FBGAMMA(1)

NAME

fbgamma - load a gamma correcting colormap

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SYNOPSIS
DESCRIPTION
OPTIONS
THEORY
EXAMPLES
SEE ALSO
AUTHOR
COPYRIGHT
BUG REPORTS

SYNOPSIS

fbgamma [-H] [-o] [-i] [-F framebuffer] val [gval bval]

fbgamma loads a gamma correcting colormap into a framebuffer. If a single value is given, that number is used for all three color channels. If three values are specified, they will be applied to the red, green, and blue channels respectively.

Gamma values larger than one will boost dim pixels with respect to bright ones. Values less than one will exaggerate the difference in brightness. The former has the effect of desaturating the displayed colors ("washing them out"), the latter will saturate the colors. An uncorrected monitor is usually too saturated.

Note that since this program is intended for gamma correction, the exponent to which pixel values are raised is the inverse of the given value. To linearize a display then, the given value will be equal to the gamma of the display (typically 2.0-3.0, see below). If you wish to directly apply an exponent to a picture (rather than its inverse), this can be done by giving a negative value (which means reciprocal in the usual mathematical sense).

OPTIONS

-F framebuffer: Specify the framebuffer device to use.
-H: Sets the default framebuffer size to 1024x1024 instead of 512x512. This size distinction is important for devices like the Adage framebuffers which have different operating modes based on the display size.
-i: Display a standard gamma calibration image on the framebuffer. This image consists of intensity ramps with patches of solid color, and vertically adjacent patches containing alternating black and colored scanlines. Adjust gamma values until vertically adjacent ramps of the same color appear equal in intensity for all patches.
-o: Overlay mode. Reads the current colormap from the framebuffer and applies the gamma correction to it, instead of starting from a linear ramp. Useful for saturating or desaturating an arbitrary colormap.
-h or -?: Print usage information and exit.

THEORY

The response of a typical monitor is non-linear with respect to input voltage. To a fairly high degree of accuracy, the response can be expressed as:

display_intensity = input_voltage ^ gamma

where gamma will usually lie in the 2.0 to 3.0 range (2.2 is the NTSC standard gamma, 2.5 is a commonly used "typical" value). Most of the time, pixel values of images have been computed assuming a linear mapping between pixel value and displayed intensity. This linear mapping can be approximated by loading a colormap into the display which raises the pixel values to the inverse gamma power.

It should be noted that getting the gamma values "right" involves careful calibration and measurement of your display device. The values are likely to be different for each color channel, and will depend on the brightness setting, etc.

A gamma value greater than unity will saturate colors by pushing them away from the white point and closer to the primaries. A gamma less than unity will desaturate colors by moving them closer to the white point.

EXAMPLES

Example 1. Calibrate a framebuffer interactively

fbgamma -i 2.2

Display the calibration image and load a gamma 2.2 correcting colormap. Adjust the value (re-running fbgamma with the image still on screen) until vertically adjacent ramps of the same color match in intensity across all patches. Once matched, that value is the display gamma to use for the session.

Example 2. Display a pix image with proper gamma correction

fbgamma 2.2
pix-fb render.pix

Load a gamma 2.2 correcting colormap into the default framebuffer, then send a pix(5) image to it with pix-fb(1). The image, which was computed assuming a linear intensity mapping, is now displayed correctly on a non-linear monitor without modifying the pixel data.

Example 3. Apply separate per-channel gamma to a named device

fbgamma -F /dev/X 2.3 2.1 2.4

Load independent red, green, and blue gamma values into the X11 framebuffer, compensating for a display whose channels respond differently.