Illumination and Shading

Transcription

1 Illumintion nd hding In order to produce relistic imges, we must simulte the ppernce of surfces under vrious lighting conditions. Illumintion models: given the illumintion incident t point on surfce, wht is reflected? hding lgorithms: determine when nd how to pply the illumintion model, in order to provide color for every visible surfce point. 1 ight ource Models Generl (rel life) light sources often hve complex geometry nd emission chrcteristics. In computer grphics, the following simplified models re commonly used: Directionl light source: ll light rys re prllel to prticulr direction. Point light source: ll light rys originte t prticulr point in the scene, in ll directions. potlight: lie point lie source, but ry directions re limited to cone. 2

2 The BDF Bidirectionl eflection Distribution Function - describes the rtio of light intensity leving point in some outgoing direction to the differentil irrdince from some incoming direction: 3 Diffuse eflection Diffuse (mbertin) surfces pper eqully bright from ll directions. θ The intensity of light reflected by point x on diffuse reflector is I I cosθ I r p d p d ( ) 4

3 The Ambient Term In the rel world, objects receive light both from light sources nd from other surfces in the scene. This indirect illumintion is very corsely modeled by dding n mbient term into the shding model: I r I I p d ( ) 5 Exmples: mbertin model Diffuse term only: Diffuse Ambient: 6

4 ight-source ttenution The irrdince due to physicl light source flls off proportionlly to the squre of the distnce. This is ccounted for by introducing light-source ttenution fctor into the shding eqution: I r I f tt I p d ( ) Typiclly, f tt 1 min, 1 2 bd cd 7 peculr eflection Phong Bui-Tuong introduced term for simulting speculr highlights on non-idel (glossy) reflectors: θ θ V I I r f tt I p [ ( ) ( V ) ] n d s 8

5 The peculr Exponent The Phong exponent n determines how concentrted the speculr pe is: 9 Exmple: Phong model 10

6 11 The eflected Vector The Phong model mes use of the reflected vector, which is computed s follows: ( ) 2 ( ) ( ) ( ) ( ) ( ) ( ) Multiple light sources ( ) ( ) [ ] 1 i n i s i d p tt r V I f I I i i The shding model esily extends to the cse of multiple light sources:

7 Polygon hding Flt (constnt) shding Evlute the shding model once per polygon, use resulting color for ll of its pixels. Gourud shding Evlute the shding model t ech vertex, nd linerly interpolte resulting vlues inside the polygon Phong shding Evlute the norml t ech vertex, nd linerly interpolte it inside the polygon. Hving the interpolted norml t ech point inside the polygon, we cn use it to clculte the shding model in ech point. 13 Gourud shding mooth surfces re commonly represented s collection of polygonl fcets for the purposes of interctive disply. If ech fcet is shded individully, it is esy to see the shding discontinuities, which result in fceted ppernce 14

8 Gourud shding To eliminte fceted ppernce we cn use Gourud shding (linerly interpolted shding). For the resulting shding to be continuous we need ech vertex in the polygon mesh to hve the sme norml for ll fces incident on it. 15 Gourud shding Clculte normls on ech vertex of the polygon 16

9 Gourud shding Cculte shding model on ech vertex nd interpolte it long the edges. 17 Gourud shding Interpolte shding model long the scnlines 18

10 Gourud shding esult, smooth shding with slow (liner) speculr effect 19 Phong hding Better results cn be obtined by linerly interpolting the normls between the vertices, nd recomputing the shding t every pixel: Gourud shding Phong shding 20

11 Phong hding Evlute the normls t ech vertex 21 Phong hding Interpolte normls on the polygon s edges 22

12 Phong hding Interpolte normls on ech pixel on the scnline 23 Phong hding Clculte shding model on ech pixel of the scnline 24

13 Phong hding esult, shows speculr highlight clerly

14 Comprisons peculr highlights re different, liner nd exponentil 27 Comprisons peculr highlight completely missing since it didn t occur close enough to ny of the vertices 28

15 Comprisons ormls estimtion, should not be bsed only on the polygon s plne. eighboring polygons should be ten in to ccount. 29