Plot a Colour Wheel scale

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JMS 2014-4-8

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评论： Image Analyst 2014-4-15

采纳的回答： Image Analyst

Colour Wheel.JPG

Hello! Please, can anyone tell how I can plot a Colour Wheel scale from specific data?

For example, if I have a [y,z] = meshgrid(-5: .35: 5, -5: .35: 5); Or [y,z] = meshgrid(-2: .1: 2, -2: .1: 2);

I also have attached file for example. Thanks in advance.

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Image Analyst 2014-4-8

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lab_hsv_display.m

See my attached demo. It does that.

It's one of the images it plots. If you want to mask it by a circle to make it round instead of square, you can use code from the FAQ to do that: http://matlab.wikia.com/wiki/FAQ#How_do_I_create_a_circle.3F

JMS 2014-4-10

That is better and helps more.

Thanks.

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Kelly Kearney 2014-4-9

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在 MATLAB Online 中打开

A simpler way:

r = linspace(0,1,10);
theta = linspace(0, 2*pi, 100);
[rg, thg] = meshgrid(r,theta);
[x,y] = pol2cart(thg,rg);
pcolor(x,y,thg);
colormap(hsv);
shading flat;
axis equal;

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Image Analyst 2014-4-11

编辑：Image Analyst 2014-4-11

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JMS, yes you can get r like that. That's exactly what I did in my demo. Look at it again below. See where I calculated radius inside the for loop. This time I also applied the circular mask for you.

fontSize = 20;
% Compute HSV image
rows = 300;
columns = 300;
midX = columns / 2;
midY = rows / 2;
% Construct v image as uniform.
v = 0.95 * ones(rows, columns);
s = zeros(size(v)); % Initialize.
h = zeros(size(v)); % Initialize.
% Initialize circle mask
circleMask = zeros(rows, columns);
% Construct the h image as going from 0 to 1 as the angle goes from 0 to 360.
% Construct the S image going from 0 at the center to 1 at the edge.
for c = 1 : columns
  for r = 1 : rows
    % Radius goes from 0 to 1 at edge, a little more in the corners.
    radius = sqrt((r - midY)^2 + (c - midX)^2) / min([midX, midY]);
    s(r, c) = min(1, radius); % Max out at 1
    h(r, c) = atan2d((r - midY), (c - midX));
    if radius <= 1
      circleMask(r, c) = 1;
    end
  end
end
% Flip h right to left.
h = fliplr(mat2gray(h));
% Construct the hsv image.
hsv = cat(3, h, s, v);
% Display the hue image.
subplot(2, 2, 1);
imshow(h, []);
title('H (Hue) Image', 'FontSize', fontSize);
% Display the saturation image.
subplot(2, 2, 2);
imshow(s, []);
title('S (Saturation) Image', 'FontSize', fontSize);
% Display the value image.
subplot(2, 2, 3);
imshow(v, []);
title('V (Value) Image (with V = 0.95)', 'FontSize', fontSize);
% Construct the RGB image.
rgbImage = hsv2rgb(hsv);
% Mask with circle.
% Mask the image using bsxfun() function
maskedRgbImage = bsxfun(@times, rgbImage, cast(circleMask, class(rgbImage)));
% Display the RGB image.
subplot(2, 2, 4);
imshow(maskedRgbImage, []);
title('RGB Image, with V = 0.95', 'FontSize', fontSize);
drawnow;
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);

Kelly Kearney 2014-4-11

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Okay, I'm pulling out my crystal ball, because you really haven't been clear about how this hypothetical color wheel corresponds to the data and quiver plot you keep referencing.

My guess is that perhaps, rather than a quiver plot, you want to show the directional field via colors, and rather than a traditional colorbar, you want a reference color wheel. If all of that is correct, then your steps will be:

1) Calculate the direction of your velocity field, and plot this via pcolor or similar

2) Plot the color wheel as I showed above in its own axis.

3) Make sure you choose an appropriate colormap that begins and ends with the same value, such as hsv.

Here's an example:

% Your data
[x, y] = meshgrid(-2: .1: 2, -2: .1: 2);
u = y./(x.^2 + y.^2); % x-components
v = x./ (x.^2 + y.^2); % y-components
% Calculate the angle
ang = atan2(v,u);
% Plot angle by color, overlaid with quiver
figure;
ax = axes('position', [0.1 0.1 0.7 0.7]);
pcolor(x,y,ang);
hold on;
quiver(x, y, u, v, 5, 'k');
shading flat;
% Plot color wheel for reference
cbax = axes('position', [0.8 0.8 0.15 0.15]);
r = linspace(0,1,10);
theta = linspace(-pi, pi, 100);
[rg, thg] = meshgrid(r,theta);
[x,y] = pol2cart(thg,rg);
pcolor(x,y,thg);
colormap(hsv);
shading flat;
axis equal;
set(cbax, 'visible', 'off');
set([ax cbax], 'clim', [-pi pi]);

Kelly Kearney 2014-4-15

Aaaaaah!

You've used the phrase "have the color wheel in RGB from the quiver plot data" several times now, and it remains just as meaningless as before.

Your quiver plot has absolutely nothing to do with a color wheel. You can create a pretty picture of a color wheel several ways (such as the examples ImageAnalyst and I gave) using Matlab. Or you could pull out a piece of paper and some crayons and draw one there. But it's still just a pretty picture, and it has no connection to your quiver plot data.

I gave an example of how to plot directional data by color, overlaid with a quiver plot, and then I drew a small wheel next to it as a guide. Is that even remotely close to what you're looking for?

Image Analyst 2014-4-15

在 MATLAB Online 中打开

(Sound of hand slapping forehead) Yeah, I'm pretty much baffled too, and about to give up. I don't even know that he has vector field data. He just takes a vector spanning a certain range in x and certain range in Y, calls meshgrid and calls that his vector field. It's totally made up/synthesized. JMS do you have any actual real world data?

It sounds like JMS wants to create a color wheel but instead of using the normal values and ranges required to produce one (like I did), he wants to use some special specified range (-2 to +2) for some unknown reason. OK, well, if you insist on doing that, then you're going to have to transform those into the range that is required, and then create the wheel picture. Of course that could all be wrong since I don't know what he wants.

Maybe he wants to create an image of the velocity according to the equations, and just simply apply a colormap to it:

% First create velocityImage from equations.  Then:
imshow(velocityImage);
colormap('hsv');
colorbar;

You don't get a pretty perfect color wheel but what you do get is an image where the color corresponds to the magnitude of the velocity.

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