LUTs can be used to express non-linear relationships of individual color channels just like color profiles. There are tools to build them out there. There's a website somewhere that can convert ICC profiles to 3D LUTs for compositing systems like Nuke or smoke for instance. Still, not sure what you would do with them what you couldn't achieve with normal PS stuff. Unless you have a workflow that requires you to use a specific LUT to integrate with one of those aforementioned systems (matte painting and that) it seems like a lot of trouble for little benefit.
You can find a sample default LUT texture on UDN, the Unreal Developer's Network: http://udn.epicgames.com/Three/ColorGrading.html
A LUT varies from a gradient map in the amount of colors it adjusts. A gradient map will take the value of the pixel, and remap it to the 0-255 range across a linear gradient. A black to white gradient applied as a gradient map to an image will essentially turn it grayscale. The left most portion of the gradient in photoshop is 0, and the right most is 255, or going from black to white. Conversely, a LUT remaps the color of a pixel on the texture to the corresponding color in your image. As such, it allows you do a lot more than just remap grayscale values to color. You can do just about anything to the color of an image.
A technique I use:
- Take a screenshot of the subject you want to Color Grade, and bring it into photoshop.
- Bring the LUT texture in to the same PSD on its own layer, and create an Alpha mask based on its selection.
- Beging adding Adjustment layers! I would stick to adjustment layers, as they are non destructive.
- Once you have the results you want, create a new layer above the adjustment layers and Merge Visible into that new layer.
- Grab selection off of the alpha you created earlier, and copy the LUT texture out of the flattened layer and merge into a new document. Save this file, and load it into the application you are using.
There is also a plugin for Unity that will send screenshots to photoshop and back to the engine: http://amplify.pt/unity/amplify-color/
It would be awesome to have LUT generation directly in Photoshop, either via a script of other plugin.
So, A gradient map would degrade an image by limiting it to 256 levels or shades. But a LUT is higher fidelity?
A 3D LUT maps every color to another color - that's why it is a 3D LUT (as opposed to levels, curves, and gradient map that are 1D LUTs).
There is also an app out called 3D LUT Creator - it's a beta, and constantly evolving, but interesting.