Render the visible area of the current page

function getPageVisibleRect(pageIndex) {
  // Page DOM element.
  const pageEl = instance.contentDocument.querySelector(
    `.PSPDFKit-Page[data-page-index="${pageIndex}"]`
  );
  const pageBoundingClientRect = pageEl.getBoundingClientRect();
  // Viewport DOM element.
  const viewportEl = instance.contentDocument.querySelector(
    ".PSPDFKit-Viewport"
  );
  // Toolbar DOM element, which needs offsetting.
  const toolbarEl = instance.contentDocument.querySelector(
    ".PSPDFKit-Toolbar"
  );
  // Get the visible page area in page units.
  return instance.transformContentClientToPageSpace(
    new NutrientViewer.Geometry.Rect({
      left: Math.max(pageBoundingClientRect.left, 0),
      top: Math.max(pageBoundingClientRect.top, toolbarEl.scrollHeight),
      width: Math.min(pageEl.clientWidth, viewportEl.clientWidth),
      height: Math.min(pageEl.clientHeight, viewportEl.clientHeight)
    }),
    pageIndex
  );
}

async function cropVisiblePageArea() {
  const pageIndex = instance.viewState.currentPageIndex;
  const { width } = instance.pageInfoForIndex(pageIndex);
  const imageUrl = await instance.renderPageAsImageURL(
    { width: width * devicePixelRatio },
    pageIndex
  );
  const img = document.createElement("img");
  img.onload = () => {
    const canvas = document.createElement("canvas");
    const pageRect = getPageVisibleRect(pageIndex);
    // Use an HDPI canvas to get a good resolution image on any device.
    const HDPIPageRect = pageRect.scale(devicePixelRatio);
    canvas.width = HDPIPageRect.width;
    canvas.height = HDPIPageRect.height;
    const ctx = canvas.getContext("2d");
    ctx.drawImage(
      img,
      HDPIPageRect.left,
      HDPIPageRect.top,
      HDPIPageRect.width,
      HDPIPageRect.height,
      0,
      0,
      HDPIPageRect.width,
      HDPIPageRect.height
    );
    // The canvas now contains the cropped page area that is currently visible.
  };
  img.src = imageUrl;
}