USAGE: color2gray [-r red] [-g green] [-b blue] [-f form] [-c clrspace]
[-B bright] [-C contrast] infile outfile

USAGE: color2gray [-h or -help]

-r ... red ......... red mix percent; float; default=29.9
-g ... green ....... green mix percent; float; default=58.7
-b ... blue ........ blue mix percent; float; default=11.4
-f ... form ........ form of combining channels; add or rms
.................... (root mean squared channel combination) or colorspace
.................... intensity channel; default=add
-c ... clrspace .... colorspace in which to extract intensity channel;
.................... rec709luma, rec601luma (or HCL), OHTA (or HSI), sGray,
.................... LAB, HSL, BCH, HSB; default=HSL
-B ... bright ...... percent brightness change of grayscale image;
.................... -100<=float<=100; default=0
-C ... contrast .... percent contrast change of grayscale image;
.................... -100<=float<=100; default=0

PURPOSE: To mix the color channels from an image into a single grayscale image using a weighted combination.

DESCRIPTION: COLOR2GRAY mixes the color channels from an image into a single grayscale image using a weighted combination. The weighted channels can be combined by simple addition or by root mean squared combination. The image may also be converted to grayscale from the intensity-like channel of various colorspaces.

Arguments:

-r red ... RED is the red mix percent (weight) in forming the grayscale image. Values are (pos. or neg.) floats. The default=29.9 (equivalent to IM non-linear grayscale. Nominally the r,g,b,o weights should total 100%. Weights are not relevant to form=colorspace.

-g green ... GREEN is the red mix percent (weight) in forming the grayscale image. Values are (pos. or neg.) floats. The default=58.7 (equivalent to IM non-linear grayscale. Nominally the r,g,b,o weights should total 100%. Weights are not relevant to form=colorspace.

-b blue ... BLUE is the red mix percent (weight) in forming the grayscale image. Values are (pos. or neg.) floats. The default=11.4 (equivalent to IM non-linear grayscale. Nominally the r,g,b,o weights should total 100%. Weights are not relevant to form=colorspace.

-f form ... FORM of combining channels. The choices are: add (a) for a simple channel addition or rms (r) for a root mean squared channel combination or colorspace (c) for intensity-like channel of specified colorspace. The default=add.

-c clrspace ... CLRSPACE is the colorspace in which to get the intensity-like channel. The choices are: rec709luma, rec601luma (or HCL), OHTA (or HSI), sGray, LAB, HSL, BCH, HSB. The default=HSL. Note that rec709luma is the current -colorspace gray, while rec601luma is the older -colorspace gray before about IM 6.7.6.7. OHTA or HSI produce results the same as an equal weighted add. Also, sGray is a perceptual gray that tries to maintain the overall brightness of the color image. The colorspaces have been listed approximately in order of darkest to lightest results, though ordering will vary from image to image. Colorspace is not relevant to form=add or to form=rms.

-B bright ... BRIGHT is the percent change in brightness of the grayscale. image. Values are floats in the range of -100 to 100. The default=0.

-C contrast ... CONTRAST is the percent change in contrast of the grayscale. image. Values are floats in the range of -100 to 100. The default=0.

REQUIREMENTS: IM 6.7.9.0 or higher when using HCL or HSI colorspaces. IM 6.6.0.4 for BCH due to the use of -evaluate-sequence. IM 6.8.3.10 for sGray due to the use of -intensity.

REFERENCES:
http://en.wikipedia.org/wiki/HSL_and_HSV#Hue_and_chroma
http://www.kweii.com/site/color_theory/2007_LV/BrightnessCalculation.pdf

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.