Measurement, Inverse Rendering COMS , Lecture 4

Motivations True knowledge of surface properties Accurate models for graphics Augmented reality, scene editing

Photorealistic Rendering Materials/Lighting (Texture Reflectance[BRDF] Lighting) Realistic input models required Arnold Renderer: Marcos Fajardo Rendering Algorithm 80’ s,90’ s : Physically based Geometry 70’ s, 80’ s : Splines 90’ s : Range Data

Flowchart Photographs Geometric model Inverse Rendering Algorithm Lighting BRDF

Flowchart Photographs Geometric model Forward Rendering Algorithm Lighting BRDF Rendering

Flowchart Photographs Geometric model Forward Rendering Algorithm BRDF Novel lighting Rendering

Next 3 slides courtesy George Drettakis

Taxonomy 1 General function = 12D Scattering function = 9D Assume time doesn’t matter (no phosphorescence) Assume wavelengths are equal (no fluorescence) Single-wavelength Scattering function = 8D Assume wavelength is discretized or integrated into RGB (This is a common assumption for computer graphics)

Taxonomy 2 Single-wavelength Scattering function = 8D Bidirectional Texture Function (BTF) Spatially-varying BRDF (SVBRDF) = 6D Ignore subsurface scattering (x,y) in = (x,y) out Bidirectional Subsurface Scattering Distribution Function (BSSRDF) = 6D Ignore dependence on position Light Fields, Surface LFs = 4D Ignore direction of incident light Texture Maps = 2D Assume Lambertian 3D Assume isotropy BRDF = 4D Ignore subsurface scatteringIgnore dependence on position 2D Measure plane of incidence 0D Low-parameter BRDF model

Outline Motivation Taxonomy of measurements BRDF measurement Highlights from recent work Next week: paper presentations

Definition of BRDF Source  src  src Detector  det  det dA Next several slides courtesy Szymon Rusinkiewicz

Measuring BRDFs A full BRDF is 4-dimensional Simpler measurements (0D/1D/2D/3D) often useful Start with simplest, and get more complex

Measuring Reflectance 0º/45º Diffuse Measurement 45º/45º Specular Measurement

Integrating Spheres Sphere with diffuse material on inside Geometry ensures even illumination More accurate measure of diffuse reflectance

Gloss Measurements Standardized for applications such as paint manufacturing Example: “contrast gloss” is essentially ratio of specular to diffuse “Sheen” is specular measurement at 85°

Gloss Measuements “Haze” and “distinctness of image” are measurements of width of specular peak

BRDF Measurements Next step up in complexity: measure BRDF in plane of incidence (1- or 2-D)

Gonioreflectometers Three degrees of freedom spread among light source, detector, and/or sample

Gonioreflectometers Three degrees of freedom spread among light source, detector, and/or sample

Gonioreflectometers Can add fourth degree of freedom to measure anisotropic BRDFs

Image-Based BRDF Measurement Reduce acquisition time by obtaining larger (e.g. 2-D) slices of BRDF at once Idea: Camera can acquire 2D image Requires mapping of angles of light to camera pixels

Marschner’s Image-Based BRDF Measurement For uniform BRDF, capture 2-D slice corresponding to variations in normals

Marschner’s Image-Based BRDF Measurement Any object with known geometry

BRDF Measurement is Hard!

Reflectance modeling (diff +specular texture) InputSynthesized Sato, Wheeler, Ikeuchi 97

Image-based measurement of skin Marschner et al. 2000

Inverse Global Illumination Yu et al. 99

From a single image Boivin and Gagalowicz 01 Original Photo

Assignment (by tomorrow) Brief of proposed project, partners, plan of action (milestones) Iterate by or schedule appointments later in the week 1-2 page proposal due next Wed. including an intermediate milestone (Oct. 23)