Chapter 15
Light Models

15.1 Reflected Light

We see objects mainly by the light that they reflect.

Ambient/background Reflection: c_aI_a

• Comes from all directions

• I_a = intensity of ambient light
• c_a = fraction of ambient light reflected back by P (coefficient of reflectivity).

  Depends on color and smoothness of surface at P.

  

Diffuse Reflection: c_d I_p (N ^. L)

Light from point source reflected equally in all directions (e.g., at dull surfaces).

• I_p = Intensity of source light
• N = Unit vector normal to the plane
• L = Unit vector directed to light source
• N ^. L = cos : light reflected decreases by this factor (Lambert's cosine law)

• Major contribution to color of point.

Specular Reflection (Phong): c_sI_p(V ^. R)ⁿ

Light from point source deflected back (e.g., at flat and smooth surfaces).

• V = Unit vector directed to viewer

• R = Unit vector symmetric to L
• R ^. V = cos : light reflected decreases by this factor (Lambert’s cosine law)

  

• Major contributor to shine of surface

Notes

• Formulas of the form c_a I_a + c_dI_p(N ^. L) + c_sI_p(V ^. R)ⁿ apply to each principal color with possible variations in the values of the coefficients.

• The intensity of light reduces with the distance from the source. Theoretically, I_p should be replaced with I_p D² , but I_p ___ D+D₀ give better results.

• Scattering of lights (e.g. by dust, transparent materials) not considered here.