pdf.js 入门使用指南2-显示篇

一：如何控制pdf缩放

 var viewport = page.getViewport(scale);
 参数：scale,0到1之间的小数，比如0.8，比如100%显示，填写1，参考如下
 var viewport = page.getViewport(1);

部分代码参考：

 var pdfDoc = null,
        pageNum = 1,
        pageRendering = false,
        pageNumPending = null,
        scale = 1,//缩放比例
        canvas = document.getElementById('the-canvas'),
        ctx = canvas.getContext('2d');

    /**
     * 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 viewport = page.getViewport(scale);

2：显示效果

ps：很显然，即便是100%显示，仍然比较小，模糊

3：dpi 的原因
如上所描述，pdf显示小，不够清晰。有一个重要的原因是默认dpi为96，然后网页一般使用dpi为72。
具体dpi ,请搜索百度，这里不再解释。
代码调整：

var CSS_UNITS = 96.0/ 72.0;

var renderContext = {
                transform: [CSS_UNITS, 0, 0, CSS_UNITS, 0, 0],
                canvasContext: ctx,
                viewport: viewport
            };

transform：就是和缩放相关的参数

网页完成代码如下：

<html>
<head>
    <title>pdf.js展示1，上一页，下一页</title>
    <meta charset="UTF-8">
</head>
<h1>PDF.js Previous/Next example</h1>

<div>
    <button id="prev">Previous</button>
    <button id="next">Next</button>
    &nbsp; &nbsp;
    <span>Page: <span id="page_num"></span> / <span id="page_count"></span></span>
</div>

<canvas id="the-canvas"></canvas>
<script src="js/pdf.js"></script>
<script src="js/pdf.worker.js"></script>
<script>
    var url = '/pdf/doc/demo1.pdf';

    var pdfDoc = null,
        pageNum = 1,
        pageRendering = false,
        pageNumPending = null,
        scale = 1,//缩放比例
        canvas = document.getElementById('the-canvas'),
        ctx = canvas.getContext('2d');

    /**
     * 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 viewport = page.getViewport(scale);
            var CSS_UNITS = 96.0 / 72.0;
            canvas.height = Math.floor(viewport.height * CSS_UNITS);
            canvas.width = Math.floor(viewport.width * CSS_UNITS) ;

            // Render PDF page into canvas context
            var renderContext = {
                transform: [CSS_UNITS, 0, 0, CSS_UNITS, 0, 0],
                canvasContext: ctx,
                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.
     */
    PDFJS.getDocument(url).then(function(pdfDoc_) {
        pdfDoc = pdfDoc_;
        document.getElementById('page_count').textContent = pdfDoc.numPages;

        // Initial/first page rendering
        renderPage(pageNum);
    });
</script>
</html>

4：显示效果如下：
转换后

转换后，如上图

转换前

转换前，如上图，作为对比

pdf.js 入门使用指南2-显示篇

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