### 6.10.5 Advanced Techniques

#### 6.10.5.1 Autostop

Example of the photon autostop option

To understand the `autostop` option, you need to understand the way photons are shot from light sources. Photons are shot in a spiral pattern with uniform angular density. Imagine a sphere with a spiral starting at one of the poles and spiraling out in ever-increasing circles to the equator. Two angles are involved here. The first, phi, is the how far progress has been made in the current circle of the spiral. The second, theta, is how far we are from the pole to the equator. Now, imagine this sphere centered at the light source with the pole where the spiral starts pointed towards the center of the object receiving photons. Now, photons are shot out of the light in this spiral pattern.

Normally, POV does not stop shooting photons until the target object’s entire bounding box has been thoroughly covered. Sometimes, however, an object is much smaller than its bounding box. At these times, we want to stop shooting if we do a complete circle in the spiral without hitting the object. Unfortunately, some objects (such as copper rings), have holes in the middle. Since we start shooting at the middle of the object, the photons just go through the hole in the middle, thus fooling the system into thinking that it is done. To avoid this, the `autostop` keyword lets you specify how far the system must go before this auto-stopping feature kicks in. The value specified is a fraction of the object's bounding box. Valid values are 0.0 through 1.0 (0% through 100%). POV will continue to shoot photons until the spiral has exceeded this value or the bounding box is completely covered. If a complete circle of photons fails to hit the target object after the spiral has passed the autostop threshold, POV will then stop shooting photons.

The `autostop` feature will also not kick in until at least one photon has hit the object. This allows you to use `autostop 0` even with objects that have holes in the middle.

Note:If the light source is within the object’s bounding box, the photons are shot in all directions from the light source.

#### 6.10.5.2 Adaptive Search Radius

Unless photons are interacting with media, POV-Ray uses an adaptive search radius while gathering photons. If the minimum number of photons is not found in the original search radius, the radius is expanded and searched again. Using this adaptive search radius can both decrease the amount of time it takes to render the image, and sharpen the borders in the caustic patterns.

Sometimes this adaptive search technique can create unwanted artefacts at borders. To remove these artefacts, a few thresholds are used, which can be specified by `expand_thresholds`. For example, if expanding the radius increases the estimated density of photons by too much (threshold is percent_increase, default is 20%, or 0.2), the expanded search is discarded and the old search is used instead. However, if too few photons are gathered in the expanded search (`expand_min`, default is 40), the new search will be used always, even if it means more than a 20% increase in photon density.

#### 6.10.5.3 Photons and Dispersion

When dispersion is specified for interior of a transparent object, photons will make use of that and show "colored" caustics.

#### 6.10.5.4 Saving and Loading Photon Maps

It is possible to save and load photon maps to speed up rendering. The photon map itself is view-independent, so if you want to animate a scene that contains photons and you know the photon map will not change during the animation, you can save it on the first frame and then load it for all subsequent frames.

To save the photon map, put the line

     save_file "myfile.ph"

into the `photons { }` block inside the `global_settings` section.

Loading the photon map is the same, but with `load_file` instead of `save_file`. You cannot both load and save a photon map in the POV file. If you load the photon map, it will load all of the photons. No photons will be shot if the map is loaded from a file. All other options (such as gather radius) must still be specified in the POV scene file and are not loaded with the photon map.