Ben Chambers <ben### [at] pacificwebguycom> wrote:
> So, my question is, does someone need both?

> I've been in the habit of using specular for my finishes, and I've only 
> seen phong used a couple of times.  I know that they use slightly 
> different formulas but, for artistic usage, is there any reason that I 
> should choose one over the other?

  As you can see from the image you posted, they use slightly different
algorithms for computing the highlights. The phong highlights tend to
look sharper while the specular highlights tend to look smoother. Which
one you prefer for a certain object depends on your artistic eye, I suppose,
although I'd say you might also make the choice depending on whether you
want to make the material look more mirror-like or more glossy.

> Or, more interestingly, is there any reason I might want to mix them in 
> the same scene (some objects using specular, and some using phong)?

  Materials are different and have different reflectivity properties.
Some produce sharper hightlights while others produce smoother ones.
It depends on what you want to make them look like.

> BTW, I don't know the specifics of the internal functions in POV, but I 
> remember Phong being a really cool way of shading polygons in a scan 
> line rasterizer.  Back in the '90s, the big thing for a while was 
> Gouraud shading, which was used to interpolate color values between two 
> points (and, later, interpolate the light values, and apply those to a 
> texture map).  However, "Phong shading" was used to describe the process 
> of interpolating the normals, and performing the lighting on a per-pixel 
> basis.

  Regardless of the name, "Phong shading" and "Phong reflection model"
(which is the 'phong' finish property in POV-Ray) are two completely
different and unrelated things. (This is so in rendering in general;
it's not something exclusive to POV-Ray or raytracing.)

  Phong shading is a concept of rendering polygons and means that normal
vectors are interpolated from vertex normals to the rest of the polygon.

  Phong reflection model (which produce the so-called phong highlights)
is a formula for calculating the illumination of a surface given certain
parameters (viewer direction, light source direction, normal vector,
phong factor and exponent).

  Although they share the name, they are independent things, not really
related to each other. (In other words, eg. Phong shading can be used
even if Phong highlights aren't.)

  In fact, POV-Ray basically uses "Phong shading" with triangle meshes
because it interpolates vertex vectors for the rest of the triangle
surface. This is, of course, done even if you don't use Phong highlights,
which is something different. :)

-- 
                                                          - Warp

Post a reply to this message