Hi all!
Given a PDF with gray images, I have found that an easy way to adjust its
colors is to modify its Image objects. The dictionary associated with this
type of object contains a key, /ColorSpace whose value is usually assigned
to "/DeviceGray". What I have done (manually) has been to substitute this
value with the colorspace
"[ /CalGray << /WhitePoint [0.9505 1.0000 1.0890] /Gamma 2.9 >>]"
and varying the value of the gamma parameter I can tweak the image to the
desired density. Deleting the cross-reference table(s) and "startxref" is
enough for the PDF to work properly, with better looking images. Acrobat
will rebuild those tables the first time it opens the document.
Now, with iText:
I am trying to use iText low level functions to apply the change described
above (even though I´ve read iText is better suited for generation rather
than modification of PDF files). So far, I am able to traverse the
cross-reference table and locate all the gray Image objects contained in the
PDF.
reader = new PdfReader(args[0]);
for (int i = 0; i < reader.XrefSize; i++)
{
PdfObject pdfobj = reader.GetPdfObject(i);
if (pdfobj != null)
{
if (pdfobj.IsStream()) {
PdfStream pdfdict = (PdfStream) pdfobj;
PdfObject pdfsubtype = pdfdict.Get(PdfName.SUBTYPE);
if (pdfsubtype == null) {
continue;
}
if (!pdfsubtype.ToString().Equals(PdfName.IMAGE.ToString())) {
continue;
}
PdfObject pdfcolorspace = pdfdict.Get(PdfName.COLORSPACE);
if (pdfcolorspace.ToString().Equals(PdfName.DEVICEGRAY.ToString()))
{
// Act here!
// PdfArray GrayObj;
// ...
// pdfdict.Put(PdfName.COLORSPACE, GrayObj);
}
}
}
My next step will be building the GrayObj object, that defines the new color
space; it does not look difficult to do. But what I am not sure how to do is
the writer part of the document. I mean, how to copy or stamp the new file
so that the original file, obtained through the reader, is maintained,
except for the Image objects, that I want to modify as explained above. I am
assuming this is feasible with iTextSharp, isn´t it?
Any help?
Thanks,
Ignacio
PS:
I have not seen in iTextSharp the tools that exist in the Java version of
iText. I´m particulary interested in the PDF Analyser / Treeviewer. Hints?
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