3.3.3 Global Settings

The global_settings statement is a catch-all statement that gathers together a number of global parameters. The statement may appear anywhere in a scene as long as it is not inside any other statement. You may have multiple global_settings statements in a scene. Whatever values were specified in the last global_settings statement override any previous settings.

Note: some items which were language directives in earlier versions of POV-Ray have been moved inside the global_settings statement so that it is more obvious to the user that their effect is global. The old syntax is permitted but generates a warning.

The new syntax is:

    global_settings { [GLOBAL_SETTINGS_ITEMS...] }
    adc_bailout Value | ambient_light COLOR | assumed_gamma Value | 
    hf_gray_16 [Bool] | irid_wavelength COLOR |
    charset GLOBAL_CHARSET | max_intersections Number |
    max_trace_level Number | number_of_waves Number |
    noise_generator Number | radiosity { RADIOSITY_ITEMS... } |
    photon { PHOTON_ITEMS... }
    ascii | utf8 | sys

Global setting default values:

charset            : ascii
adc_bailout        : 1/255
ambient_light      : <1,1,1>
assumed_gamma      : No gamma correction
hf_gray_16         : off
irid_wavelength    : <0.25,0.18,0.14>
max_trace_level    : 5
max_intersections  : 64
number_of_waves    : 10
noise_generator    : 2

  adc_bailout      : 0.01
  always_sample    : on
  brightness       : 1.0
  count            : 35  (max = 1600)
  error_bound      : 1.8
  gray_threshold   : 0.0
  low_error_factor : 0.5
  max_sample       : non-positive value
  minimum_reuse    : 0.015
  nearest_count    : 5   (max = 20)
  normal           : off 
  pretrace_start   : 0.08
  pretrace_end     : 0.04
  recursion_limit  : 3

Each item is optional and may appear in any order. If an item is specified more than once, the last setting overrides previous values. Details on each item are given in the following sections. ADC_Bailout

In scenes with many reflective and transparent surfaces, POV-Ray can get bogged down tracing multiple reflections and refractions that contribute very little to the color of a particular pixel. The program uses a system called Adaptive Depth Control (ADC) to stop computing additional reflected or refracted rays when their contribution is insignificant.

You may use the global setting adc_bailout keyword followed by float value to specify the point at which a ray's contribution is considered insignificant. For example:

  global_settings { adc_bailout 0.01 }

The default value 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.

See section "Max_Trace_Level" for details on how ADC and max_trace_level interact. Ambient_Light

Ambient light is used to simulate the effect of inter-diffuse reflection that is responsible for lighting areas that partially or completely lie in shadow. POV-Ray provides the ambient_light keyword to let you easily change the brightness of the ambient lighting without changing every ambient value in all finish statements. It also lets you create interesting effects by changing the color of the ambient light source. The syntax is:

 global_settings { ambient_light COLOR }

The default is a white ambient light source set at rgb <1,1,1>. Only the rgb components are used. The actual ambient used is: Ambient = Finish_Ambient * Global_Ambient.

See section "Ambient" for more information. Assumed_Gamma

Many people may have noticed at one time or another that some images are too bright or dim when displayed on their system. As a rule, Macintosh users find that images created on a PC are too bright, while PC users find that images created on a Macintosh are too dim.

The assumed_gamma global setting works in conjunction with the Display_Gamma INI setting (see section "Display Hardware Settings") to ensure that scene files render the same way across the wide variety of hardware platforms that POV-Ray is used on. The assumed gamma setting is used in a scene file by adding

 global_settings { assumed_gamma Value }

where the assumed gamma value is the correction factor to be applied before the pixels are displayed and/or saved to disk. For scenes created in older versions of POV-Ray, the assumed gamma value will be the same as the display gamma value of the system the scene was created on. For PC systems, the most common display gamma is 2.2, while for scenes created on Macintosh systems should use a scene gamma of 1.8. Another gamma value that sometimes occurs in scenes is 1.0.

Scenes that do not have an assumed_gamma global setting will not have any gamma correction performed on them, for compatibility reasons. If you are creating new scenes or rendering old scenes, it is strongly recommended that you put in an appropriate assumed_gamma global setting. For new scenes, you should use an assumed gamma value of 1.0 as this models how light appears in the real world more realistically.

Before we go to the following sections, that explain more thoroughly what gamma is and why it is important, a short overview of how gamma works in POV-Ray:

no assumed_gamma in scene :
No gamma correction is applied to output file.
assumed_gamma 1 :
Gamma Display_Gamma is applied to output file.
If Display_Gamma is not specified, 2.2 is used.
assumed_gamma G :
Gamma Display_Gamma/G is applied to output file.
If Display_Gamma is not specified, 2.2/G is used.
Recommended value for assumed_gamma is 1. Monitor Gamma

The differences in how images are displayed is a result of how a computer actually takes an image and displays it on the monitor. In the process of rendering an image and displaying it on the screen, several gamma values are important, including the POV scene file or image file gamma and the monitor gamma.

Most image files generated by POV-Ray store numbers in the range from 0 to 255 for each of the red, green and blue components of a pixel. These numbers represent the intensity of each color component, with 0 being black and 255 being the brightest color (either 100% red, 100% green or 100% blue). When an image is displayed, the graphics card converts each color component into a voltage which is sent to the monitor to light up the red, green and blue phosphors on the screen. The voltage is usually proportional to the value of each color component.

Gamma becomes important when displaying intensities that are not the maximum or minimum possible values. For example, 127 should represent 50% of the maximum intensity for pixels stored as numbers between 0 and 255. On systems that do not do gamma correction, 127 will be converted to 50% of the maximum voltage, but because of the way the phosphors and the electron guns in a monitor work, this may be only 22% of the maximum color intensity on a monitor with a gamma of 2.2. To display a pixel which is 50% of the maximum intensity on this monitor, we would need a voltage of 73% of the maximum voltage, which translates to storing a pixel value of 186.

The relationship between the input pixel value and the displayed intensity can be approximated by an exponential function obright = ibright ^ display_gamma where obright is the output intensity and ibright is the input pixel intensity. Both values are in the range from 0 to 1 (0% to 100%). Most monitors have a fixed gamma value in the range from 1.8 to 2.6. Using the above formula with display_gamma values greater than 1 means that the output brightness will be less than the input brightness. In order to have the output and input brightness be equal an overall system gamma of 1 is needed. To do this, we need to gamma correct the input brightness in the same manner as above but with a gamma value of 1/display_gamma before it is sent to the monitor. To correct for a display gamma of 2.2, this pre-monitor gamma correction uses a gamma value of 1.0/2.2 or approximately 0.45.

How the pre-monitor gamma correction is done depends on what hardware and software is being used. On Macintosh systems, the operating system has taken it upon itself to insulate applications from the differences in display hardware. Through a gamma control panel the user may be able to set the actual monitor gamma and Mac will then convert all pixel intensities so that the monitor will appear to have the specified gamma value. On Silicon Graphics machines, the display adapter has built-in gamma correction calibrated to the monitor which gives the desired overall gamma (the default is 1.7). Unfortunately, on PCs and most UNIX systems, it is up to the application to do any gamma correction needed. Image File Gamma

Since most PC and UNIX applications and image file formats do not understand display gamma, they do not do anything to correct for it. As a result, users creating images on these systems adjust the image in such a way that it has the correct brightness when displayed. This means that the data values stored in the files are made brighter to compensate for the darkening effect of the monitor. In essence, the 0.45 gamma correction is built in to the image files created and stored on these systems. When these files are displayed on a Macintosh system, the gamma correction built in to the file, in addition to gamma correction built into MacOS, means that the image will be too bright. Similarly, files that look correct on Macintosh or SGI systems because of the built-in gamma correction will be too dark when displayed on a PC.

The PNG format files generated by POV-Ray 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. Scene File Gamma

The image file gamma problem itself is just a result of how scenes themselves are generated in POV-Ray. When you start out with a new scene and are placing light sources and adjusting surface textures and colors, you generally make several attempts before the lighting is how you like it. How you choose these settings depends upon the preview image or the image file stored to disk, which in turn is dependent upon the overall gamma of the display hardware being used.

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 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. HF_Gray_16

The hf_gray_16 setting is useful when using POV-Ray to generate heightfields for use in other POV-Ray scenes. The syntax is... global_settings { hf_gray_16 [Bool] }

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. Irid_Wavelength

Iridescence calculations depend upon the dominant wavelengths of the primary colors of red, green and blue light. You may adjust the values using the global setting irid_wavelength as follows...

 global_settings { irid_wavelength COLOR }

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. Charset

This allows you to specify the assumed character set of all text strings. If you specify ascii only standard ASCII character codes in the range from 0 to 127 are valid. You can easily find a table of ASCII characters on the internet. The option utf8 is a special Unicode text encoding and it allows you to specify characters of nearly all languages in use today. We suggest you use a text editor with the capability to export text to UTF8 to generate input files. You can find more information, including tables with codes of valid characters on the Unicode website The last possible option is to use a system specific character set. For details about the sys character set option refer to the platform specific documentation. Max_Trace_Level

In scenes with many reflective and transparent surfaces POV-Ray can get bogged down tracing multiple reflections and refractions that contribute very little to the color of a particular pixel. The global setting max_trace_level defines the integer maximum number of recursive levels that POV-Ray will trace a ray.

 global_settings { max_trace_level Level }

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.

This reduction in color contribution is a result of scaling by the reflection amount and/or the filter values of each surface, so a perfect mirror or perfectly clear surface will not be optimizable by ADC. You can see the results of ADC by watching the Rays Saved and Highest Trace Level displays on the statistics screen.

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 has not 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 can be set up to a maximum of 256. If there is no max_trace_level set and during rendering the default value is reached, a warning is issued. Max_Intersections

POV-Ray uses a set of internal stacks to collect ray/object intersection points. The usual maximum number of entries in these I-Stacks is 64. Complex scenes may cause these stacks to overflow. POV-Ray does not stop but it may incorrectly render your scene. When POV-Ray finishes rendering, a number of statistics are displayed. If you see I-Stack Overflows reported in the statistics you should increase the stack size. Add a global setting to your scene as follows:

 global_settings { max_intersections Integer }

If the I-Stack Overflows remain increase this value until they stop. Number_Of_Waves

The waves and ripples patterns are generated by summing a series of waves, each with a slightly different center and size. By default, ten waves are summed but this amount can be globally controlled by changing the number_of_waves setting.

 global_settings { number_of_waves Integer }

Changing this value affects both waves and ripples alike on all patterns in the scene. Noise_generator

There are three noise generators implemented.

The default is noise_generator 2

Note: The noise_generators can also be used within the pigment/normal/etc. statement. Radiosity Basics

Important notice: The radiosity features in POV-Ray 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 this version will render identically in future releases or that full backwards compatibility of language syntax can be maintained.

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 calculations are only made when a radiosity{} block is used inside the global_settings{} block.

The following sections describes how radiosity works, how to control it with various global settings and tips on trading quality vs. speed.

More about "Ambient"