The new PNG format files generated by POV-Ray 3.0 overcome the problem of too much or not enough gamma correction by storing the image file gamma (which is 1.0/display_gamma) inside the PNG file when it is generated by POV-Ray. When the PNG file is later displayed by a program that has been set up correctly, it uses this gamma value as well as the current display gamma to correct for the potentially different display gamma used when originally creating the image.
Unfortunately, of all the image file formats POV-Ray supports, PNG is the only one that has any gamma correction features and is therefore preferred for images that will be displayed on a wide variety of platforms.
This means that as the artist you are doing gamma correction in the POV-Ray scene file for your particular hardware. This scene file will generate an image file that is also gamma corrected for your hardware and will display correctly on systems similar to your own. However, when this scene is rendered on another platform, it may be too bright or too dim, regardless of the output file format used. Rather than have you change all the scene files to have a single fixed gamma value (heaven forbid!), POV-Ray 3.0 allows you to specify in the scene file the display gamma of the system that the scene was created on.
The assumed_gamma global setting, in conjunction with the Display_Gamma INI setting lets POV-Ray know how to do gamma correction on a given scene so that the preview and output image files will appear the correct brightness on any system. Since the gamma correction is done internally to POV-Ray, it will produce output image files that are the correct brightness for the current display, regardless of what output format is used. As well, since the gamma correction is performed in the high-precision data format that POV-Ray uses internally, it produces better results than gamma correction done after the file is written to disk.
Although you may not notice any difference in the output on your system with and without an assumed_gamma setting, the assumed gamma is important if the scene is ever rendered on another platform.
The boolean value turns the option on or off. If the keyword is specified without the boolean value then the option is turned on. If hf_gray_16 is not specified in any global_settings statement in the entire scene then the default is off.
When hf_gray_16 is on, the output file will be in the form of a heightfield, with the height at any point being dependent on the brightness of the pixel. The brightness of a pixel is calculated in the same way that color images are converted to grayscale images:
height = 0.3 * red + 0.59 * green + 0.11 * blue
Setting the hf_gray_16 option will cause the preview display, if used, to be grayscale rather than color. This is to allow you to see how the heightfield will look because some file formats store heightfields in a way that is difficult to understand afterwards. See section "Height Field" for a description of how POV-Ray heightfields are stored for each file type.
The default value is rgb <0.25,0.18,0.14> and any filter or transmit values are ignored. These values are proportional to the wavelength of light but they represent no real world units.
In general, the default values should prove adequate but we provide this option as a means to experiment with other values.
This is used when a ray is reflected or is passing through a transparent object and when shadow rays are cast. When a ray hits a reflective surface, it spawns another ray to see what that point reflects. That is trace level one. If it hits another reflective surface another ray is spawned and it goes to trace level two. The maximum level by default is five.
One speed enhancement added to POV-Ray in version 3.0 is Adaptive Depth Control (ADC). Each time a new ray is spawned as a result of reflection or refraction its contribution to the overall color of the pixel is reduced by the amount of reflection or the filter value of the refractive surface. At some point this contribution can be considered to be insignificant and there is no point in tracing any more rays. Adaptive depth control is what tracks this contribution and makes the decision of when to bail out. On scenes that use a lot of partially reflective or refractive surfaces this can result in a considerable reduction in the number of rays fired and makes it safer to use much higher max_trace_level values.
The point at which a ray's contribution is considered insignificant is controlled by the adc_bailout value. The default is 1/255 or approximately 0.0039 since a change smaller than that could not be visible in a 24 bit image. Generally this setting is perfectly adequate and should be left alone. Setting adc_bailout to 0 will disable ADC, relying completely on max_trace_level to set an upper limit on the number of rays spawned.
If max_trace_level is reached before a non-reflecting surface is found and if ADC hasn't allowed an early exit from the ray tree the color is returned as black. Raise max_trace_level if you see black areas in a reflective surface where there should be a color.
The other symptom you could see is with transparent objects. For instance, try making a union of concentric spheres with a clear texture on them. Make ten of them in the union with radius's from 1 to 10 and render the scene. The image will show the first few spheres correctly, then black. This is because a new level is used every time you pass through a transparent surface. Raise max_trace_level to fix this problem.
Note that raising max_trace_level will use more memory and time and it could cause the program to crash with a stack overflow error, although ADC will alleviate this to a large extent. Values for max_trace_level are not restricted, so it can be set to any number as long as you have the time and memory. However, increasing its setting does not necessarily equate to increased image quality unless such depths are really needed by the scene.
If the I-Stack Overflows remain increase this value until they stop.
Changing this value affects both waves and ripples alike on all patterns in the scene.
Radiosity is an extra calculation that more realistically computes the diffuse interreflection of light. This diffuse interreflection can be seen if you place a white chair in a room full of blue carpet, blue walls and blue curtains. The chair will pick up a blue tint from light reflecting off of other parts of the room. Also notice that the shadowed areas of your surroundings are not totally dark even if no light source shines directly on the surface. Diffuse light reflecting off of other objects fills in the shadows. Typically ray-tracing uses a trick called ambient light to simulate such effects but it is not very accurate.
Radiosity is more accurate than simplistic ambient light but it takes much longer to compute. For this reason, POV-Ray does not use radiosity by default. Radiosity is turned on using the Radiosity INI file option or the +QR command line switch.
| - | Diffuse, the effect that makes the side of things facing the light brighter; |
| - | Specular, the effect that makes shiny things have dings or sparkles on them; |
| - | Reflection, the effect that mirrors give; and |
| - | Ambient, the general all-over light level that any scene has, which keeps things in shadow from being pure black. |
POV's radiosity system, based on a method by Greg Ward, provides a way to replace the last term - the constant ambient light value - with a light level which is based on what surfaces are nearby and how bright in turn they are.
The first thing you might notice about this definition is that it is circular: the light of everything is dependent on everything else and vice versa. This is true in real life but in the world of ray-tracing, we can make an approximation. The approximation that is used is: the objects you are looking at have their ambient values calculated for you by checking the other objects nearby. When those objects are checked during this process, however, a traditional constant ambient term is used.
How does POV-Ray calculate the ambient term for each point? By sending out more rays, in many different directions, and averaging the results. A typical point might use 200 or more rays to calculate its ambient light level correctly.
Now this sounds like it would make the ray-tracer 200 times slower. This is true, except that the software takes advantage of the fact that ambient light levels change quite slowly (remember, shadows are calculated separately, so sharp shadow edges are not a problem). Therefore, these extra rays are sent out only once in a while (about 1 time in 50), then these calculated values are saved and reused for nearby pixels in the image when possible.
This process of saving and reusing values is what causes the need for a variety of tuning parameters, so you can get the scene to look just the way you want.
Each item is optional and may appear in and order. If an item is specified more than once the last setting overrides previous values. Details on each item is given in the following sections.
The default value is 3.3.
When this value is too low, the light level will tend to look a little bit blotchy, as if the surfaces you're looking at were slightly warped. If this is not important to your scene (as in the case that you have a bump map or if you have a strong texture) then by all means use a lower number.
The default value is 100.