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An Introduction to Light Fields Mel Slater

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Outline Introduction Rendering Representing Light Fields Practical Issues Conclusions

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Introduction L(p, ) is defined over the set of all all rays (p, ) with origins at points on surfaces – Domain of L is 5D ray space. L is constant in ‘free space’, space free of occluders: ray occluded ray in ‘free space’

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Introduction: Free Space Radiance is constant along each ray in free space emanating from the convex hull around the scene

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Introduction: Restrict L to 4D In free space the domain of L can be restricted to 4D L(s,t,u,v) can represent all rays between two parallel planes – a ‘light slab’. st plane uv plane Here radiance is flowing in direction uv to st.

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Introduction: Light Field = 6 Light Slabs 6 light slabs can represent rays in all directions: – Near-far, Far-near – Left-right, Right-left – Bottom-top, Top-bottom Discretisation provides approximation to ‘all rays’.

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Introduction:Assigning Radiance to Each Ray st plane uv plane cop Image plane Place the cop at each grid point on st plane Use uv plane as an image plane. Each ray is assigned a radiance.

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Introduction: Synthesising a New Image st plane uv plane Image plane cop New image plane s0s0 s1s1 s2s2 s3s3 s4s4 u0u0 u1u1 u2u2 u3u3 u4u4 The image ray is approximated By L(s 3, u 1 )

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Introduction: Summary Light field is a discrete representation of L restricted to the 4D free space domain. It relies on production of a large set of images under strict viewing conditions. Most often used for virtual images of real scenes – based on digital photographs. Can be used for synthetic scenes – not much point unless globally illuminated Supports real-time walkthrough of scenes.

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Rendering Each st grid-point is associated with a uv image s t u v Let a texture map consisting of the image be associated with st grid-point.

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Rendering: Texture Mapping Each st grid-point square is projected to the uv plane t u v a b c d cop The st square is rendered with the texture map and texture coords abcd s

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Rendering: Quadrilinear Interpolation Perform linear interpolation on s Repeat expansion for t,u,v. s0s0 s1s1 u0u0 u1u1 v0v0 t0t0 t1t1 v1v1 L(s,t 0,u 0,v 0 )= 0 L(s 0,t 0,u 0,v 0 )+ 1 L(s 1,t 0,u 0,v 0 ) (s,t) (u,v)

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Efficient Rendering: Linear- Bilinear Interpolation Gortler shows that texture mapping cannot be used to reproduce quadrilinear interpolation, but linear (on st), bilinear (on uv). P(s,t) A BC D EF For the grid-point P render each of the triangles ABP, BCP, CDP, DEP, EFP,FAP with alpha-blending and texture mapping: =1 at P and =0 at each vertex.

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Rendering: Depth Correction Use approximate polygon mesh to provide depth information. cop s0s0 s1s1 s2s2 u0u0 u1u1 u2u2 u3u3 u4u4 object surface Normally s 1 u 4 would be used to approximate viewing ray. s 1 u 3 is more accurate.

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Representation The 2PP has many computational advantages: – Efficient representation of images – Texture mapping hardware used for rendering – Linear properties of the representation. It is not a ‘uniform’ representation – As the viewpoint moves so the density and distribution of close-by rays will change, and thus image quality will change during walkthrough.

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Representation: Using Spheres Camahort and Fussel, 1999 carried out study of 3 representations: – 2PP – 2-plane – 2SP – 2-sphere – DPP – direction and point DPP theoretically shown to introduce less sampling bias, greater uniformity.

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Practical Issues Idea placement of st and uv planes? – st outside the scene, and uv ideally through ‘centre’ of scene. – Ideally uv grid-points as close as possible to scene geometry Implication is that scenes with large depth range cannot be adequately represented.

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Practical Issues: Resolution What resolution should be used for st, uv? – st (M M) can afford to be sampled at lower rate, since the accuracy depends on uv (N N), close to scene surfaces. – Typical values are M=32, N=256 for synthesis of 256 256 resolution images

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Practical Issues: Memory Suppose N=256, M=32 – Each light slab requires 2 8 2 8 2 5 2 5 =2 26 rays – Each ray carries 3Bytes (RGB) – There are 6 light slabs. Full representation therefore requires: – 6 3 2 26 > 1Gb memory. Compression scheme needed (vector quantization used in practice).

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Conclusions LF approach offers a different paradigm with many disadvantages, but interesting possibilities: – Exploits current hardware for a brute-force ‘solution’ of radiance equation. – Image of real scenes or synthetic scenes (and combinations). – Rendering time independent of scene complexity. – Promise of real-time walkthrough for global illumination

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Global Illumination with a Virtual Light Field Mel Slater Jesper Mortensen Pankaj Khanna Insu Yu Dept of Computer Science University College London

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