If you calculate just enough samples, but no more, you will get an image which has slightly blotchy lighting. What you want is just a few extra interspersed, so that the blending will be nice and smooth. The solution to this is the mosaic preview: it goes over the image one or more times beforehand, calculating radiosity values. To ensure that you get a few extra, the radiosity algorithm lowers the error bound during the pre-final passes, then sets it back just before the final pass. This tuning value sets the amount that the error bound is dropped during the preliminary image passes. If your low error factor is 0.8 and your error bound is set to 0.4 it will really use an error bound of 0.32 during the first passes and 0.4 on the final pass.

The default value is 0.8.


The minimum effective radius ratio is set by minimum_reuse. This is the fraction of the screen width which sets the minimum radius of reuse for each sample point (actually, it is the fraction of the distance from the eye but the two are roughly equal). For example, if the value is 0.02 the radius of maximum reuse for every sample is set to whatever ground distance corresponds to 2% of the width of the screen. Imagine you sent a ray off to the horizon and it hits the ground at a distance of 100 miles from your eyepoint. The reuse distance for that sample will be set to 2 miles. At a resolution of 300*400 this will correspond to (very roughly) 8 pixels. The theory is that you don't want to calculate values for every pixel into every crevice everywhere in the scene, it will take too long. This sets a minimum bound for the reuse. If this value is too low, (which is should be in theory) rendering gets slow, and inside corners can get a little grainy. If it is set too high, you don't get the natural darkening of illumination near inside edges, since it reuses. At values higher than 2% you start getting more just plain errors, like reusing the illumination of the open table underneath the apple.

Remember that this is a unitless ratio.

The default value is 0.015.


The nearest_count value is the maximum number of old ambient values blended together to create a new interpolated value. It will always be the n geometrically closest reusable points that get used. If you go lower than 4, things can get pretty patchy. This can be good for debugging, though. Must be no more than 10, since that is the size of the array allocated.

The default value is 6.


This feature is not yet implemented.


This value determines how many recursion levels are used to calculate the diffuse inter-reflection. Valid values are one and two.

The default value is 1.

Tips on Radiosity

If you want to see where your values are being calculated set radiosity_count down to about 20, set radiosity_nearest_count to 1 and set radiosity_gray to 0. This will make everything maximally patchy, so you'll be able to see the borders between patches. There will have been a radiosity calculation at the center of most patches. As a bonus, this is quick to run. You can then change the radiosity_error_bound up and down to see how it changes things. Likewise modify radiosity_reuse_dist_min and max.

One way to get extra smooth results: crank up the sample count (we've gone as high as 1300) and drop the low_error_factor to something small like 0.6. Bump up the reuse_count to 7 or 8. This will get better values, and more of them, then interpolate among more of them on the last pass. This is not for people with a lack of patience since it is like a squared function. If your blotchiness is only in certain corners or near certain objects try tuning the error bound instead. Never drop it by more than a little at a time, since the run time will get very long.

If your scene looks good but right near some objects you get spots of the right (usually darker) color showing on a flat surface of the wrong color (same as far away from the object), then try dropping reuse_dist_max. If that still doesn't work well increase your ray count by 100 and drop the error bound just a bit. If you still have problems, drop reuse_nearest_count to about 4.

Appendix A

The following sections contain the legal information and license for the Persistence of Vision(tm) Ray-Tracer, also called POV-Ray(tm).

Before you use this program you have to read the sections below.

Appendix A.1
General License Agreement


This document pertains to the use and distribution of the Persistence of Vision(tm) Ray-Tracer a. k. a POV-Ray(tm). It applies to all POV-Ray program source files, executable (binary) files, scene files, documentation files, help file, bitmaps and INI files contained in official POV-Ray Team(tm) archives. All of these are referred to here as the software.

All of this software is Copyright 1991,1997 by the POV-Ray Team(tm). Although it is distributed as freeware, it is NOT Public Domain.

The copyrighted package may ONLY be distributed and/or modified according to the license granted herein. The spirit of the license is to promote POV-Ray as a standard ray-tracer, provide the full POV-Ray package freely to as many users as possible, prevent POV-Ray users and developers from being taken advantage of, enhance the life quality of those who come in contact with POV-Ray. This license was created so these goals could be realized. You are legally bound to follow these rules, but we hope you will follow them as a matter of ethics, rather than fear of litigation.

Appendix A.2
Usage Provisions

Permission is granted to the user to use the software and associated files in this package to create and render images. The use of this software for the purpose of creating images is completely free. The creator of a scene file and the image created from the scene file, retains all rights to the image and scene file they created and may use them for any purpose commercial or noncommercial.

The user is also granted the right to use the scenes files, fonts, bitmaps, and include files distributed in the include, texsamps and pov3demo sub- directories in their own scenes. Such permission does not extend to files in the povscn sub-directory. povscn files are for your enjoyment and education but may not be the basis of any derivative works.

Appendix A.3
General Rules for All Distributions

The permission to distribute this package under certain very specific conditions is granted in advance, provided that the following conditions are met.

These archives must not be re-archived using a different method without the explicit permission of the POV-Team. You may rename the archives only to meet the file name conventions of your system or to avoid file name duplications but we ask that you try to keep file names as similar to the originals as possible (for example: to

Ready-to-run unarchived distribution on CD-ROM is also permitted if the files are arranged in our standard directory or folder structure as though it had been properly installed on a hard disk.

You must distribute a full package of files as described in the next section. No portion of this package may be separated from the package and distributed separately other than under the conditions specified in the provisions given below.

Non-commercial distribution in which no money or compensation is charged (such as a user copying the software for a personal friend or colleague) is permitted with no other restrictions.

Teachers and educational institutions may also distribute the material to students and may charge minimal copying costs if the software is to be used in a course.

Appendix A.4
Definition of "Full Package"

POV-Ray is contained in two sets of archives for each hardware platform. A full package consists of either:

1) End user executable archives containing an executable program, documentation, and sample scenes but no source.

- or -

2) Programmer archives containing full source code but no executable. Also you must include an archive containing documentation, and sample scenes. On some platforms, the documentation and sample scenes are archived separately from the source. Source alone is not sufficient. You must have docs and scenes.

POV-Ray is officially distributed for MS-Dos; Windows 32-bit; Linux for Intel x86 series; Apple Macintosh; Apple PowerPC; SunOS; and Amiga. Other systems may be added in the future.

Distributors need not support all platforms but for each platform you support you must distribute a full package. For example a Macintosh only BBS need not distribute the Windows versions.

This software may only be bundled with other software packages according to the conditions specified in the provisions below.

{/HEADER 1 Conditions for Shareware/Freeware Distribution Companies/}

Shareware and freeware distribution companies may distribute the software included in software-only compilations using media such as, but not limited to, floppy disk, CD-ROM, tape backup, optical disks, hard disks, or memory cards. This section only applies to distributors of collected programs. Anyone wishing to bundle the package with a shareware product must use the commercial bundling rules. Any bundling with books, magazines or other print media should also use the commercial rules.

You must notify us that you are distributing POV-Ray and must provide us with information on how to contact you should any support issues arise.

No more than five dollars U.S. ($5) can be charged per disk for the copying of this software and the media it is provided on. Space on each disk must be used as fully as possible. You may not spread the files over more disks than are necessary.

Distribution on high volume media such as backup tape or CD-ROM is permitted if the total cost to the user is no more than $0.08 U.S. dollars per megabyte of data. For example a CD-ROM with 600 meg could cost no more than $48.00.

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