I have a few ‘wonderful’ things to say about JAI (Java Advanced Imaging).
Been having a few issues with it recently. The first one was that the users were able to view B&W (i.e. 2-bit) images just fine, but when they tried to view GreyScale images that had been submitted, it exploded with an error message regarding the class javax.media.jai.operator.SubsampleBinaryToGrayDescriptor. Looking at the name, it’s not awfully surprising that it couldn’t cope with Greyscale Images.
I mean, you provide it with a Greyscale Image, tell it it’s a B&W image, and then expect it to convert it to a Greyscale Image… No, that doesn’t quite work. The entire reason we were using that Descriptor, as far as I can tell, is that it produced reasonable-quality scaled-down images. And it was good, until the client started scanning images in Greyscale.
That was the problem. Finding that was fairly simple. Finding a solution, on the other hand, proved considerably more elusive. I initially tried the javax.media.jai.operator.ScaleDescriptor class, but that generated scaled-down images of appalling quality, which is probably why we switched to using SubsampleBinaryToGrey in the first place.
So I wasn’t able to use that, and looking at the JAI JavaDocs, nothing leaped out at me as being a reasonable solution. Took quite some time until I eventually found this post, which pointed me here. The second link provides a reasonable overview of Image Scaling issues, and made some amount of sense. The first link provides links between the operations discussed and the JAI Descriptors to use. Unfortunately, I feel that it needs a slightly better description of how these operations map, especially for people like me, who aren’t terribly knowledgeable about image processing.
For those who are curious, I eventually managed to make the link between the Area Mapping description in the second link, and javax.media.jai.operator.SubsampleAverageDescriptor. That seems to have solved my problems, but more testing needs to be done. Anyway, if you need to scale images with Java, use SubsampleAverageDescriptor. It will save you huge amounts of pain. And hopefully this post will save you huge amounts of pain too. JAI is just not an awful lot of fun.
I mentioned at the start that I’ve had a ‘few’ problems with JAI. The above is one, and the other is a really, really bizzare one.
Basically, most of the time, people were able to upload images without a problem. Most of the time. Sometimes, however, they would run into problems where an image would simply fail to upload. Looking at the errors, I saw that it was failing in TIFFFaxDecompressor (all our images have to be uploaded as TIFF files, and a thumbnail is generated when the image is uploaded).
Bugger, I thought, and started investigating. Initially, I had no idea why it was even using a Fax decompresser. Those well-versed in their TIFF lore will probably have guessed that it is because they were compressed with the Group 3/4 Fax Encoding. When I figured that out, I tried uploading with different encodings. They all worked. It was only Group 3/4 that was failing. That wasn’t good. Sent me off on a wild goose chase (and I really, really hope that anyone with the same problem finds this post and magically solves all their problems).
After following that pattern for a while, I eventually noticed that when I re-deployed to our application server (Glassfish 2.1.1), it broke. If I shut down and restarted Glassfish, everything was OK. But if I undeployed the application and then redeployed it while the Application Server was still running, then this error started happening again.
Beat head against brick wall.
Something that should have been extremely obvious when we started using JAI, but that wasn’t picked up (I wasn’t with the company when we started making use of JAI) is that it should be installed the system JRE. That’s important. The System JRE.
It should not be included as part of an Application.
And that’s where we went wrong, originally. Installing JAI into the System JRE (Sun Java 1.6, for reference) magically fixed the problem…
Moral of the story?
Always read the goddamn instructions.