Hdri Lighting

[Squash-n-Stretch]

I have a HDRI related question too, if you don't mind me budging in ;) (didn't want to start a new thread). I'm also really new to HDRI and I am not exactly convinced of their 'neccesity'. I must be using them wrong. (I use 3D max) I have lots of HDRI maps from the internet that have the image on only half the map...the rest is a black band. Is that getting distributed in the lighting? How do I get rid of it? I try to distribute it as an environment image, and try all sorts of alignments but it won't go away. Also, how do I use them with a spotlight instead of a skylight? (The last issue of 3D world said you could do that).

Thanks in advance.

[Rens]

HDR images or High Dynamic Range images store more information than you can see at a given time. Regular (8-bit) images don't have more information stored inside of it than what you can see on the screen. This higher range has several benefits, here are three:


1). Adjustable intensity. Because an HDR image stores more information you can adjust the intensity of the image and still have a realistic image.

Let's compare it with how a real camera works. Say you take a picture or a clear lightbulb in a room. At a standard camera setting the lightbulb will most likely appear as a white glowing blob while the roomn will look normal. If you were to considerably decrease the amount of light hitting the film you'll eventually see a glass bulb with glowing wires inside of it. The room will now look dark, but you can clearly see what's going on inside of the bulb.

Now compare to how images on a computer work. Say I take that first bright image of a lightbulb (the white glowing blob) and make it into a BMP file. Now if I were to decrease the intensity of the image beacause I think it's too bright I will run into a problem. Instead of seeing a dark room with a clear bulb and glowing wires I see a dark room with a grey glowing blob. There is no extra information present in the image so the bright glowing lightbulb just gets scaled down to a grey mess which looks like crap.
An HDR image has the information of several exposure settings inside of it so when you want to decrease the intensity of it you can. When you decrease it you will go from a white glowing blob to a clear glass bulb with glowing wires inside of it.

I have attached three images. The first one is an image I took of a lightbulb, regular intensity. Second is an image I took from the same lightbulb but with different exposure/f-stop settings. Notice the amount of deatil in the bulb. Third is the first image of which I decresed the brightness in Photoshop. Compare the second and the third and notice the colour in the center of the bulb.


2). Realistic reflections. Most materials and especially non-metals do not reflect back 100% of the light that is cast on the surface. In effect this means that the intensity of the light is decreased. Say we have a sheet of glass against a black background. Head-on the glass will only reflect about 4% of the light back to the viewer. This is the same problem as described in 1). If the image that is being reflected (in your 3D software) has no extra information available the reflections will look like crap. Same thing really, if the bright lightbulb which gets reflected in the glass will not look like a grey mess you're in business.


3). Realistic image based lighting. The sun will give more light than a lightbulb, but when you take a picture of them they can both appear as white. Regular low range images like BMP or JPEG will not distinguish between a white lightbulb and a white sun. Say I have an image of a lightbulb and the sun wrapped around my scene in such a way that the bulb is on one side and the sun on the other.

Now if the image is a JPEG file the scene will be lit equally from both sides because white is white, nothing extra is stored.

If the image is a high range image like an HDR or EXR file the extra information stored might say that the sun is a thousad times brighter than the bulb. Now at a given exposure setting they both might still appear as white but when I light a scene with that image the side of the sun will be lit much more intensly than the side of the bulb, which of course is more realistic.


So to sum it up: the main reason to use high range images is realism.

[Rens]

Now as for the problems.

specialk12:
- Here (http://www.splutterfish.com/sf/sf_gen_page.php3?printer=1&page=downloads/max/hdri/_contents/docs/html/hdri_i) and here (http://www.splutterfish.com/sf/contrib/tutorials/gabry_hdri_tut/_contents/index.html) are two pages explaining more about how to work with HDR images in max.
- You need to save an image as an HDR image first before you can open it as one.

Squash-n-Stretch:
- Generally the black band won't be a problem as most of the time you'll have a ground plane. The black band should fall below that and won't have any effect on the lighting of your scene. If you don't have a ground plane then you need to find some images which don't have the black band.
- I'm not sure what you mean by using them in a spotlight. Could you explain a bit more?

[ME3D]

BTW. Instead of searching the net for more HDRI images you can now use the latest pshop (CS2) to open and remove the black line.

[pldIII]

Hi All,
another quick question.
HDR images are stored linearly. I assume therefore that what we see is not exactly correct since we would need to apply gamma correction (to linearize for the display). When we use HDRIs in 3D (for reflection or even Image-based-lighting), should I apply gamma correction?


thanks

[Squash-n-Stretch]

Thanks Ren's, those tuts cleared it up. Muchos gracias.

[dcr0ss]

Hi. I've been playing around with a scene using HDRI lighting and I have a question.
I've set up a basic scene with the HDRI file set on a skylight, why is it that whenever I add any other lights into the scene the HDRI lighting doesn't seem to work? Is this one of the limitations of HDRI or is there a way to get around this, I want a realistic scene yet to be able to add lights into the scene.


cheers,
-dc

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