How to add alternative text for an image in Tagged PDF?
I know that iText can generate tagged PDF documents from scratch, but is it possible to insert alternative text for 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.
Posted on StackOverflow on Dec 2, 2015 by tsforsure
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
Structure element without /Alt key
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)
Structure element with /Alt key
We add this description by walking through the structure tree, looking for structural elements marked as /Figure
elements:
public void manipulatePdf(String dest) throws IOException {
PdfDocument pdfDoc = new PdfDocument(new PdfReader(SRC), new PdfWriter(dest));
PdfDictionary catalog = pdfDoc.getCatalog().getPdfObject();
// Gets the root dictionary
PdfDictionary structTreeRoot = catalog.getAsDictionary(PdfName.StructTreeRoot);
manipulate(structTreeRoot);
pdfDoc.close();
}
public void manipulate(PdfDictionary element) {
if (element == null) {
return;
}
// If an element is a figure, adds an /Alt entry.
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;
}
// Loops over all the kids
for (int i = 0; i < kids.size(); i++) {
manipulate(kids.getAsDictionary(i));
}
}
You can easily port this Java example to C#:
Get the root dictionary from the
PdfDocument
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.
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