Image-based lighting, often abbreviated as IBL, is a common approach to illuminate a scene by a high-dynamic range image taken from a real-world environment. Such an image is able to capture and transport the original light energy and colors from the physical world to the renderer. mental ray provides a native IBL solution for image-based lighting or lighting from procedurally generated environment. It delivers results with superior quality and optimized performance compared to traditional ways, like using final gathering combined with explicitly modeled geometry surrounding the scene with the IBL image mapped onto its surface.
Once enabled, and a regular environment shader is attached to the render camera, mental ray will create an internal special light source which returns the illumination from the environment. For rendering, mental ray is pre-sampling the environment and creating lookup acceleration data structures to support importance-driven adaptive sampling during the lighting computations in shaders. This light contribution is automatically added to all shaders performing regular direct light sampling, thus allowing existing shaders to benefit from this feature.
The IBL mode can be used in conjunction with progressive rendering.
For interactive use cases, mental ray provides optimization features which allow to generate "convincing", but not necessarily physically accurate, results much faster. The approximate IBL lighting mode can be used to trade speed for quality. This feature may be coupled with the occlusion cache which stores pre-computed occlusion information for the geometry in the scene. This data can be used during rendering to approximate shadowing and render more quickly by avoiding actual shadow tracing. These technique allow to produces images with much less noise at the cost of pre-processing time and visible artifacts. Optionally, the exact IBL solution can be blended in over time, fading out the interpolation errors and finally converging to the exact solution. Existing shaders don't need to be adjusted to take benefit of these optimization features.
The IBL can be controlled with scene options.
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