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Tone Mapping

The photographic tone mapper converts actual pixel luminances (in candelas per meter squared) into image pixels as seen by a camera, applying camera-related parameters (like f-stops and shutter speeds) for the exposure and applying tone mapping that emulates film/camera-like effects.

The photographic tone mapper has two basic modes: Photographic and Arbitrary.

In “Photographic” mode, the tone mapper assumes that input values are or can be converted to candelas per meter squared.

In “Arbitrary” mode, scene pixels are not considered to be in any particular physical unit but are simply scaled by a factor to fit in the display range of the screen.

  • Exposure Value - A single setting that corresponds to a combination of the three photographic exposure values: Shutter Speed, F/Stop, and Film ISO. Each increment or decrement in this value corresponds to halving or doubling, respectively, the effective exposure as expressed in the resultant change in the Shutter Speed value. Higher values yield darker images, and lower values yield brighter images.

    For example, the combination of a Shutter Speed of 125 (1/125th of a second), F/Stop of 16, and Film Speed (ISO) of 100 results in an EV of 15. While the same EV results from halving the Shutter Speed to 250 (1/250th of a second) and doubling the F/Stop to 11.
  • Shutter Speed (1/x) - Controls the duration, in fractions of a second, that the “shutter” is open; e.g., the value 100 means that the “shutter” is open for 1/100th of a second. The higher this value, the greater the exposure1).

    This setting has no effect in “Arbitrary” mode.
  • F/Stop - The fractional aperture number; e.g., 11 means aperture “f/11.” It adjusts the size of the opening of the “camera iris” and is expressed as a ratio. The higher this value, the lower the exposure.

    Aperture numbers on cameras are in a specific standard series, e.g., “f/8,” “f/11,” “f/16,” “f/22,” etc. Each of these are referred to as a “stop” (from the fact that aperture rings on real lenses tend to have physical divets for these values), and each such “stop” represents halving the amount of light hitting the film when compared to the previous “stop.”

    This setting has no effect in “Arbitrary” mode.
  • Film ISO - Controls the sensitivity of the “camera film” and is expressed as an index; the ISO number of the film, also known as “film speed.” The higher this value, the greater the exposure. If this is set to a non-zero value, “Photographic” mode is enabled. If this is set to 0, “Arbitrary” mode is enabled, and all color scaling is then strictly defined by the value of cm^2 Factor.
  • cm^2 Factor - In “Photographic” mode (i.e., non-zero Film ISO), this is the conversion factor between pixel values and candelas per meter squared. In “Arbitrary” mode, this is simply the multiplier applied to scale rendered pixel values to screen pixels.
  • Vignetting - Controls the image brightness in the image periphery compared to the image center, resulting in a circular fully exposed area in the center with darker edges. In a real camera, the angle with which the light hits the film impacts the exposure, causing the image to go darker around the edges. When set to 0.0, vignetting is disabled. The higher this value, the stronger the darkening around the edges.

    This effect is based on the cosine of the angle with which the light ray would hit the film plane and is hence affected by the field-of-view of the camera. It will not work at all for orthographic renderings. A value of 3.0 is similar to what a compact camera would generate.
  • White Point - Specifies the main color temperature of the light sources; the color that will be mapped to “white” on output, e.g., an incoming color of this hue/saturation will be mapped to grayscale, but its intensity will remain unchanged. This is similar to white balance controls on digital cameras.

    For example, photographs taken indoors might be lit by incandescent lights, which are relatively orange compared to daylight. Defining “white” as daylight will give unacceptable results when attempting to color-correct a photograph taken with incandescent lighting.
  • Burn Highlights - Adjusts the upper part of the “tone mapping” of the image; how much “over exposure” is allowed. This setting controls exactly how the high dynamic range imagery is adapted to fit into the white-to-black range of a display device. As the value is decreased from 1 towards 0, high intensities will become more and more “compressed” into lower intensities. When the value is 0, an infinite input value maps to white output value; i.e., over-exposure is no longer possible. A good default value is 0.5.
  • Crush Blacks - Adjusts the lower part of the “tone mapping” of the image. When the upper part of the dynamic range becomes compressed, it naturally loses some of its contrast. This often results in a desire to regain some “punch” in the image by increasing the intensity of the lower range values. When this value is 0, the lower intensity range is linear. When raised towards 1, a strong “toe” region is added to the transfer curve so that low intensities get (softly) pushed more towards black.
  • Saturation - Controls the intensity of colors in the rendered image. Compressing bright colors inherently causes them to become less saturated. Sometimes very strong compressions can cause the image to become unappealing. This setting allows artistic control over the final image saturation. A value of 1.0 is the standard “unmodified” saturation. Higher values increase the intensity of colors, while lower values decrease the intensity of colors.
  • Gamma - Applies a display gamma correction.