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The final part of a POV-Ray texture is the finish. It
controls the properties of the surface of an object. It can make it shiny and
reflective, or dull and flat. It can also specify what happens to light that
passes through transparent pigments, what happens to light that is scattered
by less-than-perfectly-smooth surfaces and what happens to light that is
reflected by surfaces with thin-film interference properties. There are
twelve different properties available in POV-Ray to specify the finish of a
given object. These are controlled by the following keywords: ambient,
diffuse, brilliance,
phong, specular,
metallic, reflection,
crand and iridescence.
Let's design a couple of textures that make use of these parameters.
Since objects in POV-Ray are illuminated by light sources, the portions of
those objects that are in shadow would be completely black were it not for
the first two finish properties, ambient and
diffuse. Ambient is used to simulate the light that is scattered
around the scene that does not come directly from a light source. Diffuse
determines how much of the light that is seen comes directly from a light
source. These two keywords work together to control the simulation of ambient
light. Let's use our gray sphere to demonstrate this. Let's also
change our plane back to its original green and white checkered pattern.
plane {
y, -1.5
pigment {checker Green, White}
}
sphere {
<0,0,0>, 1
pigment { Gray75 }
finish {
ambient .2
diffuse .6
}
}
In the above example, the default values for ambient and diffuse are used. We render this to see what the effect is and then make the following change to the finish.
ambient 0 diffuse 0
The sphere is black because we have specified that none of the light
coming from any light source will be reflected by the sphere. Let's
change diffuse back to the default of 0.6.
Now we see the gray surface color where the light from the light source
falls directly on the sphere but the shaded side is still absolutely black.
Now let's change diffuse to 0.3 and ambient to
0.3.
The sphere now looks almost flat. This is because we have specified a fairly
high degree of ambient light and only a low amount of the light coming from
the light source is diffusely reflected towards the camera. The default
values of ambient and diffuse are pretty good
averages and a good starting point. In most cases, an ambient value of 0.1
... 0.2 is sufficient and a diffuse value of 0.5 ... 0.7 will usually do the
job. There are a couple of exceptions. If we have a completely transparent
surface with high refractive and/or reflective values, low values of both
ambient and diffuse may be best. Here is an example:
sphere {
<0,0,0>, 1
pigment { White filter 1 }
finish {
ambient 0
diffuse 0
reflection .25
specular 1
roughness .001
}
interior { ior 1.33 }
}
This is glass, obviously. Glass is a material that takes nearly all of its
appearance from its surroundings. Very little of the surface is seen because
it transmits or reflects practically all of the light that shines on it. See
glass.inc for some other examples.
If we ever need an object to be completely illuminated independently of the
lighting situation in a given scene we can do this artificially by specifying
an ambient value of 1 and a diffuse value of 0.
This will eliminate all shading and simply give the object its fullest and
brightest color value at all points. This is good for simulating objects that
emit light like light bulbs and for skies in scenes where the sky may not be
adequately lit by any other means.
Let's try this with our sphere now.
sphere {
<0,0,0>, 1
pigment { White }
finish {
ambient 1
diffuse 0
}
}
Rendering this we get a blinding white sphere with no visible highlights or shaded parts. It would make a pretty good streetlight.
In the glass example above, we noticed that there were bright little
hotspots on the surface. This gave the sphere a hard, shiny appearance.
POV-Ray gives us two ways to specify surface specular highlights. The first
is called Phong highlighting. Usually, Phong highlights are
described using two keywords: phong and
phong_size. The float that follows
phong determines the brightness of the highlight while the float
following phong_size determines its size. Let's try this.
sphere {
<0,0,0>, 1
pigment { Gray50 }
finish {
ambient .2
diffuse .6
phong .75
phong_size 25
}
}
Rendering this we see a fairly broad, soft highlight that gives the sphere
a kind of plastic appearance. Now let's change phong_size
to 150. This makes a much smaller highlight which gives the sphere the
appearance of being much harder and shinier.
There is another kind of highlight that is calculated by a different means
called specular highlighting. It is specified using the keyword
specular and operates in conjunction with another keyword
called roughness. These two keywords work together in much the
same way as phong and phong_size to create
highlights that alter the apparent shininess of the surface. Let's try
using specular in our sphere.
sphere {
<0,0,0>, 1
pigment { Gray50 }
finish {
ambient .2
diffuse .6
specular .75
roughness .1
}
}
Looking at the result we see a broad, soft highlight similar to what we
had when we used phong_size of 25. Change roughness
to .001 and render again. Now we see a small, tight highlight similar to what
we had when we used phong_size of 150. Generally speaking, specular
is slightly more accurate and therefore slightly more realistic than phong but
you should try both methods when designing a texture. There are even times when
both phong and specular may be used on a finish.
There is another surface parameter that goes hand in hand with highlights,
reflection. Surfaces that are very shiny usually have a degree
of reflection to them. Let's take a look at an example.
sphere {
<0,0,0>, 1
pigment { Gray50 }
finish {
ambient .2
diffuse .6
specular .75
roughness .001
reflection {
.5
}
}
}
We see that our sphere now reflects the green and white checkered plane
and the black background but the gray color of the sphere seems out of place.
This is another time when a lower diffuse value is needed. Generally, the
higher reflection is the lower diffuse should be.
We lower the diffuse value to 0.3 and the ambient value to 0.1 and render
again. That is much better. Let's make our sphere as shiny as a polished
gold ball bearing.
sphere {
<0,0,0>, 1
pigment { BrightGold }
finish {
ambient .1
diffuse .1
specular 1
roughness .001
reflection {
.75
}
}
}
That is close but there is something wrong, the colour of the reflection and the highlight. To
make the surface appear more like metal the keyword metallic
is used. We add it now to see the difference.
sphere {
<0,0,0>, 1
pigment { BrightGold }
finish {
ambient .1
diffuse .1
specular 1
roughness .001
reflection {
.75
metallic
}
}
}
The reflection has now more of the gold color than the color of its enviroment. Last detail, the highlight. We add another metallic statement, now to the finish and not inside the reflection block.
sphere {
<0,0,0>, 1
pigment { BrightGold }
finish {
ambient .1
diffuse .1
specular 1
roughness .001
metallic
reflection {
.75
metallic
}
}
}
We see that the highlight has taken on the color of the surface rather than the light source. This gives the surface a more metallic appearance.
Iridescence is what we see on the surface of an oil slick when
the sun shines on it. The rainbow effect is created by something called
thin-film interference (read section "Iridescence" for
details). For now let's just try using it. Iridescence is specified by
the irid statement and three values: amount,
thickness and turbulence. The amount is the contribution
to the overall surface color. Usually 0.1 to 0.5 is sufficient here.
The thickness affects the "busyness" of the effect. Keep this between
0.25 and 1 for best results. The turbulence is a little different from
pigment or normal turbulence. We cannot set octaves, lambda
or omega but we can specify an amount which will affect the thickness
in a slightly different way from the thickness value. Values between 0.25 and 1
work best here too. Finally, iridescence will respond to the surface normal since
it depends on the angle of incidence of the light rays striking the surface.
With all of this in mind, let's add some iridescence to our glass sphere.
sphere {
<0,0,0>, 1
pigment { White filter 1 }
finish {
ambient .1
diffuse .1
reflection .2
specular 1
roughness .001
irid {
0.35
thickness .5
turbulence .5
}
}
interior{
ior 1.5
fade_distance 5
fade_power 1
caustics 1
}
}
We try to vary the values for amount, thickness and turbulence to see what
changes they make. We also try to add a normal block to see what
happens.
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