How to read ‘JPEG’ data inside a PDF in Java

How is the data stored?

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 keyword here because you may well need to interpret 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.

Step 1 Read the JPEG data

//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);

Step 2 Read the pixels

//this is the actual pixel data
Raster ras=iir.readRaster(0, null);

Are you working with JPEG Images in Java?

You might like to check out our JDeli image library. It offers lots of advantages over ImageIO and free alternatives such as:-

• prevent heap related JVM crashes
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• create smaller files
• control over output
• support threading
• superior image scaling algorithms

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