Ambient Aperture Lighting using Precomputed Radiance Transfer

  • By @shailen@
  • 25 September, 2013
  • Comments Off on Ambient Aperture Lighting using Precomputed Radiance Transfer

Generation of soft shadows + hard shadows using a combination of global illumination techniques

  • Precomputed Radiance Transfer
  • Ambient Aperture Lighting
Overview of Precomputer Radiance Transfer
(Sloan et. al. 02)
Precompute transfer vectors which map source radiance to exit radiance were computed. These vectors were used to calculate exit radiance in real time.
Overview Of Ambient Aperture Lighting
Terrain - Ambient Aperture Lighting
(Christopher & Sander, 2007)
  • Shading model that uses apertures to approximate a visibility function
  • Precomputed visibility is used.
  • Dynamic spherical area light sources are supported
  • Dynamic point light sources are supported
  • The “ambient” comes from the use of a modified ambient occlusion calculation to find an aperture of average visibility
 The Technique
  • Perform a Precomputed Radiance Transfer calculation to determine the spherical harmonic coefficients of transfer vectors which map source radiance to exit radiance
  • Use these coefficients per-vertex to determine % visibility and average visibility direction
  • Modulate lighting generated using PRT with this visbility data to generate hard shadows
  • Use the precomputed transfer vectors to represent low frequency shadowing which simulated soft shadows
  • Modulate the lighting computed by transfer vectors with Ambient Aperture Lighting calculation for hard shadows
Transfer Vector Precomputation
  • A metric of occlusion at each vertex
    • Projected to a Spherical Harmonic 2nd Order Basis. Store in a buffer.
    • Compress the buffer. Use PCA basis vectors to reduce dimension of the stored vectors. Cluster the vertices for compression
  • Statically calculated
  • Dynamically calculate on the GPU the exit radiance
Ambient Aperture Lighting
(Christopher & Sander, 2007)
  • Ambient aperture lighting works in 2 stages
    • Precomputation Stage
      • Visibility function is computed at every point on mesh
        • Per-vertex or per-pixel
      • Visibility function is stored using a spherical cap
      • Spherical cap stores an average, contiguous region of visibility
        • A spherical cap is a portion of a sphere cut off by a plane (a hemisphere itself is a spherical cap)
    • Rendering Stage
      • Spherical cap acts as an aperture
      • Aperture is used to restrict incoming light so that it only enters the from visible (un-occluded) directions
      • Area light sources are projected onto the hemisphere and are clipped against the aperture
      • This determines how much of their light passes through the aperture
Results
Ambient Aperture Lighting + PRT
Ambient Aperture Lighting
PRT
Limitations
  • Cannot account for deformable surfaces
  • Need a unified framework for both rigid and non-rigid geometry
  • Uses a single averaged view vector for the apertures which is not true in multiple light source scenario
Conclusion
  • The proposed technique aims at realistic and efficient soft shadows and hard shadows due to global illumination
  • Next phase deals with issues regarding making even more efficient use of the GPU Architecture
  • Extend to the domain of the deformable surfaces.
Biblography
Cristopher, O., & Sander, P. V. (2007). Ambient Aperture Lighting. I3D.
Precomputed Radiance Transfer for Real-Time Rendering in Dynamic, Low-Frequency Lighting Environments P.-P. Sloan, J. Kautz, J. Snyder SIGGRAPH 2002
Clustered Principal Components for Precomputed Radiance Transfer P.-P. Sloan, J. Hall, J. Hart, J. Snyder SIGGRAPH 2003
Frank, L., & Hughes, H. (2004). Geometry Clipmaps: Terrain Rendering Using Nested Regular Grids. ACM Transactions on Graphics (TOG) , 769 – 776 .
Kontkanen J., L. S. Ambient occlusion fields. In ACM Symposium.
Michael, B. Dynamic Ambient Occlusion and Indirect Lighting. In Gpu Gems 2.
Microsoft DirectX SDK ( Nov 2007 )
Categories: Projects

Comments are closed.