Getting DPI of a JPEG with JAI

Question

I need to get width / height in cm or inches and the DPI of a JPEG using JAI.
I'm currently getting the width and height in pixels using getWidth() and getHeight() of a RenderOp, but it apparently hasn't similar methods to get the DPI (which will suffice).
I'm still seeking into all of the JAI classes, but i'm kind of lost within it.

Answer 1

The reason why i want to try to do it with JAI is the following.
We have a huge collection of JPEG images, that our customer creates daily.
To retrieve image size and resolution, i initially had used a public domain class, whose source is identified by this header:
* ImageInfo.java
* Version 1.5
* A Java class to determine image width, height and color depth for
* a number of image file formats.
* Written by Marco Schmidt
* <http://www.geocities.com/marcoschmidt.geo/contact.html>.
* Contributed to the Public Domain.
* Last modification 2004-02-29
*/It worked fine, but we discovered that it fails with JPEG our customer created before a certain date.
Then i tried with javax.imageio.ImageIO, like this:
Iterator readers = ImageIO.getImageReadersByFormatName("jpeg");
ImageReader reader = (ImageReader)readers.next();
FileInputStream source = new FileInputStream("my.jpeg");
ImageInputStream iis = ImageIO.createImageInputStream(source);
reader.setInput(iis, true);
IIOMetadata meta = reader.getImageMetadata(0);
IIOMetadataNode nodes = (IIOMetadataNode) meta.getAsTree("javax_imageio_1.0");
NodeList nl = nodes.getElementsByTagName("HorizontalPixelSize");
if ((nl != null) && (nl.getLength() > 0))
    xDPI = Math.round(25.4f / Float.parseFloat(nl.item(0).getAttributes().item(0).getNodeValue()));
nl = nodes.getElementsByTagName("VerticalPixelSize");
if ((nl != null) && (nl.getLength() > 0))
    yDPI = Math.round(25.4f / Float.parseFloat(nl.item(0).getAttributes().item(0).getNodeValue()));Same identical problem. The new JPEGs work, the old ones don't.
In this case, javax.imageio.ImageIO throws an exception saying:
javax.imageio.IIOException: Not a JPEG file: starts with 0x55 0x72Quite explicit. The problem is that these JPEGs are correctly rendered, without a bit of complaining, by all the browsers, by all the imaging programs we tried (Adobe Photoshop, Adobe ImageReady, Paint, whatever) and also by the preview feature of Windows Explorer. We never found a single application which complains about these JPEGs. Also the size and resolution are correctly retrieved by these imaging programs.
If javax.imageio.ImageIO says it's a bad JPEG, it's a bad JPEG; however i'd need to make the whole thing work because of the above reason. That's why i'd like to try with JAI too.