Fred's ImageMagick Scripts



    Licensing:

    My scripts are available free of charge for non-commercial use, ONLY.

    For use of my scripts in commercial (for-profit) environments or non-free applications, please contact me (Fred Weinhaus) for licensing arrangements. My email address is fmw at alink dot net.

    If you: 1) redistribute, 2) incorporate any of these scripts into other free applications or 3) reprogram them in another scripting language, then you must contact me for permission, especially if the result might be used in a commercial or for-profit environment.

    Usage, whether stated in the script or not, is restricted to the above licensing arrangements. It is also subject to the ImageMagick license, which can be found at: http://www.imagemagick.org/script/license.php

AUTOGAMMA


Modifies an image to automatically apply a gamma correction.

Download Script

last modified: August 27, 2013



USAGE: autogamma [-c colormode] infile outfile
USAGE: autogamma [-h or -help]

-c .... colormode ..... colorspace/channel to use to compute
....................... gamma statistics; choices are: gray,
....................... intensity, luminance, lightness, brightness,
....................... average, magnitude, rgb; default=luminance
-m .... midrange ...... midrange value from which to compute gamma;
....................... 0<float<1; default=0.5

PURPOSE: To modify an image to automatically apply a gamma correction.

DESCRIPTION: AUTOGAMMA modifies an image to automatically apply a gamma correction. The gamma value may be computed from various graylevel representations of the image or individually channel-by-channel. The script then passes the gamma value to the IM function -gamma. If the minimum and maximum image statistics are not pure black and white, then you may want to use the autolevel script, so that you get the full benefit of the dynamic range stretch as well.

ARGUMENTS:

-c colormode ... COLORMODE is the colorspace/channel to use to compute the minimum, maximum and gamma values. The choices are: gray, intensity, luminance, lightness, brightness, average, magnitude and rgb. Values of gray and intensity are equivalent. The default is luminance.

-m midrange ... MIDRANGE value from which to compute gamma. Values are 0<floats<1. Default=0.5.

Gray or Intensity uses statistics from -colorspace Gray.
Luminance uses statistics from -colorspace Rec709Luma.
Lightness uses statistics from the lightness channel of -colorspace HSL.
Brightness uses statistics from the brightness channel of -colorspace HSB.
Average uses statistics from the first channel of -colorspace OHTA.
Magnitude uses aggregate statistics from all the channels.
RGB uses statistics independently from each channel of -colorspace sRGB/RGB.
See definitions at: http://www.imagemagick.org/script/command-line-options.php#colorspace

Note: generally there are only slight differences between the various non-rgb colormode results. Colormode=rgb can cause color balance shifts.

Gamma = log(mean)/log(mid-dynamic-range)

Note: there is one internal parameter, midrange, that can be adjusted if you want to bias the gamma slightly. See the default values section. You can also change the default colormode in the default values.

CAVEAT: No guarantee that this script will work on all platforms, nor that trapping of inconsistent parameters is complete and foolproof. Use At Your Own Risk.


EXAMPLES


Colormode Results On Dark Image

 

Original Image

Arguments:
-c rgb

 
   

Arguments:
-c luminance

Arguments:
-c gray (or intensity)

Arguments:
-c average

Arguments:
-c lightness

Arguments:
-c magnitude

Arguments:
-c brightness



The Following Images Were Obtained At http://images.google.com/


Original Image

Arguments:
-c luminance



Original Image

Arguments:
-c luminance



Original Image

Arguments:
-c luminance



Original Image

Arguments:
-c luminance



Original Image

Arguments:
-c luminance



Original Image

Arguments:
-c luminance



Original Image

Arguments:
-c luminance



What the script does is as follows:

  • Converts the image to RGB
  • Generates a copy of the image in the desired colorspace/channel
  • Accesses the mean statistics for this image
  • Computes a gamma value from the mean
  • Passes the gamma value to -gamma and applies to
    each channel of the RGB image

This is equivalent to the following IM commands for the luminosity colormode.

  • convert $infile -colorspace RGB $tmp1
  • convert $tmp1 -colorspace Rec709Luma $tmp2
  • mean=`convert $tmp2 -format "%[mean]" info:`
  • mean=`convert xc: -format "%[fx:100*$mean/quantumrange]" info:`
  • gammaval=`convert xc: -format "%[fx:log($mean/100)/log($midrange)]" info:`
  • convert $tmp1 -gamma ${gammaval} $outfile