使用 pdf.js 以高质量呈现 pdf

Question

我正在尝试使用 pdf.js 显示 pdf。 我能够在所有设备上正确显示它。但是，在视网膜显示器和 4k 显示器上它是模糊的。

我发现here、here 和here，我必须使用值window.devicePixelRatio 但我不是 Javascript 专家，我不知道如何使用它。

另外here，mozilla 对如何设置像素比率有一个简单的（不适合我）解释。但是由于我刚开始使用 JS，所以我被卡住了。

这是我的代码：

    
    // If absolute URL from the remote server is provided, configure the CORS
// header on that server.
var url = 'https://www.tecmasolutions.com/clients/pdf/pdf-doppia-2.pdf';


// Loaded via <script> tag, create shortcut to access PDF.js exports.
var pdfjsLib = window['pdfjs-dist/build/pdf'];

// The workerSrc property shall be specified.
pdfjsLib.GlobalWorkerOptions.workerSrc = '//mozilla.github.io/pdf.js/build/pdf.worker.js';

var pdfDoc = null,
    pageNum = 1,
    pageRendering = false,
    pageNumPending = null,
    canvas = document.createElement('the-canvas');
    //ctx = canvas.getContext('2d');
    canvas.style.height = "1000px";
    canvas.style.width = "900px";
    scale = 2.5;

/**
 * Get page info from document, resize canvas accordingly, and render page.
 * @param num Page number.
 */
function renderPage(num) {
  pageRendering = true;
  // Using promise to fetch the page
  pdfDoc.getPage(num).then(function(page) {
    var container = document.getElementById('the-container');
        var canvas = document.getElementById('the-canvas');
        var context = canvas.getContext('2d');

        var viewport = page.getViewport(1);
        var scale = container.clientWidth / viewport.width;
        viewport = page.getViewport(scale);

        canvas.height = viewport.height;
        canvas.width = viewport.width;


        var renderContext = {
            canvasContext: context,
            viewport: viewport 
        };
    var renderTask = page.render(renderContext);

    // Wait for rendering to finish
    renderTask.promise.then(function() {
      pageRendering = false;
      if (pageNumPending !== null) {
        // New page rendering is pending
        renderPage(pageNumPending);
        pageNumPending = null;
      }
    });
  });

  // Update page counters
  document.getElementById('page_num').textContent = num;
}

/**
 * If another page rendering in progress, waits until the rendering is
 * finised. Otherwise, executes rendering immediately.
 */
function queueRenderPage(num) {
  if (pageRendering) {
    pageNumPending = num;
  } else {
    renderPage(num);
  }
}

/**
 * Displays previous page.
 */
function onPrevPage() {
  if (pageNum <= 1) {
    return;
  }
  pageNum--;
  queueRenderPage(pageNum);
}
document.getElementById('prev').addEventListener('click', onPrevPage);

/**
 * Displays next page.
 */
function onNextPage() {
  if (pageNum >= pdfDoc.numPages) {
    return;
  }
  pageNum++;
  queueRenderPage(pageNum);
}
document.getElementById('next').addEventListener('click', onNextPage);

/**
 * Asynchronously downloads PDF.
 */
pdfjsLib.getDocument(url).promise.then(function(pdfDoc_) {
  pdfDoc = pdfDoc_;
  document.getElementById('page_count').textContent = pdfDoc.numPages;

  // Initial/first page rendering
  renderPage(pageNum);
});

    </script>

<script src="//mozilla.github.io/pdf.js/build/pdf.js"></script>
  
<div class="html-embed w-embed w-script" id="the-container">
  <canvas id="the-canvas"></canvas>
  <div id="text-layer"></div>
</div>
<div class="pdf-wrapper">
  <div id="prev" class="prev-next">&lt;</div>
  <div id="next" class="prev-next">&gt;</div>
</div>
<div class="numero-pagina">pagina: <span id="page_num">1</span> / <span id="page_count">45</span></div>

Answer 1

这是使用一点 jQuery 来创建画布元素。

var canvas = $('<canvas/>').get(0),
    context = canvas.getContext('2d'),
    scale = window.devicePixelRatio || 1,
    viewport = page.getViewport({scale: scale}),
    view = page.view, // [x1, y1, x2, y2]
    width = view[2] - view[0],
    height = view[3] - view[1];

    context.scale(scale, scale);
    canvas.width = width * scale;
    canvas.height = height * scale;
    var renderContext = {
        canvasContext: context,
        viewport: viewport
    };
    page.render(renderContext).then(...)

Answer 2

此代码修复了 pdf 模糊问题：

    var loadingTask = pdfjsLib.getDocument(this.url);
loadingTask.promise.then(function (pdf) {
    var scale = 1.3;
    inst.number_of_pages = pdf.numPages;

    for (var i = 1; i <= pdf.numPages; i++) {
        pdf.getPage(i).then(function (page) {
            var canvas = document.createElement('canvas');
            document.getElementById(inst.container_id).appendChild(canvas);             
            let ctx = canvas.getContext('2d')
            let dpr = window.devicePixelRatio || 1
            let bsr = ctx.webkitBackingStorePixelRatio ||
              ctx.mozBackingStorePixelRatio ||
              ctx.msBackingStorePixelRatio ||
              ctx.oBackingStorePixelRatio ||
              ctx.backingStorePixelRatio || 1
            let ratio = dpr / bsr
            let  originalviewport = page.getViewport({ scale: 1.5, });
            var viewport = page.getViewport({scale:screen.availWidth / originalviewport.width,})
            viewport = originalviewport
            canvas.width = viewport.width * ratio
            canvas.height = viewport.height * ratio
            canvas.style.width = viewport.width + 'px'
            canvas.style.height = viewport.height + 'px'
           ctx.setTransform(ratio, 0, 0, ratio, 0, 0)

            var renderContext = {
                canvasContext: ctx,
                viewport: originalviewport
            };
            var renderTask = page.render(renderContext);
            renderTask.promise.then(function () {
                $('.pdf-canvas').each(function (index, el) {
                    $(el).attr('id', 'page-' + (index + 1) + '-canvas');
                });
                inst.pages_rendered++;
                if (inst.pages_rendered == inst.number_of_pages) inst.initFabric();
            });
        });
    }
}, function (reason) {
    console.error(reason);
});