dear SPMers,
I recently asked a question about the colour additivity rule for adding
multiple blobs when displaying a structural in SPM2.
My basic problem was that i wanted to add 4 blobs such that the resultant
colours when overlapping could unambigously
identify which combination of blobs were overlapping. It was suggested that
that SPM2 used an additive colour model where simply
"I believe it is an additive color model. So there are only 3 colors- r, g,
b & their various
combinations. r+g = yellow, r+b = purple, g+b = cyan, r+g+b = white. So if
you have a 4th color it
will bias the overall color cast depending on the various hues.
However, once something has combined to white it cannot get more white, so
there should be no
change with a 4th color. In practice however the blending is not perfect so
I think 4 colors takes
on a cast of whatever color you use. "
After trying for some time without success to find a suitable 4 dimensional
colour space which is intuitive to visually apprehend i resorted to just
using the 3 colours where the additivity rule is straight forward.
however looking at the images where all 3 r g and blue overlap, the
resultant colour is black. maybe whether it is white or black depends on
the colour mode SPM uses. this is not a problem ... but what i need for
publication purposes is some reference to the formal colour mixing rule
that SPM is applying.
looking into the spm_orthviews code it says that SPM uses a "full colour"
mode for adding multiple blobs....
% spm_orthviews('AddColouredBlobs',handle,XYZ,Z,mat,colour)
% Adds blobs from a pointlist to the image specified by the handle(s).
% handle - image number to add blobs to
% XYZ - blob voxel locations (currently in millimeters)
% Z - blob voxel intensities
% mat - matrix from millimeters to voxels of blob.
% colour - the 3 vector containing the colour that the blobs should be
% Several sets of blobs can be added in this way, and it uses full colour.
% Although it may not be particularly attractive on the screen, the colour
% blobs print well.
%_______________________________________________________________________
% @(#)spm_orthviews.m 2.38 John Ashburner, Matthew Brett, Tom Nichols and
Volkmar Glauche 03/04/17
however it does not say how full colour works in terms of additivity, or in
terms of its interaction with intensity. can
someone explain how colours add together in full colour mode, what the rule
is for saturation/intensity scaling of each colour and how
this interacts with the additivity rule itself.
any help on this would be most appreciated.
best wishes
Ollie Hulme
Oliver Hulme
Post-doctoral research fellow
Laboratory of Neurobiology
Department of Anatomy
University College London
Gower Street
London
WC1E 6BT
tel: 0207 679 2187
mob: 07771 693 608
email: [log in to unmask]
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