Iridescence, or Newton's thin film interference, simulates the effect of light on surfaces with a microscopic transparent film overlay. The effect is like an oil slick on a puddle of water or the rainbow hues of a soap bubble (see also "Irid_Wavelength").

The syntax is:

finish { irid { AMOUNT thickness FLOAT turbulence VECTOR } }

This finish modifies the surface color as a function of the angle between the light source and the surface. Since the effect works in conjunction with the position and angle of the light sources to the surface it does not behave in the same ways as a procedural pigment pattern.

The AMOUNT parameter is the contribution of the iridescence effect to the overall surface color. As a rule of thumb keep to around 0.25 (25% contribution) or less, but experiment. If the surface is coming out too white, try lowering the diffuse and possibly the ambient values of the surface.

The thickness keyword represents the film's thickness. This is an awkward parameter to set, since the thickness value has no relationship to the object's scale. Changing it affects the scale or busy-ness of the effect. A very thin film will have a high frequency of color changes while a thick film will have large areas of color.

The thickness of the film can be varied with the turbulence keyword. You can only specify the amount of turbulence with iridescence. The octaves, lambda, and omega values are internally set and are not adjustable by the user at this time.

In addition, perturbing the object's surface normal through the use of bump patterns will affect iridescence.

For the curious, thin film interference occurs because, when the ray hits the surface of the film, part of the light is reflected from that surface, while a portion is transmitted into the film. This subsurface ray travels through the film and eventually reflects off the opaque substrate. The light emerges from the film slightly out of phase with the ray that was reflected from the surface.

This phase shift creates interference, which varies with the wavelength of the component colors, resulting in some wavelengths being reinforced, while others are cancelled out. When these components are recombined, the result is iridescence.

The concept used for this feature came from the book Fundamentals of Three-Dimensional Computer Graphics by Alan Watt (Addison-Wesley).

Section 7.6.4

Important notice: The halo feature in POV-Ray 3.0 are somewhat experimental. There is a high probability that the design and implementation of these features will be changed in future versions. We cannot guarantee that scenes using these features in 3.0 will render identically in future releases or that full backwards compatibility of language syntax can be maintained.

A halo is used to simulate some of the atmospheric effects that occur when small particles interact with light or radiate on their own. Those effects include clouds, fogs, fire, etc.

Halos are attached to an object, the so called container object, which they completely fill. If the object is partially or completely translucent and the object is specified to be hollow (see section "Hollow" for more details) the halo will be visible. Thus the halo effects are limited to the space that the object covers. This should always be kept in mind.

What the halo actually will look like depends on a lot of parameters. First of all you have to specify which kind of effect you want to simulate. After this you need to define the distribution of the particles. This is basically done in two steps: a mapping function is selected and a density function is chosen. The first function maps world coordinates onto a one-dimensional interval while the later describes how this linear interval is mapped onto the final density values.

The properties of the particles, such as their color and their translucency, are given by a color map. The density values calculated by the mapping processes are used to determine the appropriate color using this color map.

A ray marching process is used to volume sample the halo and to accumulate the intensities and opacity of each interval.

The following sections will describe all of the halo parameters in more detail. The complete halo syntax is given by:

halo { attenuating | emitting | glowing | dust [ constant | linear | cubic | poly ] [ planar_mapping | spherical_mapping | cylindrical_mapping | box_mapping ] [ dust_type DUST_TYPE ] [ eccentricity ECCENTRICITY ] [ max_value MAX_VALUE ] [ exponent EXPONENT ] [ samples SAMPLES ] [ aa_level AA_LEVEL ] [ aa_threshold AA_THRESHOLD ] [ jitter JITTER ] [ turbulence <TURBULENCE> ] [ octaves OCTAVES ] [ omega OMEGA ] [ lambda LAMBDA ] [ colour_map COLOUR_MAP ] [ frequency FREQUENCY ] [ phase PHASE ] [ scale <VECTOR> ] [ rotate <VECTOR> ] [ translate <VECTOR> ] }

Halo Mapping

As described above the actual particle distribution and halo appearance is influenced by a lot of parameters. The steps that are performed during the halo calculation will be explained below. It will also be noted where the different halo keywords will have an effect on the calculations.

1.Depending on the current sampling position along the ray, point P (coordinates x, y, z) inside the halo container object is calculated. The actual location is influenced by the jitter keyword, the number of samples and the use of anti-aliasing (aa_level and aa_threshold).
2.Point P is transformed into point Q using the (current) halo's transformation. Here all local halo transformations come into play, i.e. all transformations specified inside the (current) halo statement.
3.Turbulence is added to point Q. The amount of turbulence is given by the urbulence keyword. The turbulence calculation is influenced by the octaves, omega and lambda keywords.
4.Radius r is calculated depending on the specified density mapping (planar_mapping, spherical_mapping, cylindrical_mapping, box_mapping). The radius is clipped to the range from 0 to 1, i.e. 0 <= r <= 1.
5.The density d is calculated from the radius r using the specified density function (constant, linear, cubic, poly) and the maximum value given by max_value. The density will be in the range from 0 to max_value.
6.The density d is first multiplied by the frequency value, added to the phase value and clipped to the range from 0 to 1 before it is used to get the color from the color_map . If an attenuating halo is used the color will be determined by the total density along the ray and not by the sum of the colors for each sample.

All steps are repeated for each sample point along the ray that is inside the halo container object. Steps 2 through 6 are repeated for all halos attached to the halo container object.

It should be noted that in order to get a finite particle distribution, i. e. a particle distribution that vanishes outside a finite area, a finite density mapping and a finite density function has to be used.

A finite density mapping gives the constant value one for all points outside a finite area. The box and spherical mappings are the only finite mapping types.

A finite density function vanishes for all parameter values above one (there are no negative parameter values). The only infinite density function is the constant function.

Finite particle distributions are especially useful because they can always be transformed to stay inside the halo container object. If particles leave the container object they become invisible and the surface of the container will be visible due to the density discontinuity at the surface.

Multiple Halos

It is possible to put more than one halo inside a container object. This is simply done by putting more than one halo statement inside the container object statement like:

sphere { 0, 1 pigment { Clear } halo { here comes halo nr. 1 } halo { here comes halo nr. 2 } halo { here comes halo nr. 3 } ... }

The effects of the different halos are added. This is in fact similar to the CSG union operation.

You should note that currently multiple attenuating halos will use the color map of the last halo only. It is not possible to use different color maps for multiple attenuating halos.

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