The colors of the halo in the above image are somewhat dim. There is too much of the background visible through the halo. That does not look much like fire, does it? An easy way to fix this is to decrease the transparency of the particles in the areas of high density. Just use the following color map instead of the old one (the negative transmittance is correct).

color_map { [ 0 color rgbt <1, 0, 0, 1> ] [ 1 color rgbt <1, 1, 0, -1> ] }

Looking at the result of halo02.pov we will see that the halo is indeed much brighter.

The halo is much brighter now.

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This is done by using the turbulence keyword together with the amount of turbulence we want to add. Just like in the following example.

Adding turbulence to the halo moves all points inside the halo container in a pseudo-random manner. This results in a particle distribution that looks like there was some kind of flow in the halo (depending on the amount of turbulence you'll get a laminar or turbulent flow). The hight turbulence value is used because an explosion is highly turbulent.

Looking at the example image ( halo03.pov ) you'll see that this looks more like a fiery explosion than the glowing ball we got until now.

Adding some turbulence makes the fiery explosion more realistic.

You'll notice that the time it took to render the image increased after we added the turbulence. This is due to the fact that for every sample taken from the halo the slow turbulence function has to be evaluated.

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This is done by adding the scale keyword inside the halo statement.

The scale 0.5 command tells POV-Ray to scale all points inside the halo by this amount. This effectively scales the radius we get after the density mapping to a range of 0 to 0.5 instead of 0 to 1 (without turbulence). If we now add the turbulence the points are allowed to move half a unit in every direction without leaving the container object. That is excactly what we want.

To compensate for the smaller halo we would get we scale the sphere (and the halo inside) by 1.5.

Looking at the new example image ( halo04.pov ) you will no longer see any signs of the container sphere. We finally have a nice fiery explosion.

Resizing the halo makes it look much better.

The amount by which to scale the halo depends on the amount of turbulence you use. The higher the turbulence value the smaller the halo has to be scaled. That is something to experiment with.

Another way to avoid that points move out of the sphere is to use a larger sphere, i. e. a sphere with a radius larger than one. It is important to resize the sphere before the halo is added because otherwise the halo will also be scaled.

You should note that this only works for spherical and box mapping (and a non-constant density function). All other mapping types are (partially) infinite, i.e. the resulting particle distribution covers an infinite space (see also "Halo Mapping" ).

The rather mathematical explanation used there doesn't help much in understanding how this feature is used. It is quite simple though. The frequency value just tells the program how many times the color map will be repeated in the density range from 0 to 1. If a frequency of one (the default) is specified the color map will be visible once in the density field, e. g. the color at 0 will be used for density 0, color at 0.5 will be used for density 0.5 and the color at 1 will be used for density 1. Simple, isn't it?

If you choose a frequency of two, the color at 0 will be used for density 0, the color at 0.5 will be used for density 0.25 and the color at 1 will be used for density 0.5. What about the densities above 0.5? Since there are no entries in the color map for values above 1 we just start at 0 again. Thus the color at 0.1 will be used for density 0.55 ((2*0.55) mod 1 = 1.1 mod 1 = 0.1), the color at 0.5 will be used for density 0.75 and the color at 1 will be used for density 1.

If you are good at mathematics you'll note that the above example is not quite right because (1 * 2) mod 1 = 0 and not 1. Just think that we used a value slightly smaller than one and everything will be fine.

You may have noticed that in order to avoid sudden changes in the halo color for frequencies larger than one you'll have to used a periodic color map, i.e. a color map whose entries at 0 and 1 are the same.

We'll change our example by using a periodic color map and changing the frequency value to two.

Using a periodic color map and a frequency of two gives a much nicer explosion.

Looking at the result of ( halo05.pov ) we can be quite satisfied with the explosion we just have created, can't we?

There's one thing left you should be aware of when increasing the frequency value. It is often necessary to increase the sample rate in (nearly) the same way as you change the frequency. If you don't do this you'll probably get some severe aliasing artefacts (like color jumps or strange bands of colors). If this happens just change the samples value according to the frequency value (twice sampling rate for a doubled frequency).

We have a nice fiery explosion but we want to try to add some science fiction touch to it by using different colors. How about a nice green, less turbulent explosion that gets red at its borders?

Nothing easier than that!

sphere { 0, 1.5 pigment { color rgbt <1, 1, 1, 1> } halo { emitting spherical_mapping linear turbulence 0.5 color_map { [ 0 color rgbt <0, 1, 0, 1> ] [ 1 color rgbt <1, 0, 0, -1> ] } samples 10 scale 0.75 } hollow scale 1.5 }

Using red and green colors gives an unexpected result.

This should do the trick. Looking at the result of halo06.pov you may be disappointed. Where is the red center of the explosion? The borders are green as expected but there is a lot of yellow in the center and only a little bit red. What is happening?

We use an emitting halo in our example. According to the corresponding section in the halo reference chapter (see "Emitting" ) this type of halo uses very small particles that do not attenuate light passing through the halo. Especially particles near the viewer do not attenuate the light coming from particles far away from the viewer.

During the calculation of the halo's color near the center of the container sphere, the ray steps through nearly all possible densities of the particle distribution. Thus we get red and green colors as we march on, depending on the current position in the halo. The sum of these colors is used which will gives as a yellow color (the sum of red and green is yellow). This is what is happening here.

How can we still get what we want? The answer is to use a glowing halo instead of the emitting halo. The glowing halo is very similar to the emitting one except that it attenuates the light passing through. Thus the light of particles lying behind other particles will be attenuated by the particles in front.

For the results of the glowing halo see "The Glowing Halo" .

The gowing halo is very similar to the emitting halo except that it also absorbs light. You can view it as a combination of the emitting and the attenuating halo described in section "The Attenuating Halo" .

By just replacing the emitting keyword in the example in section "Changing the Halo Color" with the glowing keyword we get the desired effect as shown in the example image ( halo11.pov ).

Using a glowing halo gives the expected result.

Even though the red color of the high density areas is not very visible because the green colored, lower density areas lying in front absorb most of the red light, you don't get yellow color where you would have expected a red one.

Due to its similarity with the emitting halo we leave it up to you to make some experiments with this halo type. You just have to keep all those things you learned in the previous sections in mind to get some satisfying results.

Attenuating halos are ideal to create clouds and smoke. In the following examples we will try to make a neat little cloud. We start again by using a unit-sized sphere that is filled with a basic attenuating halo ( halo21.pov ).

camera { location <0, 0, -2.5> look_at <0, 0, 0> } light_source { <10, 10, -10> color rgb 1 shadowless } plane { z, 2 pigment { checker color rgb 0, color rgb 1 } finish { ambient 1 diffuse 0 } scale 0.5 hollow } sphere { 0, 1 pigment { color rgbt <1, 1, 1, 1> } halo { attenuating spherical_mapping linear color_map { [ 0 color rgbt <1, 0, 0, 1> ] [ 1 color rgbt <1, 0, 0, 0> ] } samples 10 } hollow }

Even though clouds normally are not red but white or gray, we use the red color to make it more visible against the black/white checkerboard background.

The color of an attenuating halo is calculated from the total accumulated density after a ray has marched through the complete particle field. This has to be kept in mind when creating the color map. We want the areas of the cloud with a low density to have a high translucency so we use a color of rgbt<1,0,0,1> and we want the high density areas to be opaque so we choose a color of rgbt<1,0,0,0>.

The basic attenuating halo used to create a cloud.

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Another idea is to scale the container object to get an ellipsoid shape that can be used to model a cloud pretty good. This is done by the scale <1.5, 0.75, 1> command at the end of the sphere. It scales both, the sphere and the halo inside.

Looking at the results of halo22.pov you'll see that this looks more like a real cloud (besides the color).

Scaling the halo container and adding some turbulence gives a better result.

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We want to model this appearance by adding two additional halos to our current container object (see section "Multiple Halos" for more details). This is done in the following way:

The three halos used differ only in their location, i. e. in the translation vector we have used. The first two halos are used to form the base of the cloud while the last sits on top of the others. The sphere has a different radius than the previous ones because more space is needed for all three halos.

The result of halo23.pov somehwat looks like a cloud, even though it may need some work.

Using multiple halos pretty much improves our cloud.