These are descriptions of the attributes in the mental ray IBL (Image Based Lighting) node. See Image-based lighting (sky-like illumination) and also Render infinitely distant (sky-like) illumination and reflection.
These are the attributes in the Image Based Lighting Attributes section.
Hardware Exposure controls gamma for hardware display. Gamma is the overall brightness of an image. Changes to gamma not only adjust the brightness, but also adjust ratios of colors in the image.
A value of 1 means that the color is not adjusted. Increase the value to make the mid-range tones brighter. Decrease the value to make them darker.
These attributes are the same as those of Maya's texture node. They apply in similar ways to IBL.
For more information on texture nodes, see Overview of texture nodes in the Shading book.
Select whether to map a file or a procedural texture as the IBL texture. File textures can be previewed interactively. See also Texture.
The default value for this attribute is on, and results in the default IBL behavior.
If you turn this attribute off, IBL switches to Finite mode. In Finite mode, IBL behaves as a finite sphere, where translation and uniform scaling are supported. Photon and light emission behave as point lights, not directional lights, so that IBL can be used for closed environments. To avoid artifacts, ensure that your IBL sphere encloses your entire scene.
Controls the IBL texture's visibility to certain ray types. This maps to a camera environment shader in mental ray. If all controls are deactivated, then no environment shader is processed. For classical image based lighting, the IBL texture must be visible to final gather rays. and Final Gathering must be on.
These are the attributes in the Light Emission section.
You can emit light from the IBL environment radially inward toward the origin (of world space). If light emission is turned on, the IBL node acts like true light sources in the scene. The IBL texture is mapped to a mental ray light shader just like a real light source would be.
The light shader sets up a “control texture” for light emission. Every pixel in that texture virtually represents a directional light. These values specify the control texture's resolution. Higher values offer more precision but are more processor intensive. Loaded IBL textures are downsampled to the filter resolution; procedural textures are sampled at this resolution in pixel units for optimum performance.
Sampling all “directional lights” represented by the control texture is often prohibitively expensive. Therefore the shader has a built-in importance mechanism that attempts to select the primary (key) lights.
The first Samples parameter specifies the number of important lights that must be sampled. The second parameter quasi-randomly selects a certain number of secondary (fill) lights.
These are the attributes in the Photon Emission section.
Photons can be emitted from the IBL environment sphere. This requires Global Illumination and, or Caustic photons to be turned on in the Render Settings window. The emission direction is chosen randomly for individual photons. Similar to light emission, a photon picks up the color based on its emission direction from the IBL texture (file or procedural), possibly adjusted by color gain and offset. The results of this maps to a mental ray photon emission shader.
If off, photons will be stored on first hit. This is useful if IBL is to be achieved using global illumination photons alone. In terms of a real-world environment you can think of this switch as photons from the Sun being scattered in the atmosphere before reaching any surface.
Turn this option on, if you are emitting caustic photons, or if you are emitting light in combination with photons.
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