colours.ps Manual

colours.ps README

This module implements in PostScript some RGB colours and colour-handling subroutines to help an artist using PostScript. It is designed to work with the line-drawing routines in line_drawing.ps

To install: go to www.pjb.com.au/comp/free/colours.ps.txt and save the file to your local disc. Rename it to colours.ps and move it into some appropriate directory such as ~/ps/lib . . .

Peter J Billam    www.pjb.com.au

Name

colours.ps - Some useful Colour stuff in PostScript

Synopsis 

 %!PS-Adobe-2.0
 %%EndComments
 %%BeginProlog
 (/home/wherever/ps/lib/colours.ps) run
 % to send it to a printer see include_run . . .
 %%EndProlog
 %%Page: 1 1

 palegreen setrgbcolor

 blue 0.6 grey 0.4 rgbmix setrgbcolor

 /darkskin paleorange 0.4 brown 0.6 rgbmix rgbdef
 darkskin setrgbcolor

 /contourcolour [ 0 blue 50 yellow 100 green 150 grey 200 white ]
    altitude rgbinterpolate rgbdef

 paleorange rgb2gray setgray  % same brightness as the paleorange

 /equivalentgray palegreen rgb2gray def

 orange 0.9 adjustbrightness setrgbcolor
 violet palegreen rgb2gray adjustbrightness setrgbcolor

Colours

colours.ps defines the following colours:

You can use rgbmix and adjustbrightness to mix yourself any other colours that you may need, just like a painter would.

Subroutines

The following subroutines are available: rgbmix, rgbinterpolate, rgbdef, rgb2gray and adjustbrightness

colour1 weight1 colour2 weight2 rgbmix

The above invocation leaves on the stack, in r g b three-number form, a colour intermediate between colour1 and colour2. The arguments are:
colour1 is the first colour, in r g b three-number form
weight1 is the weight that will be given to the first colour in the mixing
colour2 is the second colour, in r g b three-number form
weight2 is the weight that will be given to the second colour in the mixing

The weights don't have to add up to one, they get normalised to total 1.0 automatically. rgbmix uses non-linear mixing, to conform to the way the eye sees these things.

[ a1 colour1 .. aN colourN ] a rgbinterpolate

The above invocation leaves on the stack, in r g b three-number form, a colour which depends on where the number a lies in relation to the series of numbers a1,a2..aN.

It is expected that a1,a2..aN are in ascending order !
If a is a1 or less, colour1 will be returned,
if a is between a1 and a2, rgbmix is used to interpolate between them,
If a is a2 colour2 will be returned,
if a is between a2 and a3 rgbmix is used to interpolate between them,
if a is aN or more, colourN will be returned.
For example:
[ .5 paleblue .51 white 1 mediumgray ] z rgbinterpolate setrgbcolor
would transform a brownian landscape in z into a sky with clouds ...

/name_of_colour r g b rgbdef

The above invocation causes name_of_colour to be defined as { r g b } so it's like a three-dimensional version of def.

r g b rgb2gray

The above invocation causes the r g b colour to be converted to its equivalent gray value, which is then left on the stack. The conversion follows the NTSC video standard, which determines how a colour television signal is rendered on a black and white television: gray = .3 * red + .59 * green + .11 * blue

Printers and Graphic displays differ widely, and the NTSC standard does not necessarily produce a gray value which you will perceive, on your Graphic display, as having the same luminance as the original colour. If this becomes an issue for you on some particular display, then the callibrate_rgb2gray variable will allow you to callibrate the conversion.

r g b gray adjustbrightness

The above invocation causes the r g b colour to be adjusted in brightness until it matches the gray value, as judged by the rgb2gray subroutine. The resulting colour is left on the stack in r g b form.

Where possible, all three components of the colour are scaled by the same factor; this adjusts the Brightness of the colour. But if one of those components would as a result exceed 1.0, then it is fixed at 1.0 and the result is mixed with white until it matches gray; this reduces the Saturation of the colour.

An excellent discussion of the many uses of equiluminant colours of different hues can be found in Margret Livingstone's book Vision and Art, the Biology of Seeing.

Callibrating rgb2gray to fit your display

Printers and graphic displays differ widely, and the NTSC standard which is used by rgb2gray and adjustbrightness does not necessarily produce a gray value which you will perceive, on your display, as having the same luminance as the original colour. If this becomes an issue for you, a simple one-parameter adjustment is available to help you customise the formula to fit your display. Usage: 

 (/home/wherever/ps/lib/colours.ps) run
 . . .
 /callibrate_rgb2gray 0.4 def   % between 0.0 and 1.0
 orange rgb2gray setgray ...

If you set callibrate_rgb2gray to 0.0, then the standard NTSC conversion will be performed. The NTSC formula converts evenly mixed (greyish) colours correctly. But the NTSC formula thinks that red has the same luminance as 0.3 setgray, whereas your printer or display may well show the gray as being much darker. As you increase the value of callibrate_rgb2gray, the purer colours (reddish, greeninsh, blueish) will be converted lighter, without affecting the conversion of the more neutral (greyish) colours. For example, on my CRT, /callibrate_rgb2gray 0.4 def gives the best results. If you leave callibrate_rgb2gray unset, its default value is now 0.15

A PostScript page callibrate_rgb2gray.ps is supplied to help you measure what callibrate_rgb2gray should be to suit a particular display or printer. Various values of callibrate_rgb2gray are printed down the left side of the page; choose the value which gives each grey rectangle in the row the same luminance as its neighbouring colour rectangles. Choose the best compromise value across all the colour columns.

As mentioned, for my CRT, the best setting is 0.4. I'd enjoy hearing what the optimum is for other devices; if they cluster around a particular value I'll make that the default.

Author

Peter J Billam www.pjb.com.au/comp/contact.html

See Also

As examples, see sample1.ps.txt and sample4.ps.txt . . . Save the file to your local disc, rename it to sample1.ps or sample4.ps and edit it so that the run statement(s) near the beginning point to the directory where line_drawing.ps and colours.ps are installed on your system. Then use GhostView or equivalent to view it. If you wish to print it out, feel free to use something like include_run to roll the run files in.

See also

colours.ps CHANGES

20001212 Colour-Handling Utilities for Postscript
20001213 corrected some colours after visual test
20001214 rgbmix does non-linear mixing
20030225 add nonlinearise and nonlinearrgbmix
20030303 rgbinterpolate now working :-)
20030313 roll in some stuff from line_drawing.ps
20030610 some extra colours
20031016 add rgb2gray
20031017 add adjustbrightness
20031020 add pink as synonym for palered
20031024 add callibrate_rgb2gray mechanism
20060121 adjustbrightness defends against division by zero
20060121 callibrate_rgb2gray now defaults to 0.15

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