How to convert colored images to black and white?

The Question:

You have a PDF with a colored JPG. For instance: image.pdf

If you look inside this PDF, you'll see that the filter of the image stream is /DCTDecode and the color space is /DeviceRGB.

Now you want to replace the image in the PDF, so that the result looks like this: image_replaced.pdf

In this PDF, the filter is /FlateDecode and the color space is change to /DeviceGray.

In the conversion process, you want to user a PNG format.

The Example:

I have prepared an example for you that makes this conversion: ReplaceImage

I will explain this example step by step:

Step 1: finding the image

In my example, I know that there's only one image, so I'm retrieving the PRStream with the image dictionary and the image bytes in a quick and dirty way.

PdfReader reader = new PdfReader(src);
PdfDictionary page = reader.getPageN(1);
PdfDictionary resources = page.getAsDict(PdfName.RESOURCES);
PdfDictionary xobjects = resources.getAsDict(PdfName.XOBJECT);
PdfName imgRef = xobjects.getKeys().iterator().next();
PRStream stream = (PRStream) xobjects.getAsStream(imgRef);

I go to the /XObject dictionary with the /Resources listed in the page dictionary of page 1. I take the first XObject I encounter, assuming that it is an image, and I get that image as a PRStream object.

The code you shared is better than mine, but this part of the code isn't relevant to your question and it works in the context of my example, so let's ignore the fact that this won't work for other PDFs. What you really care about are steps 2 and 3.

Step 2: converting the colored JPG into a black and white PNG

Let's write a method that takes a PdfImageObject and that converts it into an Image object that is changed into gray colors and stored as a PNG:

public static Image makeBlackAndWhitePng(PdfImageObject image) throws IOException, DocumentException {
    BufferedImage bi = image.getBufferedImage();
    BufferedImage newBi = new BufferedImage(bi.getWidth(), bi.getHeight(), BufferedImage.TYPE_USHORT_GRAY);
    newBi.getGraphics().drawImage(bi, 0, 0, null);
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    ImageIO.write(newBi, "png", baos);
    return Image.getInstance(baos.toByteArray());
}

We convert the original image into a black and white image using standard BufferedImage manipulations: we draw the original image bi to a new image newBi of type TYPE_USHORT_GRAY.

Once this is done, you want the image bytes in the PNG format. This is also done using standard ImageIO functionality: we just write the BufferedImage to a byte array telling ImageIO that we want "png".

We can use the resulting bytes to create an Image object.

Image img = makeBlackAndWhitePng(new PdfImageObject(stream));

Now we have an iText Image object, but please note that the image bytes as stored in this Image object are no longer in the PNG format. As already mentioned in the comments, PNG is not supported in PDF. iText will change the image bytes into a format that is supported in PDF

Step 3: replacing the original image stream with the new image stream

We now have an Image object, but what we really need is to replace the original image stream with a new one and we also need to adapt the image dictionary as /DCTDecode will change into /FlateDecode, /DeviceRGB will change into /DeviceGray, and the value of the /Length will also be different.

You are creating the image stream and its dictionary manually. That's brave. I leave this job to iText's PdfImage object:

PdfImage image = new PdfImage(makeBlackAndWhitePng(new PdfImageObject(stream)), "", null);

PdfImage extends PdfStream, and I can now replace the original stream with this new stream:

public static void replaceStream(PRStream orig, PdfStream stream) throws IOException {
    orig.clear();
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    stream.writeContent(baos);
    orig.setData(baos.toByteArray(), false);
    for (PdfName name : stream.getKeys()) {
        orig.put(name, stream.get(name));
    }
}

The order in which you do things here is important. You don't want the setData() method to tamper with the length and the filter.

Step 4: persisting the document after replacing the stream

I guess it's not hard to figure this part out:

replaceStream(stream, image);
PdfStamper stamper = new PdfStamper(reader, new FileOutputStream(dest));
stamper.close();
reader.close();