Why the colour bar values are not representing the vectors on the plot?

Question

muhammad choudhry 2022-1-5

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链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1623120-why-the-colour-bar-values-are-not-representing-the-vectors-on-the-plot

评论： DGM 2022-1-6

colorbar.jpg

Hi please see the attached image I am trying to use the colour bar to represent the U and V values from the graph but it is not representing it. can you guys guide me how to process it. Below is my current code.

Code:

files = { 'F:\3-PIV_Experimental_Data\Outlet_110\Data_LaserSheet_D\Data_PIV\Data_Vector\Run 13-56-31.Adaptive PIV.6tvk0mbp\Export.6tvk0mbp.000000.csv'; ...

'F:\3-PIV_Experimental_Data\Outlet_110\Data_LaserSheet_B\Data_PIV\Data_Vector\Run 17-19-03.Adaptive PIV.6ul4ynyv\Export.6ul4ynyv.000000.csv'; ...

'F:\3-PIV_Experimental_Data\Outlet_110\Data_LaserSheet_A\Data_PIV\Data_Vector\Run 12-45-51.Adaptive PIV.6tskdx6a\Export.6tskdx6a.000000.csv'};

for i = 1:numel(files)

a = readtable(files{i});

X = a{:,5}/1000;

Y = a{:,6}/1000;

U = a{:,9};

V = a{:,10};

quiver(X, Y, U, V, 10);

xlim([0.1 0.4])

ylim([0.1 0.4])

colorbar('AxisLocation','in')

hold on

end

legend('DataLaserSheet-D','DataLaserSheet-C','DataLaserSheet-B','DataLaserSheet-A')

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Answer 1

DGM 2022-1-5

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1623120-why-the-colour-bar-values-are-not-representing-the-vectors-on-the-plot#answer_868945

编辑：DGM 2022-1-5

Quiver plots and line plots do not use the colormap used by colorbar. You have to explicitly set both the axes 'colororder' and 'colormap' properties to the same map.

load('wind','x','y','u','v')

X = x(11:22,11:22,1);

Y = y(11:22,11:22,1);

hold on

quiver(X,Y,u(11:22,11:22,1),v(11:22,11:22,1));

quiver(X,Y,u(11:22,11:22,2),v(11:22,11:22,2));

quiver(X,Y,u(11:22,11:22,3),v(11:22,11:22,3));

quiver(X,Y,u(11:22,11:22,4),v(11:22,11:22,4));

colormap(get(gca,'colororder'))

colorbar

That said, that's really what the legend is for. In this case, the colorbar scale has no clear meaning.

Still, I'm not sure if that's what you're asking for. "I am trying to use the colour bar to represent the U and V values" might mean that you're trying to describe the vector fields using some solid color map (like pcolor() or something). If that's the case, you'll have to actually do that.

figure

hold on

pcolor(X,Y,hypot(u(11:22,11:22,1),v(11:22,11:22,1)));

quiver(X,Y,u(11:22,11:22,1),v(11:22,11:22,1));

shading interp

colormap(gray(256))

colorbar

Note that I only plotted one quiver plot and the pcolor() plot shows only the vector magnitude. Trying to put more things into the same plot should raise the question of how they're expected to be represented clearly.

If you want to represent the vector field (both components) with pcolor(), then you might want to look at this discussion:

https://www.mathworks.com/matlabcentral/answers/1592664-use-of-2d-colormaps-possible-in-matlab

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DGM 2022-1-6

In the given quiver plot, there is no colored representation of either U or V. The colormap wouldn't refer to anything that's visible. Even if it did, it can only refer to one thing.

a = readtable('https://www.mathworks.com/matlabcentral/answers/uploaded_files/854510/Export.6tvk0mbp.000085.csv');
Warning: Column headers from the file were modified to make them valid MATLAB identifiers before creating variable names for the table. The original column headers are saved in the VariableDescriptions property.
Set 'VariableNamingRule' to 'preserve' to use the original column headers as table variable names.
X = a{:,5}/1000;
Y = a{:,6}/1000;
U = a{:,9};
V = a{:,10};
ru = [min(U) max(U)] % range of U
ru = 1×2
   -9.4214   12.5737
rv = [min(V) max(V)] % range of V
rv = 1×2
   -9.4879   13.3122

Say you set up a colorbar and set its extents to match the range of either U, V, or some combination of their ranges.

quiver(X, Y, U, V, 10);

colorbar

%caxis(ru) % set to the range of U

%caxis(rv) % set to the range of V

caxis([min(ru(1),rv(1)) max(ru(2),rv(2))]) % set to the union of ru,rv

colormap(spring) % to avoid confusing colors

The colorbar limits are set, but they don't correspond to anything in the plot. You have to pick something. You can do something with pcolor() like I suggested, but obviously if you plot a pcolor map of U, you can't plot a pcolor map of V without covering up the other one. So you'd ether have to pick one, or plot the magnitude like the example above.

As far as I know, quiver() doesn't support colormapping. There is this thing on the File Exchange:

https://www.mathworks.com/matlabcentral/fileexchange/58527-quiver-magnitude-dependent-color-in-2d-and-3d/

though I haven't tried it. If that works, you could colormap the quiver plot itself and avoid the use of pcolor(), but you'd still only be able to describe one thing at a time (i.e. magnitude).

muhammad choudhry 2022-1-6

quiverC2D(X, Y, U, V, 10); that worked as well! cheers thanks alot for your time and effort

DGM 2022-1-6

I'm glad that tool worked out. I was worried you might have issues with such a large dataset. Getting good visualization out of quiver plots is kind of maddening, as you can see.

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