A new HDR file format seemingly popped out of nowhere, just when we were discussing HDPhoto here. XDepth is aiming at the same thing: efficient compression of HDR images, so they can be distributed over the web. Or take up less storage space - we all know how HDR files eat through your drive, especially when you're dealing with image sequences.

Admittedly, I was a little skeptical about this format. The publisher is very secretive about their encoding approach, sounds like the format is proprietary. And the publisher's name, "Trellis Management Co. Ltd.", doesn't really give me a warm fuzzy feeling either. They actually had another HDR format out for almost a year, called "CLARITY-HDR^TM but aside from the initial announcement I haven't seen it anywhere.

That being said, here is the scoop: XDepth is amazing! It combines the best things of JPEG-HDR and HDPhoto: It's backwards-compatible with JPEG and instantly usable with a free Photoshop Plugin. And after an initial test, it even seems to beat both of them in the amount of compression. Well done, Trellis!

But first things first - here is what the Photoshop saver looks like:

XDepth's Photoshop Saver Dialog: Well organized and usable by mere mortals.

Notice the separation of HDR and LDR compression. When you save your image as XDepth file, the saver will actually make a regular 8-bit JPG, and hide the HDR data in the matadata. That's right - even though the native file extension is .xdp, you can just rename it to .jpg and it will open anywhere. In fact, the XDepth website itself is made of nothing but these renamed .xdp files, making it the first truly HDR online presence on the web.
In the context of this dialog, LDR quality sets the compression of this base-JPG, and the Gamma value is your crude tonemapping control, letting you choose how contrasty you want in this JPEG. The default value 2.2 corresponds to the default settings in Photoshop's 32-bit Preview.

The interesting part comes in when you re-open the file in Photoshop. Then all the hidden HDR data comes back, and find yourself in 32-bit mode.

So let's see how the format compares, using the good old Kitchen Window again.
To include my current favorite, Ward's JPEG-HDR, I pulled it up in Photosphere (for the lack of a Photoshop Plugin, which is in real life a deal breaker by itself).

It turns out, XDepth beats JPEG-HDR and HDPhoto in filesize, although it doesn't look quite as clean. "Lossless" in the saver dialog apparently means "perceptually lossless", not absolutely lossless. In that regard, XDepth will certainly not make OpenEXR obsolete for images that we still need to work with.

However, that isn't what XDepth is aiming at - it is more of a final output format, like JPEG. It is supposed to crunch stuff as small as it gets, while still looking acceptable.

And this is where XDepth's strength is at: Just a tiny wiggle on the quality slider, to 90/95%, will shrink that EXR from 5.7 MB to 344 kB. That's only 1/17th of the size!! And it still looks fine.
Just for the sake of it, I tried to get the HDPhoto to the same file size, and ended up at 48% quality. At this point, the image turns into cubist masterpiece of artifacts.

Bottom line is: XDepth offers an impressive compression ratio, and usability is very straight-forward. The format has all it takes to become the standard for high dynamic range JPEG, clearing the path for a smooth transition to the promised land of All Things HDRI.
However, the catch is Licensing and application support. For now, Trellis Management Co. Ltd. seems to be friendly enough to provide us the tools for free. My wild guess is, they won't make it all open like OpenEXR, instead they will tackle one application at a time and implement import/export plugins themselves. And then, in stage 2, when everyone calls for direct hardware support, they will ask camera and screen makers for licensing fees. I might be entirely wrong here, but if I'm not: Good luck, Trellis, I really hope you succeed! Your format is simply too nice to get lost in some legal grind.

So go grab the Photoshop Plugin from the XDepth website and start playing with it!