Dave Green's `cubehelix' colour scheme
Astronomical Packages (AIPS/casa/DS9/karma kvis),
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:
(right) The Crab Nebula at 347 GHz observed with SCUBA on the JCMT
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
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
This colour scheme is described in more detail in:
The parameters that control this colour scheme are:
- 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
- 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;
- 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);
- 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
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).
As in the published paper, here is the Fortran 77
source of a subroutine that implements this scheme.
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
- DS9: it is possible to read in specific colour maps
from text files, using
ds9 -cmap file <cmap-filename>
and several version of the scheme are
available, or you can make others
- 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).
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
of gnuplot, as shown in this demo).
- 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;
- The cubehelix scheme is available from the 1.1.0 release of matplotlib.
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).