使用PDFBox转换含电子签名PDF为图片时缺失内容求助

问题现象 #

使用PDFBox 3.0.2（及旧版本）渲染含电子签名的PDF时，签名区域的William文本无法被渲染，其余内容正常显示。

原因分析 #

电子签名的显示内容（如William）属于PDF的AcroForm签名域外观，默认情况下：

• PDFRenderer在RenderDestination.PRINT模式下可能会过滤部分屏幕优先的元素；
• PDFBox加载文档时，若未正确保留AcroForm相关数据，会导致签名外观无法被加载渲染；
• 签名域的外观可能被标记为仅在特定场景显示，PRINT模式下被隐藏。

解决方案 #

1. 调整渲染目标参数 #

将RenderDestination.PRINT替换为RenderDestination.SCREEN，该模式会优先保留屏幕显示的所有元素，包括签名外观：

try (PDDocument document = Loader.loadPDF(new File("test-signed.pdf"))) {
    PDFRenderer pdfRenderer = new PDFRenderer(document);
            
    // 替换RenderDestination为SCREEN
    BufferedImage bufferedImage = pdfRenderer.renderImage(0, 300 / 72f, ImageType.RGB, RenderDestination.SCREEN);

    File file = new File("output.png");
    ImageIO.write(bufferedImage, "png", file);
}

2. 确保AcroForm被正确加载 #

如果上述方法无效，可显式设置加载参数保留AcroForm，并强制渲染所有注释：

// 设置加载参数，保留AcroForm
LoadOptions options = new LoadOptions();
options.setPreserveAcroForm(true);

try (PDDocument document = Loader.loadPDF(new File("test-signed.pdf"), options)) {
    PDFRenderer pdfRenderer = new PDFRenderer(document);
    // 启用注释渲染
    pdfRenderer.setRenderAnnotations(true);
            
    BufferedImage bufferedImage = pdfRenderer.renderImage(0, 300 / 72f, ImageType.RGB, RenderDestination.SCREEN);

    File file = new File("output.png");
    ImageIO.write(bufferedImage, "png", file);
}

3. 手动绘制签名域外观 #

若前两种方法仍失效，可手动遍历签名域，提取其外观并绘制到渲染后的图片上：

try (PDDocument document = Loader.loadPDF(new File("test-signed.pdf"))) {
    PDFRenderer pdfRenderer = new PDFRenderer(document);
    BufferedImage bufferedImage = pdfRenderer.renderImage(0, 300 / 72f, ImageType.RGB, RenderDestination.SCREEN);
    Graphics2D g2d = bufferedImage.createGraphics();

    // 获取AcroForm及签名域
    PDAcroForm acroForm = document.getDocumentCatalog().getAcroForm();
    if (acroForm != null) {
        for (PDField field : acroForm.getFields()) {
            if (field instanceof PDSignatureField) {
                PDSignatureField signatureField = (PDSignatureField) field;
                PDAppearanceDictionary appearance = signatureField.getAppearance();
                if (appearance != null) {
                    PDAppearanceStream appearanceStream = appearance.getNormalAppearance();
                    if (appearanceStream != null) {
                        // 转换外观流为BufferedImage并绘制
                        float scale = 300 / 72f;
                        Rectangle2D rect = signatureField.getWidgets().get(0).getRectangle().getRectangle2D();
                        BufferedImage sigImage = appearanceStream.getImage(document);
                        g2d.drawImage(sigImage, 
                            (int)(rect.getX() * scale), 
                            (int)((document.getPage(0).getMediaBox().getHeight() - rect.getY() - rect.getHeight()) * scale), 
                            (int)(rect.getWidth() * scale), 
                            (int)(rect.getHeight() * scale), 
                            null);
                    }
                }
            }
        }
    }

    g2d.dispose();
    File file = new File("output.png");
    ImageIO.write(bufferedImage, "png", file);
}

内容的提问来源于stack exchange，提问作者dat pham cao hoang