使用 iText 为标记的 pdf (PDF/UA) 中的图像添加替代文本 [英] Add alternative text for an image in tagged pdf (PDF/UA) using iText

问题描述

我在

代码无法识别狐狸或狗，因此我们创建了一个带有 Alt 属性的新文档，上面写着没有 Alt 描述的图形":

我们通过遍历结构树来添加这个描述，寻找标记为/Figure元素的结构元素:

public void handlePdf(String src, String dest)抛出 IOException，文档异常 {PdfReader 阅读器 = 新 PdfReader(src);PdfDictionary catalog = reader.getCatalog();PdfDictionary structTreeRoot =catalog.getAsDict(PdfName.STRUCTTREEROOT);操作(structTreeRoot)；PdfStamper 压模 = 新 PdfStamper(阅读器，新的 FileOutputStream(dest));压模.关闭()；}公共无效操作(PdfDictionary 元素){如果(元素==空)返回;if (PdfName.FIGURE.equals(element.get(PdfName.S))) {element.put(PdfName.ALT,new PdfString("没有Alt描述的图"));}PdfArray kids = element.getAsArray(PdfName.K);if (kids == null) 返回；for (int i = 0; i < kids.size(); i++)操纵(kids.getAsDict(i))；}

您可以轻松地将此 Java 示例移植到 C#:

• 从 PdfReader 对象中获取根字典，
• 获取结构树的根(一个字典)，
• 遍历那棵树的每个分支的所有孩子，
• 如果线索是人物，请添加 /Alt 条目.

完成后，使用 PdfStamper 保存更改后的文件.

I've looked up some documentations and examples under the http://developers.itextpdf.com/examples.

I know iText is able to generate tagged pdf from scratch, but is it possible to insert alternative text to images in an existing tagged pdf (without changing anything else)? I need to implement this feature in a program without using GUI applications (such as Adobe Acrobat Pro). Thanks.

解决方案

Please take a look at the AddAltTags example.

In this example, we take a PDF with images of a fox and a dog where the Alt keys are missing: no_alt_attribute.pdf

Code can't recognize a fox or a dog, so we create a new document with Alt attributes saying "Figure without an Alt description": added_alt_attributes.pdf)

We add this description by walking through the structure tree, looking for structural elements marked as /Figure elements:

public void manipulatePdf(String src, String dest)
    throws IOException, DocumentException {
    PdfReader reader = new PdfReader(src);
    PdfDictionary catalog = reader.getCatalog();
    PdfDictionary structTreeRoot =
        catalog.getAsDict(PdfName.STRUCTTREEROOT);
    manipulate(structTreeRoot);
    PdfStamper stamper = new PdfStamper(
        reader, new FileOutputStream(dest));
    stamper.close();
}

public void manipulate(PdfDictionary element) {
    if (element == null)
        return;
    if (PdfName.FIGURE.equals(element.get(PdfName.S))) {
        element.put(PdfName.ALT,
            new PdfString("Figure without an Alt description"));
    }
    PdfArray kids = element.getAsArray(PdfName.K);
    if (kids == null) return;
    for (int i = 0; i < kids.size(); i++)
        manipulate(kids.getAsDict(i));
}

You can easily port this Java example to C#:

• Get the root dictionary from the PdfReader object,
• Get the root of the structure tree (a dictionary),
• Loop over all the kids of every branch of that tree,
• When a lead is a figure, add an /Alt entry.

Once this is done, use PdfStamper to save the altered file.

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