Image Based Lighting (IBL)

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 and processed as high dynamic range imaging (HDRI) for greater realism. This image is then projected onto a dome or sphere analogously to environment mapping. Path tracing is then used to resolve all the visibility and light paths that illuminate the objects in the 3D scene from the IBL. According to both the materials' scattering response (BSDF) 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 renderers, IBL can be also used as a complement to the more standard techniques that try to model illumination using classic CG lights, blockers and bouncers.

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

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

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

1. use a large pixel resolution for your HDRI, ideally use 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 the final image.

• illumination
• shadowing
• reflections/refractions
• background
• atmospherics (TBD)

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 (the Hosek & Wiklie model, used in the 3DelightSky provides superior quality than the Preetham model)
• a manually crafted image, wither hand painted or created with ad-hoc authoring 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 three simple steps:

1. create the IBL node and map a hdri image via a File node or a procedural sky texture like 3DelightSky node
2. enable indirect illumination
3. render

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

1. make sure the 3Delight for Maya plugin is loaded for you version of Maya, create a new scene and select 3delight as a main renderer in render globals
   
2. create/import some geometry, for this scene you can use the beautiful and free "Ajax 3D OBJ scan model" from Jotero
3. apply the general purpose 3delightMaterial, a “phisically/visually plausible” material which uses the Cook-Torrance BSDF
4. in the preference choose to render on maya render view (mrw) or in  i-display, in the matter case you can see all the image layers (AOVs) in one go
5. Enable color management to achieve a proper linear workflow, in the Render Globals> Common section
6. In Render Globals> 3Delight's defaul render pass create a delight environment
7. apply an HDRI texture:
   1. download and use the "pisa.hdr" texture from Paul Debevec's High Res Probes which does not require any intensity tweak:
      
      
   2. or a procedural texture such as 3DelightSky:
   3. or a "hand-made" one done in e.g. Nuke or in HDRI Light Studio:
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
11. setup more render layers and render in i-display, where all additional AOVs are filled accordingly
12. optionally you can put the environment image in the background: this is for previews only, ideally the envronment should be composited in your compositor app of your choice.

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