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PDF streams
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This chapter covers
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Image and font streams Adding and extracting file attachments Creating portable collections Integrating rich media
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We ve arrived at the final chapter of part 4. In this part, we re turning PDF files inside out. In chapter 13, we explored the file structure and discussed the different objects. We focused on the content stream of pages in chapters 14 and 15. In this chapter, we ll continue working with streams: we ll look at image and font streams, and you ll find out how to add streams containing other files as attachments, and how to organize these files in a portable collection. We ll finish this chapter with some really cool examples of adding multimedia annotations to a document and integrating a Flash application into a PDF document.
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16.1 Finding and replacing image and font streams
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When you create an image using the Image class, or a font using the Font or BaseFont class, you don t have to worry about the way these objects are stored in the finished document. For example, when you use a standard Type 1 font, iText will add
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Finding and replacing image and font streams
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a font dictionary to the PDF file. When you use a font that is embedded, the font dictionary will also refer to a stream with a full or partial font program that is copied into the PDF file. In this section, we ll look at advanced techniques that address the lowest level of PDF creation and manipulation with iText. The examples that follow were inspired by questions that were posted to the mailing list (see appendix B for more info about the list).
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16.1.1 Adding a special ID to an Image
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In the previous chapter, you learned how to extract all the images from a page, but what if you want to pick one specific image programmatically An image is a stored in a stream object. Each stream consists of a dictionary followed by zero or more bytes bracketed between the keywords stream and endstream (see table 13.2). The entries of the stream dictionary are filled in by iText. In the case of images, you ll have at least entries for the width and the height of the image, and a value defining the compression filter, but there s no reference to the original filename. The original bits and bytes of the image may have been changed completely. One of the mailing-list subscribers wanted to solve the problem of retrieving specific images by adding an extra entry to the image stream dictionary. Listing 16.1 was written in answer to his question.
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Listing 16.1 SpecialId.java
Image img = Image.getInstance(RESOURCE); img.scaleAbsolute(400, 300); img.setAbsolutePosition(0, 0); PdfImage stream = new PdfImage(img, "", null); stream.put(new PdfName("ITXT_SpecialId"), new PdfName("123456789")); PdfIndirectObject ref = writer.addToBody(stream); img.setDirectReference(ref.getIndirectReference()); document.add(img);
C D E
B C D
You create an instance of the high-level Image object, and set some properties, as described in chapter 2. You use this Image object to create a low-level PdfImage object. This object extends the PdfStream class. With the second parameter, you can pass a name for the image; the third parameter can be used for the reference to a mask image.
PdfStream extends PdfDictionary. Just like with plain dictionaries, you can add key-
value pairs. In this case, you choose a name for the key using the prefix reserved for iText (ITXT): ITXT_SpecialId. The value of the entry is also a name of your choice, in this case /123456789.
You add the stream object to the body of the file that is written by the PdfWriter object. The addToBody() method returns a PdfIndirectObject. Because it s the first element that s added to the writer in this example, the reference of this object will be 1 0 R.
PDF streams
You tell the Image object that it has already been added to the writer with the method setDirectReference(). Finally, you add the image to the document. The image bytes have already been written to the OutputStream in E. Line G writes the Do operator and its operands to the content stream of the page, and adds the correct reference to the image bytes F to the page dictionary. This example unveils the mechanism that s used by iText internally to add streams. You ll use the PDF file that was created by listing 16.1 in the next example. You ll search for an image with the special ID /123456789, and you ll replace it with another image that has a lower resolution.
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