使用iText7提取pdf文件中包含的签名图像

Question

我想知道我们如何使用ITEXT7来提取与数字签名相关的图像信息。我知道过去也有人问过类似的问题，但它们大多是围绕ITEXT5提出的，在对软件进行了所有更新和修改后，这与ITEXT7大不相同。

Answer 1

可以使用低级API从签名外观中提取图像。完整的Java代码：

private void saveImageFromSignature(PdfDocument document, String fieldName) throws IOException {
    PdfAcroForm acroForm = PdfAcroForm.getAcroForm(document, false);
    PdfDictionary xObject = acroForm.getField(name)
                                    .getWidgets()
                                    .get(0)
                                    .getNormalAppearanceObject()
                                    .getAsDictionary(PdfName.Resources)
                                    .getAsDictionary(PdfName.XObject)
                                    .getAsStream(new PdfName("FRM"))
                                    .getAsDictionary(PdfName.Resources)
                                    .getAsDictionary(PdfName.XObject);
    PdfStream stream = xObject.getAsStream(new PdfName("Im1"));


    PdfImageXObject image      = new PdfImageXObject(stream);
    BufferedImage   result     = createImageFromBytes(image.getImageBytes());
    //pdf allows using masked image in the signature appearance 
    PdfStream       maskStream = (PdfStream) stream.getAsStream(PdfName.SMask);
    if (maskStream != null) {
        PdfImageXObject maskImage = new PdfImageXObject(maskStream);

        BufferedImage maskBimage = createImageFromBytes(maskImage.getImageBytes());
        String fileMask = String.format(getOutputFolder() + "/file_mask_%d.%s",
                                        image.getPdfObject().getIndirectReference().getObjNumber(),
                                        image.identifyImageFileExtension());
        ImageIO.write(maskBimage,
                      image.identifyImageFileExtension(),
                      new File(fileMask));
        //the mask defines an alfa channel 
        Image transpImg = transformToTransperency(maskBimage);
        result = applyTransperency(result, transpImg);
       


    }
    String filenameComp = String.format(getOutputFolder() + "/file_comp_%d.%s",
                                        image.getPdfObject().getIndirectReference().getObjNumber(),
                                        image.identifyImageFileExtension());


    ImageIO.write(result,
                  image.identifyImageFileExtension(),
                  new File(filenameComp));
    document.close();
}
private Image transformToTransperency(BufferedImage bi) {
    ImageFilter filter = new RGBImageFilter() {
        @Override
        public int filterRGB(int x, int y, int rgb) {
            return (rgb << 8) & 0xFF000000;
        }
    };
    ImageProducer ip = new FilteredImageSource(bi.getSource(), filter);
    return Toolkit.getDefaultToolkit().createImage(ip);

}
private BufferedImage applyTransperency(BufferedImage bi, Image mask) {
    BufferedImage dest = new BufferedImage(
            bi.getWidth(), bi.getHeight(),
            BufferedImage.TYPE_INT_ARGB);
    Graphics2D g2 = dest.createGraphics();
    g2.drawImage(bi, 0, 0, null);
    AlphaComposite ac = AlphaComposite.getInstance(AlphaComposite.DST_IN, 1.0F);
    g2.setComposite(ac);
    g2.drawImage(mask, 0, 0, null);
    g2.dispose();
    return dest;
}

Upd：这只适用于非常有限的情况。谢谢@mkl。