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# How to create custom color wheel for velocity data stored as RGB and only using R/G channels?

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Mat576 on 1 Apr 2021
Commented: Mat576 on 8 Apr 2021
I have velocity field data stored in RGB format where R = angle and G = amplitude/magnitude. I am wanting to create a custom color wheel with the following:
R: [0 1] where 0.5 = 0 degrees
G: [0 1] where the values increase going from the center to edge of circle
B: unused / zero
Does anyone have any suggestions on how to go about this? I would like it to be cyclic (i.e. smooth transition in colors at -pi and pi). Thank you in advance.
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### Accepted Answer

darova on 2 Apr 2021
What about this?
[R,G] = ndgrid(0:10:360,0:1); % polar coordinates
[X,Y] = pol2cart(R*pi/180,G); % convefrt o cartesian
Z = sind(R/2);
pcolor(X,Y,Z) ##### 3 CommentsShowHide 2 older comments
Mat576 on 8 Apr 2021
Thank you! A few small tweaks got me to what I needed:
[T,R] = ndgrid(-180:180,0:(Vmax/256):Vmax);
[X,Y] = pol2cart(T*pi/180,R);
[m,n] = size(R);
[r1,g1] = ndgrid((1:m)/m,(1:n)/n);
wheel = surf(X,Y,X*0,cat(3,r1,g1,r1*0));
axis off
wheel.EdgeColor = 'none';
view(0,90)
Now to get this plotted on the corner of the velocity-field image, haha!

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