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Class6 - Class 6. Lighting and Shading (HW4) CS580...

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Class 6. Lighting and Shading (HW4) Ulrich Neumann CS580 (Computer Graphics Rendering)

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Review: the shading Eq C = (Ks Σ L [Ie (R E) s ] ) + (Kd Σ L [Ie (N L)]) + (Ka Ia) Note – V in figure is E (eye ray)
Examine the Vectors L is the direction to an infinitely far point source constant in whole scene and specified by the application E is constant and set by camera view direction known direction in image space (0,0,-1) know world space camera position and look-at point approximate far away camera with narrow FOV a camera close to the scene requires E computed for each model vertex or pixel N is specified at triangle vertices - given in model space R must be computed for each lighting calculation R = 2(N L)N - L (next slide) L, E, N, R must be transformed to the same space for shading Eq evaluation: model, world, or image space assume any of these spaces for now…

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Compute Reflection Ray R = 2(N L)N - L R N L θ θ 2(N L)N -L (N L)N
Vertex Normals and Face Normals What are the normals to the surface? Each polygonal face has a normal N = (b - a) x (c - b) Polygon mesh is only an approximation of true curved surfaces - can we do better? use the actual surface normals sample at vertices assume smooth variation over faces a b c N

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Vertex Normals Vertex normals are duplicated at each shared vertex of adjacent-faces Two options when adjacent-faces share the same vertex position Same normal at adjacent-face vertex for smooth shading Different normal at adjacent-face vertex for sharp edges
Cases that come up in shading (1) You need to deal with these in the HW Sign of N L and N E Test the illumination and the viewing direction relative to the normal of a surface Both positive : compute lighting model Both negative : flip normal and compute lighting model on backside of surface Both different sign : light and eye on opposite sides of surface so that light contributes zero – skip it

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Cases that come up in shading (2) R E calculations must be clamped to zero to maintain [0, 1] bounded range R E may be negative for front or back surface illuminations Watch for color overflow from multiple lights Overflow causes black holes Occurs when converting float values >1.0 to 12-bit GzIntensity values written to the frame buffer R N L E
Shading Calculation Logistics Application specifies lights (Ie and Ia) and material properties (Ka, Kd, Ks, s) before we start any triangles Renderer uses these values until they are overwritten The geometry (vectors) and these parameters are all the elements needed to compute the shading equation However - we only know all this information at the model vertices where N is specified

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All pixels need color All terms of shading eq are known (and fixed) everywhere on the triangle, except for Normals Norms are specified per-vertex – they can vary to approximate
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This note was uploaded on 04/22/2010 for the course CS 580 at USC.

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Class6 - Class 6. Lighting and Shading (HW4) CS580...

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