HDRI (High Dynamic Range Image) is the magic word when it comes to creating ultra realistic images. While normal textures save their color values between (0.0, 0.0, 0.0) for black and (1.0, 1.0, 1.0) for white, an HDRI can have much brighter values than (1.0, 1.0, 1.0). These values can go up to 100.000. For example, the sun is much brighter than a white sheet of paper. Because an HDRI map catches this higher-range lighting information from the environment it can be used to illuminate the scene. The HDRI background works like a spherical area light source.
The HDRI tag extends the camera object with the ability to add an HDRI background to the scene. This HDRI background can then be used by the renderer to sample reflections, refractions, ambient occlusion or radiosity samples.
Attention: You can only add an HDRI tag to a camera object.
The quality of a HDRI rendering is highly dependent on the quality of the HDRI texture. A excellent source for affordable high end HDRI textures is www.realtexture.com . Cheetah3D comes with two small sample HDRI textures from www.realtextures.com . You can find them in the examples folder.
Advantages of the HDRI Tag:
- Illumination of the scene: The ambient occlusion or radiosity algorithms will sample the HDRI background to estimate how much light arrives from the environment. Even without a real light source a scene can be illuminated using an HDRI background alone.
- Realistic reflections/refractions: The HDRI background can be used to create very realistic reflections or refractions. Lets assume your material has a reflection intensity of 0.1. If you used a normal environment texture (like a .jpg) you will get a value of (1.0, 1.0, 1.0) when the sun is sampled. Multiplied by 0.1 the reflected sun ends up with a dark grey (0.1, 0.1, 0.1) in the reflection. But now lets use an HDRI as background. The sun might be sampled as (100000.0, 100000.0, 100000.0) for example. Multiplied by 0.1 you get (10000.0, 10000.0, 10000.0). After clamping this ends up as a nice realistic white reflection (see the head of the penguin on the image below)
- Soft shadows: Because the HDRI can be seen as a big spherical area light the shadows created by it in ambient occlusion or radiosity are always soft (see second image).
- Background: You get the background of your scene for free!
 This scene was rendered with an HDRI background and ambient occlusion. Check out the realistic reflections and the soft shadows under the penguin's wings.

Another simple scene illuminated by a HDRI. Once again check out the nice bright
reflections and the realistic soft shadows.
Properties
- .hdr image: The filename of the HDR image. You can only load HDRIs which come in the .hdr file format.
- Type: Defines the mapping of the .hdr file. If your .hdr file was saved in the probe format for example you have to select probe to get the right mapping. There are three widely used mappings available in Cheetah3D.
- Power: A scale factor for the HDRI texture. Every color sampled from the HDRI will be multiplied by this factor.
- Clamp power: Pixels in the HDRI with a very high intensity (for example in the sun) can cause serious artifacts in combination with ambient occlusion or radiosity renderings. The artifacts can be easily reduced by clamping the high intensities. Don't set the clamp value to low however, or you will lose all the benefits of using an HDRI.
- Rotate background: Rotates the HDRI texture around the worlds Y axis .Some HDRI textures look better in certain directions so it is often desired that these parts of the texture appear in the background of the rendering. With help of the rotate background property you can achieve this.
- Background: The HDRI will also be used for the scene background. Take care when using this since it will interacts with the fog tag, if you use one.
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