grayscale to isoluminant color

[Britt]

It seems that the dkl colorspace would allow one, with an appropriate
calibration, to convert a grayscale image to an isoluminant color
image, thus converting luminance contrast to color contrast, by
mapping the grayscale (-1 to 1) range to one of the axes of a dkl
isoluminant plane; by rotation one could create many isoluminant
variants of the same grayscale image with different color mappings.
But, I am having a little trouble correlating my visualizing of this
three-d colorspace with my trigonometry. Has anyone made a
transformation of this type or could they point me to what they
believe to be a useful reference for helping me to work out the
coordinate transformation? Thank you, Britt Anderson, University of
Waterloo

[Jonathan Peirce]

basically, yes. You could provide a greyscale image as a texture to a
patch stimulus and it will then be coloured for you using whatever
colour space you like. As with rgb space, dkl in psychopy allows for
modulations around a central grey point, with white in your image
representing the direction you specified, black values are considered
the negative of the direction in colour space you specified. Mid-grey
values in your image will remain mid-grey in all spaces.

In DKL space, colours are usually specified using spherical coordinates
(elevation from the isolum plane, azimuth for chromaticity, and
contrast). To get isoluminant stimuli use an elevation of zero. If you
set azimuth=0 then high intensity values (white) from you greyscale
image become +L (pinkish), dark values become +M (greenish).

Be aware that certaain colours that appear to be within this circle are
not within the gamut of your monitor. PsychoPy should spit a warning
about those, but doesn't abort the run. That's what Alex's movie that he
recently posted to the list illustrates.

Jon

--
Jonathan Peirce
Nottingham Visual Neuroscience

http://www.peirce.org.uk

This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham.

This message has been checked for viruses but the contents of an attachment
may still contain software viruses which could damage your computer system:
you are advised to perform your own checks. Email communications with the
University of Nottingham may be monitored as permitted by UK legislation.