# CAP 4703 Computer Graphic Methods Prof. Roy Levow Chapter 6.

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CAP 4703 Computer Graphic Methods Prof. Roy Levow Chapter 6

Light and Matter  Surface may –emit light –reflect light  Rendering equation –Recusrively applying the effect from a source to the viewer yields the  Equation cannot be solved in general  Approximate approaches can be computationally intensive

Approximations  Ray tracing –trace light from source to eye  most rays don’t reach the eye  Radiosity –Treat surfaces as small regions each emitting fixed light  Calculate emission independent of viewer  Simplified ray tracing –Single interaction between light source and each surface

Light and Surfaces

Interaction with Surfaces  Specular surface –Similar to mirror –Light leaves at same angle as entry

Interaction with Surfaces (cont)  Diffuse Surface –Light leaves in all directions

Interaction with Surfaces (cont)  Translucent Surface –some light penetrates surface –leaves at modified angle – refraction –some reflected

Light Sources  Six variable illumination function I(x,y,x,theta, phi, lambda) –Generally too complex to compute for all points

Light Sources  Consider four simpler light sources –Ambient lighting  Omnidirectional; constant everywhere –Point source  Radiates in all directions –Spot lights  Small cone of constant light –Distant light  rays are parallel

Color Sources  Describe color with 3-dimensional RGB vector –(Ir, Ig, Ib) T

Characteristics of Light Sources  Ambient light –every point in scene receives same light in every direction  Point source –Intensity decreases with inverse square of distance –Often approximate with form (a + bd +cd 2 ) -1 where d is distance to soften effect

Characteristics of Light Sources  Spot light –Light limited to cone of angle –diminishes toward edges of cone as cos e (theta) for some value of e –intensity as for point source  Distant source –constant intensity –fixed direction

Phong Reflection Model  Described by 4 vectors n – normal to p v – to viewer l – to source r – reflected ray

Phong Reflection Model  Supports –ambient –diffuse –specular or combination of these  Uses 3 x 3 matrix –columns for r – g – b –rows for ambient, diffuse, specular  Color source and reflection matrices

Phong Reflection Model  resulting intensity is sum or products of corresponding terms I ir = R ira L ira +R ird L ird +R irs L irs = I ira +I ird +I irs = I ira +I ird +I irs for red, etc  Sum over all points for complete effect

Ambient Reflection  Ra = k a, 0 <= k a <= 1 –a fraction of light is reflected –one coefficient for each color

Diffuse reflection  Lambertian surfaces –reflect only vertical component –R d proportional to cos(theta) –I d = k d (l. n) L d / (a + bd +cd 2 )

Specular Reflection  Phong model equation I s = k s L s (R. v) a k s is shininess coefficient

OpenGL Shading  Polygonal shading –Generate shading on individual polygons  Flat shading glShadeModel(GL_FLAT); glShadeModel(GL_FLAT); –select one vertex of polygon for normal –same color throughout polygon –tends to produce artificial looking surfaces with distinctly colored regions –eye is very sensitive to change  Mach Bands

Interpolative and Gourand Shading  GL_SMOOTH –average normals –example: plates 4 & 5

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