Image Based Lighting (IBL)

WIP - PLESE DO NOT EDIT: UNDER WRITING!!!

Image-based lighting (IBL) is a CG rendering technique/workflow which involves using an omni-directional representation of light information encoded as an image, typically captured or processed as high dynamic range imaging (HDRI) for greater realism. This image is then projected onto a dome or sphere analogously to environment mapping, and path tracing is used to resolve all the light paths that illuminate the objects in the 3D scene. According to both the scattering response (BSDF) of the objects' materials and to the lighting information encoded into the image, multiple importance sampling (MIS) is used to resolve both indirect diffuse and indirect specular effects.

IBL provides fast, easy to setup, highly detailed lighting, shadowing, reflections and refractions and is the de-facto standard lighting technique in global illumination path tracing renderers, it is a viable alternative (but can be also used as a complementary) to more standard techniques that try to model illumination using classic CG lights.

Unlike other renderers who fail at sampling the IBL efficiently, 3Delight provides a proprietary, state of the art, multiple importance sampling (MIS) algorithm which does not require users to:

• downsizing the HDRI
• blurring the HDRI
• using multiple versions of the HDRI for different ray types
• manually controlling the multiple importance sampling of the the HDRI by setting a sampling number

In 3Delight instead you should follow these two basic rules:

1. use a HDRI pixel resolution of twice the size your render output resolution e.g. if you are rendering a full HD frame @ 1920x1080, then your ideal HDRI resolution is 3840x2160.
2. never pre-blur your HDRI: 3delight can efficiently use the same image for both diffuse and specular lighting

Understanding how the IBL contributes to my image:

• illumination
• shadowing
• reflections/refractions
• background
• atmospherics

What image should I use as input for IBL?

• an image captured on set by taking multiple photographic exposures and post-processed into a single image
• a procedural sky texture (usually using the Hosek & Wiklie model)
• a manually crafted image, wither hand painted or created with tools such as "HDRI Light Studio"
•  

What mapping should I use?

• latitude/longitude
• spherical image

Step-by-step guide

Setting up IBL in 3Delight can be nailed down to two simple steps:

1. create the IBL node and map a hdri image or a procedural sky texture
2. enable indirect illumination

Here detailed step that take into account the pre-requisites:

1. create/import some geometry
2. apply the all around 3delight “phisically/visually plausible” material: 3delightMaterial, make one object diffusive, one reflective and one refractive
3. in the preference choose to render on maya render view to see the result in the MRW or in  i-display so to see all the image layers in one go
4. select 3delight as a renderer
5. renderglobals: in the common section enable linear workflow
6. renderpass: create a delight environment
7. apply a texture or a procedural
8. render, you will get mostly a black image except if on the 3Delight material you have set some reflectivity or refractivity which will be stored in the relative image layer
   
   If you do not enable indirect illumination you will not use the environment to illuminate your scene (indirect diffuse) nor receive any information from the surface-to-surface color transport. unlike relfecttion and refraction which are indirect specular effects, in order to receive indirect diffuse and colro bleed in you must turn on indirect illumination.
    
9. renderpass: enable indirect illumination
10. render again, now  your main buffer feature also the indirect diffuse and environment diffuse, additionally all  AOVs are filled accordingly
11. optionally you can put the environment image in the background: this is for previews only, ideally the envronemnt should be composited in your compositor app of choice.

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