Creating Your Own HDR Images
Creating suitable HDRI (or High Dynamic Range Image) files from rendered images is a fascinating challenge as it requires that you understand more than what virtual images can handle. By "virtual" I mean those images captured on film or rendered in CG applications. For the most part in the virtual world there are only 256 levels of gray, or luminance. These levels only approximate what we actually see in the real world. Our eyes translate these into the visual range we can see and gives us our vision of the image. But the real world doesn't work this way. In truth, the world is full of a spectrum of luminance that surpasses what we can create with a simple graphics editing program. To that end, simple renders just don't have the depth that an HDR file has.

This tutorial will focus on creating an HDR image from a standard BMP or JPG, which is considered a LDRI, or Low Dynamic Range Image.

Carrara is the focus of this tutorial, but the images created by this method can be ported to any CG program capable of using HDRI images.

Requirements:
Windows PC
     (though this may work on a Mac as well, just not with HDRShop)
Carrara Studio 3
HDRShop by Paul Debevec
     found at http://www.debevec.org/HDRShop. At current there is not a Mac version, but there is a lot of interest in one.  You may be able to run it in Vitual PC though.
A photo editing program
     I use Paintshop Pro 8, but Photoshop will work just as well. Make sure that Windows defaults JPGs and BMPs to open in this program.
Time, lots of time... :)


1st Exercise

This exercise gets us used to using HDRShop to change the luminance values of an image to produce light within the HDR Image and subsequent render.
I will provide a basic CAR file to use for a render. All necessary files are located at THIS link.
Getting started:
1) Create a totally black BMP image file of size 640x320, or click HERE to download mine. Make sure to keep your image editing program open.
2) Load this file into HDRShop and take the default Camera Response Curve.
     The Camera Response Curve isn't really important as we will be making changes to our own images instead of compiling others, so the default is usually OK for what we are doing.
3) Click "File", then "Edit in Image Editor".
     Wait a few seconds. When HDRShop prompts with a window that says "Hit OK when edit complete," switch over to the image editing program. DO NOT CLICK OK! This will return the image from the temp file and your subsequent edits will not come through. Trust me, just leave it alone for now.

4) Take a round brush (size=20, hardness=0, opacity=25, color=Pure White, blend=Dodge) and in one stroke inscribe a small circle anywhere on the canvas. Draw a second larger circle over that one and you will see the first one grow brighter. The goal of this is not to create pure white spots but to build up a sort of light sphere here. Repeat until you have a respectable sphere. Create another two somewhere else on the canvas, same way same process.
5) Close the image and overwrite the old imagefile.
     This will save the temporary file back so that HDRShop can refresh the image.
6) Go back to HDR Shop and click the OK button.
     Honest, it's OK to do it now. Pinky-swear.
     You will see your changes updated in the HDRShop window.

Now that we know that we can edit these images it is time to get started on "illuminating" the image, so to speak.
7) Hit the - [minus] key to drop the f-stop parameter down 1.
     If you look at the bottom right of the screen, the "+0.00 stops" is now showing "-1.00 stops." Understanding at this point is not necessary. Just know that this reduces the luminance of the current image in HDRShop.

8) Click "File", then "Edit in Image Editor".
     Time to bring this altered image in for editing.
9) Create a smaller sphere over the three spheres you created.
     We see that the sphere gets brighter again.
10) Close the image and overwrite the old image file.
11) Go back to HDR Shop and click the OK button.
12) Repeat steps 7-11 three or four more times making progressively smaller (but brighter) spheres.
     What we are trying to accomplish is to create three semi-point lights for the HDR image. Every time we drop the f-stop and add more white to it we are brightening the light(s) we are creating. Of course we could add pure white at 100% Opacity, but when it comes to editing precreated images or renders that kind of addition is overkill.
13) Look at your final result in HDRShop.
  If you put your mouse cursor over the light areas you will see that the numbers in the lower right corner will change according to the luminance of the area. These number are independent of the f-stop number and will remain the same no matter what stop you have it at. The minimum this number has to be to create a decent light source is 32.

         Understand that as a whole, the numbers are a reflection of the environment so they will vary widely over the image, but your light sources should have a relatively high value of no less than 32. In doing so, you may also want to make sure that large light areas not exceed the value of 1024. This will cause burning, or large overly-white hot spots, in the render. The key is to be subtle and reproduce realistic lighting. While in some cases you may want the burning associated with a bright light, in natural lighting you want to keep the values at lower levels.
     Pure White (H=255, S=255, L=255, 100% opacity) at lower f-stops will produce higher values in a doubling value. 1 at +0, 2 at -1, 4 at -2, 8 at -3, and so on.
     One word of warning, NEVER add black, or for that matter anything with a HSL-luminance value less than 32 (Paul Debevec recommends 15, but I like to be careful). Adding colors of this range will create artifacting in the image which translates into serious color banding when you change the f-stops parameters. Trust me on this one, just don't do it.

Rendertime:

Now it is time to check the HDR File.

First we must save it. Click File, then Save as. Choose Radiance Format (*.HDR, *.pic), and then name your file. Then click Save.

Click HERE for a good test render file that I use. It's a 415kb zipfile. This is also included in the full zip package above. ED209 (the robot) comes courtesy of 3DCafe.

Open Carrara and the render file. It will ask to find a specific HDR file. Just find the one we just created and double-click on it.

Go to the Render Room and click Render. This will create a 640x480 render of the scene using Skylight and Indirect Light, lighting quality at Best. Don't worry if it comes out dark. This is not real good as a test of HDR images but it will allow you to see what happens when you start adding luminance to an image, even a totally black one.

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