What is it?

High Dynamic Range Image,..........using an image to light a scene without need for external lights. Gives a more natural ambient lighting to your scene,............at the cost of much higher rendertimes.

It was originally used (and still is) as a way of lighting a 3D image with the light from a scene (ie: in a movie) so that the image will fit into the scene more perfectly; with exactly the same lighting as everything else in the scene.

Svarg nailed it and sackrat gave the gory details. Basically you use a high dynamic range image to light your scene. If you have a photographic background - think of an image that contains every exposure from pitch black to pure white. With HDRI you get to use a slider to dial in the perfect exposure. Add to that automatic soft shadows and amazing reflections and you have HDRI. Try it - and like AS says set the quality to zero unless you see banding in the shadows. Then nudge it up until the banding goes away or is acceptable. Cheers! Scott

An HDRI is a High Dynamic Range Image,
It's basically a digital image format designed specifically to be able to allow for more accurate storage of brightness values. For example:
A normal 24-bit image only gets you 256 levels of brightness, 0-0-0 RGB value being black, and 255-255-255 RGB value being white. This is sufficient to show a nice picture on your computer monintor because the monitor doesnt show more then 256 levels of brightnes anyways, but not sufficient for actual accurate storage of brightness values.
For example:
In a 24bit photo, a lightbulb and the sun could both come out white (255-255-255 RBG value). Yet, in reality, the light comming from the sun is much more intense (thus brighter) then the light comming from a lightbulb. The 24bit image is just not capable of expressing this level of contrast in the amount of brightness, it clips at 255-255-255 RGB.
To accurately store the difference in brightness of the lightbulb and the sun, one would use an HDRI image because this type of image is designed to allow for much, much more brightness values. (think towards thousands and thousands of brightness values here, maybe tens of thousands, i'm not entirely sure how much exactly)

So why would one use HDRI's if a normal monitor isnt capable of showing all the brightness values in the image? well here's the explaination for that:

In 3D renders using IBL (image based lighting), an image "wrapped around your 3d scene" can be used to simulate light cast by lightsources, or bounced off objects, surrounding your 3D scene. For instance the sky, clouds, sun, buildings in the street, etc.

To accurately simulate this light, it suddenly becomes neccesary that the image you use is in fact able to express the different degrees of for instance the brightness of a lightbulb and the sun which i gave as an example because these really do affect the way your scene gets lit. If you turn on a light in a dark room it lights the room, but never as brightly and vibrantly as the sun would, shadows stay darker, as do various more or less obscured area's, nooks and crannies. It is in effects like this that the extra brightness values in an HDRI image turn out to be extremely usefull to accurately portray/simulate the real life situation.

And another thing to add here is that HDRI IBL is mainly useful for indoor scenes or scenes involving lots of objects which have reflective surfaces, such as a car and you want reflections off various real life buildings and light sources.  If you are doing a distant lake scene then IMO it is far less usefull. As stated above though, expect very long render times if using higher quality settings - particularly if you are using glass objects.

Have a look at recent Bryce 6 renders in the galleries. Often they will state if HDRI has been used.

Generally HDRI based IBL is usefull for any scene that needs the simulation of an environment around it for lighting purposes. The reflections are cool, but non-reflective objects need to be lit too

I'd like to see a mini tut on how you guys actually add HDRI to your scenes. I've yet to see a "this is what you do" tut. I'm probably being real dense, but having never used it before, I'm not sure how to do it. 
Do you create a super large sphere and apply a 'spherical' HDRI image to it?
My eternal thanks to anyone who will tell me how to do it.
:) Ang

Ang - in Bryce 6 super easy. Just click on sky and fog and find the last tab that says IBL. This is imaged based lighting. Click open to find your HDRI image and then set the quality slider to zero and the slider below it to the brightness you want. The super large sphere is virtual, but in pre Bryce 6, Agentsmith and bunch of the other guys here used to fake it using the large sphere technique. But Bryce 6 is true HDRI, no sphere needed and the image truly lights your scene. btw - the image needs to be the light probe style. Scott

Go to the top of this page. Find the HDRI for Bryce 6 link. Click and it will take you to a page with useful info. Look for and click on the link that says,   Older Debevec HDRI's, then scroll down and click on the images of the spheres, download, then open them. These are just a few that are available on the net, But They Must Be an Angular Map to work in Bryce as the ones on this download page are.

Thanks Guys!
I've still got questions in my head, but I'll try experimenting with it first and if I'm still confused I'll come back and ask more questions.
Ang runs off to download some HDRIs

I'd make a tutorial but since i use 3dsmax its prolly not gunna help people with bryce6 LOL (haven't bothred to spend money on b6 yet. New shoes are higher up on my wishlist :tongue1:)

Hmmm...  Couldn't you buy Bryce 6 and render some shoes instead?

Shoes ?!?  You have feet !?!?

Ok, anybody ready for dumb question number 1?
 Why have an option to render as background if by unchecking it - it no longer reflects in the materials?

Good dumb question, Ang.  I'm scratching my head on that one also.  

I'm having trouble with unnatural bright spots when used as background in enclosed scene and if you turn it off then you get no hdri reflections ?????????

Thanks :P

I'd call that one a major goof that should be fixed in 6.1. We should be able to turn off the background image but still have it show up in the reflections. Just having the light doesn't do much good for reflective surfaces.

@ Ang25

See ICM's thread titled HDRI - DOF - Play 
Masking may be the answer to reflections without the unwanted effects of background enabled

Quote - Shoes ?!?  You have feet !?!?

Well yea, how else do i walk to the coffee machine?? I spent the entire day yesterday finding a pair of shoes. I tried to find Onitsuka Tiger Faile Samatore 2006 in black&white but couldnt find it anywhere. I finally ended up with another pair. Onitsuka Tiger Est. 1949's in black to be exact. Great shoes Everyone goes oh are those insert a brand 'n its never the right one :tongue1: i just love it when i find nice clothes that no one else knows. Theres nothing worse then looking like everyone else!