[PDF] Rendering on Canvas

  paintOnCanvas(canvasWrapper) {

    const renderCapability = createPromiseCapability();
    const result = {
      promise: renderCapability.promise,
      onRenderContinue(cont) {
        cont();
      },
      cancel() {
        renderTask.cancel();
      },
    };

    const viewport = this.viewport;
    const canvas = document.createElement("canvas");
    this.l10n
      .get("page_canvas", { page: this.id }, "Page {{page}}")
      .then(msg => {
        canvas.setAttribute("aria-label", msg);
      });

    // Keep the canvas hidden until the first draw callback, or until drawing
    // is complete when `!this.renderingQueue`, to prevent black flickering.
    canvas.setAttribute("hidden", "hidden");
    let isCanvasHidden = true;
    const showCanvas = function () {
      if (isCanvasHidden) {
        canvas.removeAttribute("hidden");
        isCanvasHidden = false;
      }
    };

    canvasWrapper.appendChild(canvas);
    this.canvas = canvas;

    if (
      typeof PDFJSDev === "undefined" ||
      PDFJSDev.test("MOZCENTRAL || GENERIC")
    ) {
      canvas.mozOpaque = true;
    }

    const ctx = canvas.getContext("2d", { alpha: false });
    const outputScale = getOutputScale(ctx);
    this.outputScale  = outputScale;

    if (this.useOnlyCssZoom) {
      const actualSizeViewport = viewport.clone({ scale: CSS_UNITS });
      // Use a scale that makes the canvas have the originally intended size
      // of the page.
      outputScale.sx *= actualSizeViewport.width  / viewport.width;
      outputScale.sy *= actualSizeViewport.height / viewport.height;
      outputScale.scaled = true;
    }

    if (this.maxCanvasPixels > 0) {
      const pixelsInViewport = viewport.width * viewport.height;
      const maxScale = Math.sqrt(this.maxCanvasPixels / pixelsInViewport);
      if (outputScale.sx > maxScale || outputScale.sy > maxScale) {
        outputScale.sx = maxScale;
        outputScale.sy = maxScale;
        outputScale.scaled = true;
        this.hasRestrictedScaling = true;
      } else {
        this.hasRestrictedScaling = false;
      }
    }

    const sfx = approximateFraction(outputScale.sx);
    const sfy = approximateFraction(outputScale.sy);
    canvas.width = roundToDivide(viewport.width * outputScale.sx, sfx[0]);
    canvas.height = roundToDivide(viewport.height * outputScale.sy, sfy[0]);
    canvas.style.width = roundToDivide(viewport.width, sfx[1]) + "px";
    canvas.style.height = roundToDivide(viewport.height, sfy[1]) + "px";
    // Add the viewport so it‘s known what it was originally drawn with.
    this.paintedViewportMap.set(canvas, viewport);

    // Rendering area
    const transform = !outputScale.scaled
      ? null
      : [outputScale.sx, 0, 0, outputScale.sy, 0, 0];
    const renderContext = {
      canvasContext: ctx,
      transform,
      viewport: this.viewport,
      enableWebGL: this.enableWebGL,
      renderInteractiveForms: this.renderInteractiveForms,
    };
    const renderTask = this.pdfPage.render(renderContext);
    renderTask.onContinue = function (cont) {
      showCanvas();
      if (result.onRenderContinue) {
        result.onRenderContinue(cont);
      } else {
        cont();
      }
    };

    renderTask.promise.then(
      function () {
        showCanvas();
        renderCapability.resolve(undefined);
      },
      function (error) {
        showCanvas();
        renderCapability.reject(error);
      }
    );
    return result;
  }