Computer Graphics. Shading. Page. Copyright Gotsman, Elber, Barequet, Karni, Sheffer Computer Science, Technion. The Physics

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1 Comuter Grahics Illumiatio Models & The Physics 2 Local vs. Global Illumiatio Models Examle Local model direct ad local iteractio of each object with the light. Ambiet Diffuse Global model: iteractios ad exchage of light eergy betwee differet objects. Secular Fial Image 3 Light Sources Poit source (A: All light origiates at a oit Rays hit laar surface at differet icidece agles Parallel source (B: All light rays are arallel Rays hit a laar surface at idetical icidece agles May be modeled as oit source at ifiity Also called directioal source Area source (C: Light origiates at fiite area i sace. Ibetwee the oit ad arallel sources B C Also called distributed source A Ambiet Light Assume o-directioal light i the eviromet Object illumiated with same light everywhere Looks like silhouette The Illumiatio equatio I = I a k a I a - ambiet light itesity k a - fractio of ambiet light reflected from surface. Also defies object color 5 6 Coyright Gotsma, Elber, Barequet, Kari, Sheffer Comuter Sciece, Techio

2 Comuter Grahics Diffuse Light Dull surfaces such as solid matte lastic reflects icomig light uiformly i all directios. Called diffuse or Lambertia reflectio Uderstad itesity as the umber of hotos er ich 2. If a flow of hotos assig a ich 2 widow is hittig a the red surface, how may hotos hit a ich 2 of the surface? Let θ is the agle betwee the directio of icomig light ad ormal to surface, ad let L, N be corresodig uit vectors. Diffused Ball L θ x x 2 N x 3 The legth of the segmet x is /cos θ. The amout of icidet light er uit surface area (thus reflected light is roortioal to cos θ =N L Moo Paradox Diffuse Reflectio Illumiatio equatio is ow: I = I k + I k ( N L = I k + I k cosθ a a d a a d I k d - oit light source s itesity - surface diffuse reflectio coefficiet Questio: Ca we locate the light source from a shaded image? 0 Secular Reflectio Shiy objects (e.g. metallic reflect light i referred directio R determied by surface ormal N. L θ θ N α R V Secular Reflectio (Phog Model Illumiatio equatio: a a ( k ( N L + k ( R V I = I k + I k s - Secular reflectio coefficiet - Secularity exoet d s Most objects are ot ideal mirrors also reflect i the immediate viciity of R Phog Model aroximate atteuatio by the form of cos α (o real hysical basis 2 Coyright Gotsma, Elber, Barequet, Kari, Sheffer Comuter Sciece, Techio

3 Comuter Grahics Secular Reflectio (cot d Exoet of cosie cotrols decay factor of atteuatio fuctio: No hysical basis but looks good: 3 Retroreflector seds light back where it came from regardless of the agle of isidece More o Illumiatio Equatio For multile light sources: = I aka + ( k ( N L + k ( R V I I d I of all light sources are added together Precautios should be take from overflows s shadigmodel Eve More o Illumiatio Equatio For distace/atmosheric atteuatio sources: I = ( k ( N L + k ( R V I I aka + d s d d - distace betwee surface ad light source ad/or distace betwee surface ad viewer (heuristic atmosheric atteuatio 7 8 Coyright Gotsma, Elber, Barequet, Kari, Sheffer Comuter Sciece, Techio

4 Comuter Grahics Flat Normal er Vertex Alied to iecewise liear olygoal models Simle surface lightig aroximated over olygos Illumiatio value deeds oly o olygo ormal each olygo is colored with a uiform itesity Looks o-smooth (worseed by Mach bad effect 9 If a olyhedro is a aroximatio of smooth surface: Assig to each vertex the ormal of origial surface at that oit If surface is ot available use estimated ormal (e.g. average of eighborig faces. 20 Comute illumiatio itesity at vertices usig those ormals Iterolate itesity over olygo iterior Gouraud Gouraud Liearly iterolate lightig itesities at the vertices over iterior ixels of the olygo, i the image lae 2 Questio: Ca Gouraud shadig suort secular lightig? 22 Phog Iterolate (at the vertices i image sace ormal vectors istead of illumiatio itesities Aly the illumiatio equatio for each iterior ixel with its ow (iterolated ormal Commets o Phog shadig is more exesive (why? but well worth the effort Ca achieve good lookig secular highlight effects Both the Gouraud ad Phog shadig schemes are erformed i the image lae ad fit well ito a olygoal sca-coversio fill scheme Both the Gouraud ad Phog are view deedet Ca cause artifacts durig aimatio as they are trasformatio deedet Coyright Gotsma, Elber, Barequet, Kari, Sheffer Comuter Sciece, Techio

5 Comuter Grahics Comariso 25 Coyright Gotsma, Elber, Barequet, Kari, Sheffer Comuter Sciece, Techio