| Photometry Filter | |
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| Description | Performs photometric scattering with the ray-tracing engine. |
| Threaded | Yes |
| Memory Usage | Heavy |
| Stream Support | Yes |
| Added In | 0.3.0 |
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Table of Contents
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Basics
As the name suggests, this filter performs photometry. There are a fairly large number of different options that can be set in this filter so we will go through then individually. The basic idea behind this filter comes from the model described in Salo and Karjalainen (Icarus, 2003). It uses a ray trace engine to follow rays from different sources through a set of spheres. The rays scatter off the spheres and binned in various ways. Scattered photons are also followed to the specified number of scatterings.
In addition to providing binning information that can be plotted as a rectangular surface, each input stream also has a second output stream that tells how much each element of the input contributed to the total binned light. For a ring simulation, this allows you to determine what particles are actually being seen so that you can determine if one type of particle is more or less visible than others.
There are five settings tabs for the Photometry Filter in addition to the standard three for sources, output, and label. These are Basic, Photon Sources, Selections, Bounds, and Binning.
Basics
The Basic tab contains the settings that are going to be required for all uses of this filter. It is shown in the figure below. You can enter formulas for the position and size of each of the spheres. You can also enter an albedo for the particles, the maximum number of scatter photons to calculate, and the maximum number of wraps to perform on bounds. Lastly you can specify a scattering style. Currently the only scattering style that is implemented in Lambert Scatter though this can easily be extended in the future.
Photon Source
The Photon Source tab allows you to add different sources of photons into the scene. Currently there are two type of sources that can be added. The first is a direction source. This send in all the photons on parallel rays with the specified direction. This type of source should be used to model sun light. The second is a sphere source where rays will come from points on the surface of a sphere to pass through the scene. You can specify the position and size of the sphere. This should be used for something like Saturn shine. For both source types you can specify how many photons to send from that source and the total flux those photons represent.
The way that both of these sources work is that they try to send rays through either the top plane or the bottom plane of the bounding box of the scene. Which plane is chosen depends on the direction or the point on the sphere. If the ray is heading downward, the top plane is used. If it is heading upward the bottom plane is used. If the ray is coming edge on or if the source on the sphere is between the top and bottom planes, it will pick a random point in the bounding box of the scene to pass the ray through.
Selections
You can specify one or more selections in your scene. These are boolean formulas that that should denote special groups of particles. The image below shows a selection for larger particles. When a selection is present, the output data will have two additional values in it for each selection. The first extra value is intensity in that bin for a photon that went straight from a particle meeting that selection criteria to the bin. The second is intensity for any light that hit that bin that scattered off a particle matching the selection at any point in its path. Using the selection of large particles, you will quickly and easily values for total light going into a bin as well as light that got to the bin from a large particle.
Bounds
The bounds options allow you to put in boundaries that photons hit and get moved to other locations. There are currently four types of boundaries. The first is a periodic cube. This boundary includes an option to simply use the bounds of the geometry. If this is selected, the other options are ignored. Otherwise you specify the minimum and maximum values for x, y, and z. The sliding brick option is useful for ring particles. The general plane allows you to specify a plane in space and an offset that the photon should be moved when it hits that plane. The periodic sphere will determine when a photon hits one edge and then move the photon to the opposite edge across the center of the sphere. None of the current bounds alter the direction of the photon.
Binning
The last option is to set the binning method that is used. There are three different binning methods to choose from. All three have basic settings that are basically the same. You get to specify ranges and a number of bins for the two different dimensions the binning is done in. The meaning of those dimensions varies between the binning methods.
The default is a camera binning. With this binning method you specify the location of a camera, the vector for where it is facing, and the vector for the direction that should be up. When a photon hits something, this binning determines what bin the light would come through to get from that location to the camera. If nothing is blocking it, that bin is incremented. For this binning the ranges specified are spatial distances in the up-down and right-left directions, not angles. This is done so users don't have to calculate opening angles for the geometry.
The other binning methods are cylinder and sphere binning. These spread bins around a given location based on the bounds specified. Every time that a photon hits something in the scene, a ray is sent from that intersection point to every bin. If nothing blocks it, the bin is incremented by the proper amount for how much light should scatter from that point to the bin location. These binning methods allow you to determine the total brightness that would be seen from many different orientations with a single pass of the filter.
Results
The first output stream from this filter can be sent to a plot with a Rectangular Surface Plot to produce an image of what happens with the photometry. A sample of this from a rings simulation is shown here that used the settings shown in the figures above except that the number of photons from the source has been increased to 1000000.
