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
}
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.
The following sections explain more thoroughly what gamma is and why it is important.
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 aren't 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 don't 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.
Since most PC and UNIX applications and image file formats don't understand display gamma, they don't 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 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.
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 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.