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

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Mat576 2021-4-1

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https://ww2.mathworks.cn/matlabcentral/answers/790279-how-to-create-custom-color-wheel-for-velocity-data-stored-as-rgb-and-only-using-r-g-channels

评论： Mat576 2021-4-8

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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Answer 1

darova 2021-4-2

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https://ww2.mathworks.cn/matlabcentral/answers/790279-how-to-create-custom-color-wheel-for-velocity-data-stored-as-rgb-and-only-using-r-g-channels#answer_665429

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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)

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darova 2021-4-3

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Create your own color map

[T,R] = ndgrid(0:10:360,0:5); % polar coordinates

[X,Y] = pol2cart(T*pi/180,R); % convefrt o cartesian

[m,n] = size(R);

[r1,g1] = ndgrid((1:m)/m,(1:n)/n); % red and green channels

surf(X,Y,X*0,cat(3,r1,g1,r1*0)) % surf with user colormap

view(0,90)

Mat576 2021-4-8

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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)