Update: JPDF2HTM5 has been rebranded as BuildVu and JPDFForms has been rebranded as FormVu

Reading ‘JPEG’ data inside a PDF in Java

Pdf files contain compressed raw image data. This file is sometimes equivalent to a JPEG file so if you can extract the raw data and save it as a file with a filetype .jpeg, it will open as a JPEG.

Sometimes is the key word here because you may well need to interpet the data using colour information in the PDF file. For example, the actual data may be encoded Gray or DeviceRGB data (in which case it will look correct when you open the JPEG. But it may need some additional details (such as indexed colours) or be YCCK, in which case you will see the image but the colours will be wrong.

Although it cannot always make sense of these JPEG data (because the colour detail is not in the PDF, you can still use Java to open and access the pixel data in Java using ImageIO. The actual pixel data is stored in a Raster object.

So if you want to recreate the image you will need to get the pixel data and ‘merge’ it with the colour data. Here is how you can read the actual pixel data in Java. Even if Java does not understand the colours, it can access the actual pixels themselves.

//read the image data - data is a byte[] containing the data
in = new ByteArrayInputStream(data);
 
//choose JPEG decoder
Iterator iterator = ImageIO.getImageReadersByFormatName("JPEG");
 
while (iterator.hasNext())
{
Object o = iterator.next();
iir = (ImageReader) o;
if (iir.canReadRaster())
break;
}
 
ImageIO.setUseCache(false);
iin = ImageIO.createImageInputStream((in));
iir.setInput(iin, true);
 
//this is the actual pixel data
Raster ras=iir.readRaster(0, null);

Merging the different colours is a whole series of articles – are you interested?

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Mark Stephens

System Architect and Lead Developer at IDRSolutions
Mark Stephens has been working with Java and PDF since 1999 and has diversified into HTML5, SVG and JavaFX. He also enjoys speaking at conferences and has been a Speaker at user groups, Business of Software, Seybold and JavaOne conferences. He has a very dry sense of humor and an MA in Medieval History for which he has not yet found a practical use.
Markee174

About Mark Stephens

Mark Stephens has been working with Java and PDF since 1999 and has diversified into HTML5, SVG and JavaFX.

He also enjoys speaking at conferences and has been a Speaker at user groups, Business of Software, Seybold and JavaOne conferences. He has a very dry sense of humor and an MA in Medieval History for which he has not yet found a practical use.

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