The Phong Reflection Model

1. The Phong Reflection Model

2. Lira Liga Liba Li = Lird Ligd Libd Lirs Ligs Libs Phong Reflection Model Supports three types of light-surface interactions - - ambient - diffuse - specular Each source has an ambient, a diffuse and a specular component for each of the three primary colors

3. Rira Riga Riba Ri = Rird Rigd Ribd Rirs Rigs Ribs Phong Reflection Model Supports three types of light-surface interactions - - ambient - diffuse - specular The amount of light reflected at each surface point has a has an ambient, a diffuse and a specular component for each of the three primary colors

4. We obtain the total intensity by adding the contributions of all sources and possibly a global ambient term Ir= ( Iira + Iird + Iirs ) + Iar i Phong Reflection Model We compute the contribution for each color source by adding the ambient, diffuse and specular components Lir= Rira Lira + Rird Lird + Rirs Lirs = Iira + Iird + Iirs

5. Phong Reflection Model n v l r p

6. pov: ambient 1.0 Ambient Reflection The same at every point on surface. Ra = kawhere 0 <= ka <= 1 kaindicates the percentage of ambient light reflected Ia = kaLa

7. 2 a + bd + cd we can add attenuation as a function of distance d Id = ---------------- (l n)Ld kd . pov: diffuse 1.0 Diffuse Reflection Diffuse reflections are from rough surfaces that scatter light in many directions. Also called Lambertian surfaces. Rd ~ cos(theta),where theta is the angle between n and l Id = kd(cos(theta))Ld ,where kd is the diffuse reflectivity coefficient Id = kd(l n)Ld,(assuming I & n are unit vectors) .

8. pov: specular 1.0 Specular Reflection Most reflected rays are close to the ideal reflection direction so a highlight (bright spot) appears on the surface. This occurs because the surfaces are smooth. The amount of light seen depends on the angle phi between r and v Is = ksLs cos (phi) , where a represents how tight the highlight is and 0 <= ks <= 1 Is = ksLs(r v) ,(assuming r & n are unit vectors) a . a

9. + = Id = ---------------- (kdLd(l n) + KsLs(r v) ) + kaLa 2 a + bd + cd . . 1 a Phong Reflection Model So finally the PHONG MODEL (for EACH COLOR and EACH LIGHT SOURCE) is ....

10. = Id = ---------------- (kdLd(l n) + KsLs(r v) ) + kaLa 2 a + bd + cd . . 1 a Phong Reflection Model So finally the PHONG MODEL (for EACH COLOR and EACH LIGHT SOURCE) is .... + +

11. Polygonal Shading To give a more realistic shading across the face of a flat polygon, possibly with differently calculated normals at each vertex, we want to vary the shading so the surface appears to curve properly between the vertices. Imagine a polygon on a sphere surface with vertex normals perpendicular to the sphere not to each polygon. We want it to look like a sphere.

12. Polygonal Shading • Flat Shading • Interpolative (Gouraud shading) • Phong Shading

13. Polygonal Shading • FLAT SHADING -- one normal n for the polygon, one calculation of the polygon color for the whole polygon

14. Polygonal Shading • GOURAUD (INTERPOLATIVE) SHADING -- also called SMOOTH shading, vertex normals are calculated as a normalized average of all of the polygon normals sharing that vertex, then the color at polygon point is an interpolation of the polygon vertex colors.

15. Polygonal Shading • PHONG SHADING -- instead of interpolating vertex intensities, we interpolate normals across each polygon • same calculation of vertex normals as Gouraud shading, but a new surface normal is calculated for every interior point by first interpolating normals along each polygon edge, and then interpolating interior normals from pairs of edge normals. This is almost always an OFFLINE process. • Produces smoother renderings but much more computationally intensive.