我在Swift 3.0中根据html内容创建了pdf文件：

/**
 *
 */
func exportHtmlContentToPDF(HTMLContent: String, filePath: String) {
    // let webView = UIWebView(frame: CGRect(x: 0, y: 0, width: 694, height: 603));

    // webView.loadHTMLString(HTMLContent, baseURL: nil);

    let pdfPrinter = PDFPrinter();
    let printFormatter = UIMarkupTextPrintFormatter(markupText: HTMLContent);
    // let printFormatter = webView.viewPrintFormatter();

    pdfPrinter.addPrintFormatter(printFormatter, startingAtPageAt: 0);

    let pdfData = self.drawPDFUsingPrintPageRenderer(printPageRenderer: pdfPrinter);

    pdfData?.write(toFile: filePath, atomically: true);
}

/**
 *
 */
func drawPDFUsingPrintPageRenderer(printPageRenderer: UIPrintPageRenderer) -> NSData! {
    let data = NSMutableData();

    UIGraphicsBeginPDFContextToData(data, CGRect.zero, nil);

    printPageRenderer.prepare(forDrawingPages: NSMakeRange(0, printPageRenderer.numberOfPages));

    let bounds = UIGraphicsGetPDFContextBounds();

    for i in 0...(printPageRenderer.numberOfPages - 1) {
        UIGraphicsBeginPDFPage();

        printPageRenderer.drawPage(at: i, in: bounds);
    }

    UIGraphicsEndPDFContext();

    return data;
}

除了我的base64编码图像外，其他所有内容都可以正常渲染。在Web视图中或在Safari或chrome浏览器中的HTML内容本身可以正确显示，并且可以正确显示所有图像。但是图像永远不会渲染到pdf中。

为什么不渲染图像？如何使它们渲染？