A simple gradient sky sphere.
The other is more limiting, but as we will shortly see, can be worked around quite easily. If we have worked with layered textures, we have already seen how we can pile multiple texture patterns on top of one another (as long as one texture has transparency in it). This very useful technique has a problem incorporating the special textures we have just seen as a layer. But there is an answer!
For example, say we have a layered texture called Speckled_Metal, which produces a silver metallic surface, and then puts tiny specks of rust all over it. Then we decide, for a really rusty look, we want to create patches of concentrated rust, randomly over the surface. The obvious approach is to create a special texture pattern, with transparency to use as the top layer. But of course, as we have seen, we wouldn't be able to use that texture pattern as a layer. We would just generate an error message. The solution is to turn the problem inside out, and make our layered texture part of the texture pattern instead, like this
And the ultimate effect is the same as if we had layered the rust patches on to the speckled metal anyway.
With the full array of patterns, pigments, normals, finishes, layered and special textures, there is now practically nothing we cannot create in the way of amazing textures. An almost infinite number of new possibilities are just waiting to be created!
It is easy to assign a simple color or a complex color pattern to a virtual sky sphere. You can create anything from a cloud free, blue summer sky to a stormy, heavy clouded sky. Even starfields can easily be created.
You can use different kinds of fog to create foggy scenes. Multiple fog layers of different colors can add an eerie touch to your scene.
A much more realistic effect can be created by using an atmosphere, a constant fog that interacts with the light coming from light sources. Beams of light become visible and objects will cast shadows into the fog.
Last but not least you can add a rainbow to your scene.
The background color will be visible if a sky sphere is used and if some translucency remains after all sky sphere pigment layers are processed.
In the following examples we'll start with a very simple sky sphere that will get more and more complex as we add new features to it.
You may have noticed that the color of the sky varies with the angle to the earth's surface normal. If you look straight up the sky normally has a much deeper blue than it has at the horizon.
We want to model this effect using the sky sphere as shown in the scene below (skysph1.pov).
The interesting part is the sky sphere statement. It contains a pigment that describe the look of the sky sphere. We want to create a color gradient along the viewing angle measured against the earth's surface normal. Since the ray direction vector is used to calculate the pigment colors we have to use the y-gradient.
The scale and translate transformation are used to map the points derived from the direction vector to the right range. Without those transformations the pattern would be repeated twice on the sky sphere. The scale statement is used to avoid the repetition and the translate -1 statement moves the color at index zero to the bottom of the sky sphere (that's the point of the sky sphere you'll see if you look straight down).
If you want to start one of the colors at a specific angle you'll first have to convert the angle to a color map index. This is done by using the formula
color_map_index = (1 - cos(angle)) / 2
where the angle is measured against the negated earth's surface normal. This is the surface normal pointing towards the center of the earth. An angle of 0 degrees describes the point below us while an angle of 180 degrees represents the zenith.
In POV-Ray you first have to convert the degree value to radian values as it is shown in the following example.
This scene uses a color gradient that starts with a red color at 30 degrees and blends into the blue color at 120 degrees. Below 30 degrees everything is red while above 120 degrees all is blue.
The sky sphere we use is shown below. A ground plane is also added for greater realism (skysph2.pov).
The gradient pattern and the transformation inside the pigment are the same as in the example in the previous section.
The color map consists of three colors. A bright, slightly yellowish red that is used for the sun, a darker red for the halo and a dark blue for the night sky. The sun's color covers only a very small portion of the sky sphere because we don't want the sun to become too big. The color is used at the color map values 0.000 and 0.002 to get a sharp contrast at value 0.002 (we don't want the sun to blend into the sky). The darker red color used for the halo blends into the dark blue sky color from value 0.002 to 0.200. All values above 0.200 will reveal the dark blue sky.
The rotate -135*x statement is used to rotate the sun and the complete sky sphere to its final position. Without this rotation the sun would be at 0 degrees, i.e. right below us.
Looking at the resulting image you'll see what impressive effects you can achieve with the sky sphere.
The sky sphere has one drawback as you might notice when looking at the final image (skysph3.pov). The sun doesn't emit any light and the clouds will not cast any shadows. If you want to have clouds that cast shadows you'll have to use a real, large sphere with an appropriate texture and a light source somewhere outside the sphere.
The usage of both fog types will be described in the next sections in detail.
The distance value determines the distance at which 36.8% of the background are still visible (for a more detailed explanation of how the fog is calculated read the reference section "Fog").
The fog color can be used to create anything from a pure white to a red, blood-colored fog. You can also use a black fog to simulate the effect of a limited range of vision.
The following example will show you how to add fog to a simple scene (fog1.pov).
According to their distance the spheres in this scene more or less vanish in the greenish fog we used, as does the checkerboard plane.
Using as transmittance value of 0.2 as in
the fog's translucency never drops below 20% as you can see in the resulting image (fog2.pov).
The filter value determines the amount of light that is filtered by the fog. In our example 100% of the light passing through the fog will be filtered by the fog. If we had used a value of 0.7 only 70% of the light would have been filtered. The remaining 30% would have passed unfiltered.
You'll notice that the intensity of the objects in the fog is not only diminished due to the fog's color but that the colors are actually influenced by the fog. The red and especially the blue sphere got a green hue.
The turbulence keyword is used to specify the amount of turbulence used while the turb_depth value is used to move the point at which the turbulence value is calculated along the viewing ray. Values near zero move the point to the viewer while values near one move it to the intersection point (the default value is 0.5). This parameter can be used to avoid noise that may appear in the fog due to the turbulence (this normally happens at very far away intersection points, especially if no intersection occurs, i. e. the background is hit). If this happens just lower the turb_depth value until the noise vanishes.
The following example (fog5.pov) uses a ground fog which has a constant density below y=25 (the center of the red sphere) and quickly falls off for increasing altitudes.
Just try the following example (fog6.pov).
You can combine constant density fogs, ground fogs, filtering fogs, non-filtering fogs, fogs with a translucency threshold, etc.