Dave Green's `cubehelix' colour scheme

Implementations: Fortran, Astronomical Packages (AIPS/casa/DS9/karma kvis), Other (gnuplot/IDL/matlab/python/matplotlib)

Example images using the `cubehelix' colour scheme using AIPS. (left) A portion of the Galactic plane from the VGPS survey at 1.4 GHz, which includes the supernova remnant G35.6-0.4 (see: 2009MNRAS.399..177G). (right) The Crab Nebula at 347 GHz observed with SCUBA on the JCMT (see 2004MNRAS.355.1315G).

1. Background

Many colour schemes used to display astronomical intensity images do not have an underlying increase in the perception of the brightness of the colours used (e.g. burning out to red for the high data values, but using yellow/green for intermediate data values, which are perceived as being brighter than the red).

2. A Solution

I have written up the implementation of a colour scheme -- called `cubehelix' -- which is intended to be perceived as increasing in intensity. This is a goes from black to white, deviating away from a pure greyscale (i.e. the diagonal from black to white in a colour cube) using a tapered helix in the colour cube, while ensuring a continuous increase in perceived intensity. This colour scheme prints as a monotonically increasing greyscale on black and white postscript devices.

This colour scheme is described in more detail in:

• Green, D. A., 2011, `A colour scheme for the display of astronomical intensity images', Bulletin of the Astronomical Society of India, 39, 289.
  (2011BASI...39..289G at ADS.)

The parameters that control this colour scheme are:

1. the start colour (this is the direction of the predominant colour deviation from black at the start of the colour scheme, with R=1, G=2, B=3 etc.);
2. the number of R->G->B rotations that are made from the start (i.e. black) to the end (i.e. white) of the colour scheme;
3. a hue parameter, which controls how saturated the colours are (if this parameter is zero then the colour scheme is purely a greyscale; if the parameter is larger than 1, then some R, G or B values may be out of range near the start or end colour scheme, so will have to be clipped, although if only a few colour levels are clipped, the resulting colour scheme may still be satisfactory);
4. a `gamma factor' can be used to emphasise low or high intensity values.

The image below shows what I consider my `default' scheme.

And here is a 3-D visualisation (made with Maple) of how the colour scheme spirals around the diagonal of the colour cube (see 3d-cubehelix.mpl for the maple code you want to explore this in 3-D further).

Note that since this `default' scheme uses a hue parameter of 1, no R, G or B values are clipped, and the perceived intensity is constantly increasing. If the hue parameter is increased, then the scheme becomes more colourful, but some of the R, G or B values are clipped, although the underlying perception of intensity is not exactly constantly increasing, it is still monotonically increasing. Below is a `more hue' scheme, for a hue parameter of 1.5.

For further examples of cubehelix colour schemes, see either:

• this page of colour wedges (including colour scheme files for use in DS9 or kvis, see Section 3.2 below), or
• this javascript page which allows you to visualise what other combinations of parameters produce, and also prodice colour scheme files for DS9 (this works in Firefox and Chrome, but not necessarily in other browsers).

3. Implementations

3.1 Fortran

As in the published paper, here is the Fortran 77 source of a subroutine that implements this scheme.

3.2 Astronomical packages (AIPS/casa/DS9/karma kvis)

This scheme is also currently implemented in:

• AIPS: from the 31DEC10 release as TVHELIX -- for examples, see the plots at the top of this page (note: you may have to RUN NEWPARMS so that TVHELIX is declared as verb within AIPS; and if you run a midnight job, for 31DEC11 you should have a DOPRINT option for TVHELIX to print out the four parameters used to control the colour scheme, which is probably useful).
• casa: from version 3.1.0, in viewer as a colo(u)r scheme called CubeHelix.
• DS9: it is possible to read in specific colour maps from text files, using
  
      prompt> ds9 -cmap file <cmap-filename>
  
  and several version of the scheme are available, or you can make others here.
• karma/kvis: it is possible to read in specific colour maps from .kf binary files, using the
  
      Intensity → PseudoColour (emulated) → Load
  
  menus. Appropriate .kf files for several version of the scheme are available (these work fine for me, under Linux and Solaris, but may not work with all implementations, as the .kf files are, I believe, memory dumps, so for example might need byte-swapping).

3.3 Other implementations

Other implementations I currently know of are:

• gnuplot: Ingo Thies (of Universität Bonn) has added this to his other useful colortools for gnuplot (also there is a `cubehelix' palette option is recent development versions of gnuplot, as shown in this demo).
• IDL:
  • Aaron Barth (of University of California, Irvine) has added this colo(u)r scheme to his ATV data display and analysis package for IDL;
  • James Davenport (of the University of Washington) has a written an implementation for IDL -- cubehelix.pro;
• matlab: Phillip Graff (a colleague of mine here at the University of Cambridge) has written function implementing this for matlab -- CubeHelix.m;
• python/matplotlib:
  • The cubehelix scheme is available from the 1.1.0 release of matplotlib.
  • Ian Heywood (or the University of Oxford) has implemented this for python (with matplotlib) -- cubehelix.py.
  • Oleg Smirnov (of ASTRON) has incorporated cubehelix into his .fits viewer Tigger (see screenshot and Colormaps.py, specifically see CubeHelixColormap.colorize()), which is in the latest release (Version 1.2, of 17th September 2011) of MeqTrees.
• J. J. Green (no relation!) has included several cubehelix color schemes in his compilation of colour schemes, at cpt-city, in a variety of formats (cpt: colour palette tables for use with the Generic Mapping Tools; c3g: CSS3 gradients; ggr: gradients for the GNU image manipulation program, GIMP; gpf: Gnuplot palette files; grd: (version 3) for Paint Shop Pro and Photoshop; inc: POV-Ray colour map headers; sao: the SAO format used by the astronomical image viewer, DS9; svg: scalar vector graphics gradients).

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