如何在画布上添加形状？

Question

我想要

• 将矩形形状添加到保存当前页面 pdf 的每个画布
• 每个页面中的形状可以出现 n 次。

问题

• 为什么我的形状也替换了 pdf？
• 无法向保存当前页面 pdf 的每个画布添加矩形形状

以下是我尝试过的一些内容。 https://jsfiddle.net/goteL3hn/

代码：

// Adding a rectangle
jQuery(function($) {

    $("#addRectangle").click(function() {
    fCanvas = new fabric.Canvas("the-canvas", {
        selection: false
    });
        fCanvas.add(new fabric.Rect({
            left: 100,
            top: 100,
            fill: 'red',
            width: 20,
            height: 20
        }));
    });
});

Answer 1

您在 canvas the-canvas元素中渲染 PDF，但是在添加 rect 对象时，您使用相同的元素创建了 fabricjs canvas the-canvas元素，因此它清除了画布并仅绘制了 rect 对象。

您需要在 fabricjs 画布顶部上下文中呈现 pdf 页面数据，并在图像加载后将其添加为下部画布元素中的图像对象。

演示

 // If absolute URL from the remote server is provided, configure the CORS // header on that server. var url = 'https://raw.githubusercontent.com/mozilla/pdf.js/ba2edeae/web/compressed.tracemonkey-pldi-09.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, prevPageNum = 1, pageRendering = false, pageNumPending = null, scale = 0.8, fCanvas = new fabric.Canvas("the-canvas", { selection: false }); /** * Get page info from document, resize canvas accordingly, and render page. * @param num Page number. */ var pageHistory = {}; function renderPage(num) { pageHistory[prevPageNum] = fCanvas.toJSON(); var jsonData = pageHistory[num]; fCanvas.discardActiveObject(); //if you have any object selected, then it will deselect if (jsonData) { fCanvas.loadFromJSON(jsonData, function() { renderPdfData(num); fCanvas.renderAll.bind(fCanvas) }); } else { pageHistory[num] = fCanvas.toJSON(); fCanvas.clear(); renderPdfData(num); } } function renderPdfData(num) { pageRendering = true; // Using promise to fetch the page pdfDoc.getPage(num).then(function(page) { var viewport = page.getViewport(scale); fCanvas.setDimensions({ height: viewport.height, width: viewport.width }) // Render PDF page into canvas context, use fabricjs top canvas element context var renderContext = { canvasContext: fCanvas.contextTop, viewport: viewport }; var renderTask = page.render(renderContext); // Wait for rendering to finish renderTask.promise.then(function() { pageRendering = false; var imageData = fCanvas.upperCanvasEl.toDataURL(); //create an image from top context and put it in lower canvas element fabric.Image.fromURL(imageData, function(img) { img.set({ left: 0, top: 0, evented: false, selectable: false, excludeFromExport:true }); fCanvas.add(img).sendToBack(img); fCanvas.clearContext(fCanvas.contextTop); 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; } prevPageNum = pageNum; pageNum--; queueRenderPage(pageNum); } document.getElementById('prev').addEventListener('click', onPrevPage); /** * Displays next page. */ function onNextPage() { if (pageNum >= pdfDoc.numPages) { return; } prevPageNum = pageNum; pageNum++; queueRenderPage(pageNum); } document.getElementById('next').addEventListener('click', onNextPage); /** * Asynchronously downloads PDF. */ pdfjsLib.getDocument(url).then(function(pdfDoc_) { pdfDoc = pdfDoc_; document.getElementById('page_count').textContent = pdfDoc.numPages; // Initial/first page rendering renderPage(pageNum); }); // Adding a rectangle jQuery(function($) { $("#addRectangle").click(function() { fCanvas.add(new fabric.Rect({ left: 100, top: 100, fill: 'red', width: 20, height: 20 })); }); });

 #the-canvas { border:1px solid black; }

 <script src="//mozilla.github.io/pdf.js/build/pdf.js"></script> <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script> <script src="https://rawgit.com/kangax/fabric.js/master/dist/fabric.js"></script> <h1>PDF.js Previous/Next example</h1> <div> <input type="button" id="addRectangle" value="Add Signature"> <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>