telephone network between the called party and the calling party.';
$pdf->writeHTML($html_p2, true, 0, true, 0);
$base64PdfString = $pdf->Output('', 'E');
$base64PdfArray = explode("\r\n", $base64PdfString);
$base64 = '';
for($i = 5; $i < count($base64PdfArray); $i++) 
{
    $base64 .= $base64PdfArray[$i];
}
 ?>

 <!doctype html>
  <head>
  <meta name="viewport" content="width = 1050, user-scalable = no" />
    <script type="text/javascript" src="plugin/turnjs4/extras/jquery.min.1.7.js"></script>
   <script type="text/javascript" src="plugin/turnjs4/extras/modernizr.2.5.3.min.js"></script>
   <script type="text/javascript" src="plugin/pdfjs_ex/pdf.js"></script>    
   <script>     

   const b64toBlob = (b64Data, contentType='', sliceSize=512) =>
  {
    const byteCharacters = atob(b64Data);
    const byteArrays = [];

    for (let offset = 0; offset < byteCharacters.length; offset += sliceSize)
    {
        const slice = byteCharacters.slice(offset, offset + sliceSize);
        const byteNumbers = new Array(slice.length);
        for (let i = 0; i < slice.length; i++)
        {
            byteNumbers[i] = slice.charCodeAt(i);
        }
        const byteArray = new Uint8Array(byteNumbers);
        byteArrays.push(byteArray);
    }

    const blob = new Blob(byteArrays, {type: contentType});
    return blob;
}



$( document ).ready(function() {

    const contentType   ='application/pdf';
    const b64Data       ='<?php echo $base64;?>';
    const blob          =b64toBlob(b64Data, contentType);

    const blobUrl       =URL.createObjectURL(blob);


    PDFJS.getDocument({ url: blobUrl }).then(function(pdf_doc) 
    {
            __PDF_DOC = pdf_doc;
            __TOTAL_PAGES = __PDF_DOC.numPages;
            var div_container = document.querySelector('#flipbook');
            pdf_doc.getPage(1).then(function(page) 
            {
                var scale = 1;
                var viewport = page.getViewport(scale);

                // Prepare canvas using PDF page dimensions
               var pag1 = document.createElement('canvas');
                pag1.id ='Pg_1';
                var context1 = pag1.getContext('2d');
                pag1.height = viewport.height;
                pag1.width = viewport.width;
                var renderContext = 
               {
                    canvasContext: context1,
                    viewport: viewport
                };
             page.render(renderContext).then(function()
            {
                div_container.appendChild(pag1);
                var dataUrl =pag1.toDataURL(); 

            //const b   =b64toBlob(dataUrl,'image/png');
            //const b   =b64toBlob(window.btoa(dataUrl),'image/png');
            //const burl =URL.createObjectURL(b); 
            //console.log(b);

            //var b2 =pag1.toBlob(function(blob) {}, 'image/png');

        var element                     = document.createElement("div");
        element.style.backgroundImage   = "url(" + dataUrl + ")";
        div_container.appendChild(element);
         $("#Pg_1").remove(); 

          });
            });

    })

})
 </script>
</head>

<body>
 <!--<div id="magazine">
    <canvas id="pdf"></canvas>
 </div>-->

 <div class="flipbook-viewport">
  <div class="container">
      <div class="flipbook" id="flipbook">
          <!--<canvas id="pdf"></canvas>-->
      </div>
  </div>
</div>


<script type="text/javascript">

function loadApp() {

// Create the flipbook

$('.flipbook').turn({
        // Width

        width:922,

        // Height

        height:600,

        // Elevation

        elevation: 50,

        // Enable gradients

        gradients: true,

        // Auto center this flipbook

        autoCenter: true

});
 }

  // Load the HTML4 version if there's not CSS transform

   yepnope({
    test : Modernizr.csstransforms,
    yep: ['plugin/turnjs4/lib/turn.js'],
    nope: ['plugin/turnjs4/lib/turn.html4.min.js'],
    both: ['plugin/turnjs4/samples/basic/css/basic.css'],
    complete: loadApp
   });

  </script>

 </body>

当我执行console.log(dataURL)时，它以data:image/png;base64开头。所以，我认为它是 base-64 编码的。但是，错误提示未正确解码。为什么会这样how to create imagebloburl from canvas displaying pdf page?

提前致谢。

Answer 1

这能够创建图像 bloburl。但是在函数外访问burl变量时显示undefined。

  pag1.toBlob(function(blob) 
    {
       var burl =URL.createObjectURL(blob); 

    }, 'image/png');

期待更好的解决方案。

从 canvas.toDataURL() 创建图像 bloburl

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