Complex Shading Algorithms

Transcription

1 Complex Shading Algorithms CPSC 414

2 Overview So far Rendering Pipeline including recent developments Today Shading algorithms based on the Rendering Pipeline Arbitrary reflection models (BRDFs) Bump mapping Environment mapping

3 Reflection Models In previous lecture: Reflection models in hardware limited to diffuse + Phong or Blinn-Phong Physically not plausible (no preservation of energy) Better reflection models Based on physical principles Derived by analyzing small-scale structure of objects E.g.: Microfacet models

4 Microfacet Models Model: Rough surface is composed of small, planar parts that are perfect mirrors (microfacets) Perfect mirror : Reflect incoming light from one direction to exactly one outgoing direction Correct term: Fresnel Reflector

5 Fresnel Reflector Synopsis: Perfectly smooth surface Thus light from one direction only reflected in exactly one outgoing direction Percentage of light reflected depends ONLY on the optical densities ( index of refraction ) of the materials separated by the surface (e.g. air and glass) All energy not reflected is refracted in exactly one direction (Snell s law) E.g.: Water, glass have optical densities of about 1.5: reflection only at grazing angles, not at perpendicular angles Metal: high density (several 100s) almost all light is reflected, independent of angle

6 Microfacet Models Contributing Factors to Microfacet BRDFs: Facet Distribution Function: Describes the statistics of the microfacets, I.e. how likely it is that we find a facet with a certain orientation Fresnel term: Describes the amount of light reflected for a given facet normal and light direction (see previous slide) Geometric term Describes shadowing and occlusion of facets

7 Example: Torrance-Sparrow Model Torrance-Sparrow ('67): n f r =F(θ)D(δ) G(α,β)/(π cosα cosβ) Sample factors store as 2D textures Texture coordinates: v β δ θ h α l reparameterize terms over cosines of angles allows for hardware-based texture coordinate generation

8 Torrance-Sparrow Model * +

9 Banks Model Anisotropic model (Banks '94, Heidrich '98)

10 Sampled BRDFs Factorization of sampled BRDFs (Kautz 99) measured/simulated BRDF data rendering similar to analytic models

11 Summary Material Rendering Texture-based lighting: Use texture mapping for storing complex functions like BRDF models Make sure the texture coordinates are easy to compute In our case: use cosines of angles rather than angles themselves An be computed as part of vertex program! Maximum flexibility by replacing fixed-function hardware lighting with arbitrary illumination models Implements per-fragment lighting (similar to Phong shading!)

12 Overview Shading algorithms based on the Rendering Pipeline Arbitrary reflection models (BRDFs) Bump mapping Environment mapping

13 Bump Mapping Concept: Draw very simple geometry Use texture mapping to perturb normal at every surface location before performing a per-fragment lighting operation

14 Bump Mapping Original Approach (Blinn 76): Store texture with a single channel (height value): Compute per-pixel normal as if the surface was displaced by the height value along the interpolated geometric normal Use that normal for per-fragment lighting

15 Bump Mapping Normal Mapping Approach: Skip the computation of the normal That is the expensive step Directly encode the normal into the texture map (R,G,B)= (x,y,z), appropriately scaled Then only need to perform illumination computation Interpolate light and viewing direction from the vertices of the primitive Use register combiners for doing lighting computation

16 Bump Mapping Register Combiner Lighting Diffuse: Inputs: 8Gouraud-interpolated color holds light direction 8Texture lookup yields normal for fragment Combiner operation: 8Compute dot product between the two vectors

17 Bump Mapping Register Combiner Lighting Blinn/Phong specular component: Inputs: 8Halfway vector between light and viewing direction (coded as Gouraud interpolated surface color) 8Texture lookup yields normal for fragment Combiner operation 8First combiner stage computes dot product 8Other combiner stages can successively square result to achieve different exponents

18 Bump Mapping Examples

19 Bump Mapping Issues: How do we compute per-vertex light vector, viewing vector, and halfway vector? Software or vertex programs What about other exponents or general reflection models? Dependent texture lookup: 8Use register combiners to perform per-fragment dot products 8Use the result as texture coordinates into an arbitrary reflection model.

20 Coming Up Wed, Nov 21: Shadow algorithms Fri, Nov 23 - : Ray-Tracing