Notes for February 18 class -- More Ray Tracing

Phong model for specular reflection

The first really interesting model for surface reflection was developed by Bui-Tong Phong in 1973. Before that, computer graphics surfaces were rendered using only diffuse lambert reflection. Phong's was the first model that accounted for specular highlights.

The Phong model begins by defining a reflection vector R, which is a reflection of the direction to the light source L about the surface normal N.

As we showed in class, and as you can see from the diagram on the right, it is given by:

R = 2 (N • L) N - L
Once R has been defined, then the Phong model approximates the specular component of surface reflectance as:
srgb max(0, E • R)p )
where srgb is the color of specular reflection, p is a specular power, and E is the direction to the eye (in our case, E = -W, the reverse of the ray direction). The larger the specular power p, the "shinier" the surface will appear.

To get the complete Phong reflectance, we sum over the lights in the scene:

argb + i lightColori ( drgb max(0, N • Li) + srgb max(0, E • R) p )
where argb, drgb and srgb are the ambient, diffuse and specular color, respectively, and p is the specular power.

Blinn model for specular reflection

A few years later, Jim Blinn created a variation on the Phong model which produced somewhat more realistic highlights in extreme cases (when the viewer is looking from an angle that is far from the surface normal).

The Blinn model is also appreciably faster in the special case where we approximate E by a constant vector everywhere, and we assume that the light direction L is constant throughout the object.

The basic idea is that a halfway vector H is defined as the normalized midpoint between direction to light L and direction to eye E. If E and L are constant, this needs to be done only once for each light source.

Then at each pixel, we just take the inner product between N and H:

srgb max(0, N • H)p
Because H tends to stay relatively near to N, to get the same level of specularity the power s_p needs to be about three times greater in the Blinn model than the corresponding value in the Phong model.


Casting shadows is relatively easy in ray tracing. Once we have found a surface point S, then for each light source, we shoot another ray whose origin V' is just the surface point S, and whose direction W' is the direction toward that light source Li.

We want to make sure that the ray misses the object we are starting from, so we move the origin V' of our new ray slightly off the surface. Our "shadow ray" will therefore be:

[ V' , W' ] = [ S + ε , Li ]
If this shadow ray encounters any other object, then the surface is in shadow at this pixel, and we do not add in the diffuse and specular components of surface reflectance.

To the right above is an example of the surface not being in shadow. Just below that is an example of a surface being in shadow, because its light path is blocked by another object.



Another great thing about ray tracing is that we can continue to follow the path of a light ray backward from the camera, to model the behavior of mirror reflection. Adapting the technique that we used to calculate the reflection direction R for the Phong reflectance model, but replacing L in that equation by -W (the direction back along the incoming ray):
W' = 2 (N • (-W)) N - (-W)
we can compute a new ray that starts at surface point S, and goes into that reflected direction.

As shown in the figure on the right, we want to offset the origin of this ray a bit out of the surface, so that the ray does not accidentally encounter the object itself.

Whatever color is computed by this ray, we mix it into the result of the Phong reflectance algorithm. The result is the appearance of a shaded surface with a mirror finish.


Boolean intersection

Another nice thing about ray tracing is that we can use it to do boolean modeling, as we discussed in class. For example, given two spheres A and B, we can compute the intersection (if any) of those spheres by calculating their intersection along the ray.

Suppose, along a given ray, the values of t when entering and exiting sphere A are Ain and Aout, respectively.

Also suppose that along this ray the values of t when entering and exiting sphere B are Bin and Bout, respectively.

Then the intersection of the two shapes, along the ray, is given by the maximum of the enter (in) values and the minimum of the exit (out) values:

tin of entry into intersection shape = max(Ain , Bin)
tin of exit from intersection shape = min(Aout , Bout)
If tin < tout, then the ray has intersected the intersection shape. Otherwise, the ray has missed the intersection shape.

To shade the resulting intersection shape at a pixel, we need to use the normal of the surface that was actually hit by the ray. For example, if Ain > Bin, then we need to use the surface normal of A.


Homework (due before class on Wednesday Feb 25)

  • Implement either the Phong or the Blinn reflectance model.

  • Implement shadows.

  • Implement boolean intersection between two spheres.

  • Extra credit:
    • Implement both Phong and Blinn.

    • Implement reflection.

    • Implement boolean intersection between multiple (more than two) spheres.

  • As always, make something cool and fun, try to create something interactive (using uCursor) and/or animated (using uTime).