Image synthesis and OpenGL: advanced lighting

1. Importance of natural lightings
2. Irradiance environment maps
3. Radiosity mapping
4. Radiosity normal mapping
5. Irradiance normal mapping
6. Environment influence on dynamic objects
7. Dynamic lights
8. Sources
   

Importance of natural lightings

Point light

Environment

Irradiance environment maps

• sum of all light energies received on a part of a surface
• Hypothesis
• Diffuse surface
• Distant illumination
• No shadows
• No inter-reflections
  

Radiance env map

Irradiance env map

• Hemispherical integration on each pixel
• slow!
• Approximation using frequential analysis
• Spherical harmonics
  

Original	Approx order 0 (1 coefs) RMS = 25%	Approx order 1 (4 coefs) RMS=8%	Approx order 2 (9 coefs) RMS=1%

• Spherical harmonics
• good approximation using only 9 coefficients
• per vertex or per fragment
  
• But
  
• only diffuse surfaces
  

Radiosity mapping

• Indirect lighting
• secondary light sources
• global lighting
• slow!! not real-time

computation: 3min

computation: 7h

• Solution:
• precompute solution
  
• use it in real-time applications
• Only works with view-independent materials
• diffuse surfaces (only depends on surface normal)
  
• \( \longrightarrow \)radiosity: model the transfer of energy between surfaces

Direct lighting only

Direct lighting + radiosity

• Light maps
• efficient / fast
• limited to diffuse materials
• limited to static scenes
• no high frequencies

• Problem: what if we do not know the orientation of the surface in advance (i.e. normal mapping)?
  

Radiosity normal mapping

• Combine light maps and normal maps
• light map = global illum effect per vertex (low freq)
• normal maps = geometry details
• Surface normal not known in advance
• compute radiosity for a few space directions
• interpolate in the shader according to the (detailed) normal
  

Radiosity sampling

Rendering

• 3 radiosity values per vertex (1 for each direction)
• project normal in the directional space
• interpolate radiosity value from these 3 directions

Without radiosity normal mapping
With radiosity normal mapping

Irradiance normal mapping

• Identical to Radiosity Normal Mapping execpt that
  
• hemispherical harmonics are used to store irradiance at each sample
• defined in the tangent plane
• refinable basis functions

• Efficient for indirect lighting
• hemispherical harmonics
  
• stored per vertex (about 6 coefs)
• fast to evaluate (polynomial expansion)
• Allows to deal with normal variations
• irradiance defined in the whole hemisphere in the tangent plane
• allows big normal variations
• Limitations
• static objects
• object-dependant (not reusable)
• A video here
  

Environment influence on dynamic objects

Irradiance volume

• Ambient cube
• Irradiance sampled in 6 directions
• Can be precomputed at every point of a scene
  

• Scene sampling
• coarse grid (only needs a low resolution effect)
• by designer (probes placed at different positions in the scene)
  

• Object lighting
• from a given object
• get the closest ambient cube
• approx irradiance according to surface normal and cube faces

Without ambient cube

With ambient cube

Light propagation volumes

• Sample secondary sources
• Group samples in a coarse grid
• Propagate samples in neighbor cells
• Use this radiosity information according to object positions in the scene
  

Dynamic lights

Single pass

• For each object
• render object, apply all lighting in one shader

• Limitations
• hidden surfaces: wasted shading
• hard to manage multi-light situations
• hard to integrate with shadows

Multipass lighting

• For each light
• For each object affected by light
• framebuffer += brdf(object,light)

• Limitations
• hidden surfaces: wasted shading
• high batch count (1/object/light)
  
• lots of repeated work at each pass (objects may be rendering many times)
  

Deferred shading

• For each object
• render lighting properties to G-Buffers (positions/normal/color/...)
• For each light
• framebuffer += brdf(G-Buffers,light)

• Integrated in most rendering engines (ex. CryEngine3)
• integrates well with global illum techniques, shadows, npr effects, screen space ambient occlusion, etc
• lots of variations
  

• Limitations
• aliasing (needs special post-process algorithms - screen space anti-aliasing methods)
• transparency (draw transparent objects in a second step)

Sources

• J. Baril, F. Berthaut, N. Mellado
• An Efficient Representation for Irrandiance Environment Maps [Ramamoorthi and Hanrahan, 2001]
• Shading in Valve's Source Engine [Mitchell et al. 2006]
  
• Efficient Self-Shadowed Radiosity Normal Mapping [Green, 2007]
  
• Efficient Irradiance Normal Mapping [Habel and Wimmer, 2010]
• Light Propagation Volumes in CryEngine 3 [Kaplanyan 2009]
• Deferred Shading [Nvidia 2004]
  
• Siggraph 2010 course notes