Quiver with color: Add-on

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Niklas Kurz
Niklas Kurz on 12 May 2021
Commented: Adam Danz on 13 May 2021
Sadly Matlab didn't enabled color-coded vectors in their ever so neat function quiver
Therefore there are endless detours open in Matlab, like the one I found for plotting what I think of:
It's promissing
"all vectors have the same size based on the optimum value for the grid provided"
Given that I don't understand the function itself I can't work out how . My best try is:
[x,y] = meshgrid(-2:0.2:2);
xr = -y;
yr = x;
ncquiverref(x,y,xr,yr,'col')
What's looking pretty pretty (I mean, look at that clean arrows), but monochrome.

Accepted Answer

Adam Danz
Adam Danz on 12 May 2021
Edited: Adam Danz on 12 May 2021
You can plot each quiver arrow in a loop to individually control the colors. This demo assigns color according to the magnitude of each vector.
% Demo data
[X,Y] = meshgrid(-2:0.2:2);
Z = X .* exp(-X.^2 - Y.^2);
[U,V] = gradient(Z,0.2,0.2);
% Create axes
ax = axes('Color', [.15 .15 .15]);
hold(ax,'on')
box(ax,'on')
% Define the colormap for the quiver arrows.
% cmap can have any number of rows.
cmap = autumn(255);
ax.Colormap = cmap;
% Assign colors based on magnitude of vectors
vectorMagnitude = hypot(U(:),V(:));
% Scale magnitudes to rows of colormap
vecMagNorm = (vectorMagnitude-min(vectorMagnitude))./range(vectorMagnitude);
vecColorIdx = round(vecMagNorm * (size(cmap,1)-1)) + 1;
% Plot the quiver data
for i = 1:numel(Z)
quiver(ax, X(i),Y(i),U(i),V(i), .2, ...
'Color', cmap(vecColorIdx(i),:), 'LineWidth',1)
end
% Set properties for the main axes
axis equal
xlim(ax, [-2 2])
ylim(ax, [-2 2])
% Add colorbar
cb = colorbar(ax);
% Set colorbar range
caxis(ax, [floor(vectorMagnitude(1)), ceil(vectorMagnitude(2))])
% Label the colorbars
ylabel(cb,'Vector magnitude')
  2 Comments
Adam Danz
Adam Danz on 13 May 2021
I just looked at the file exchange submission you mentioned and I see that they are using line objects which allows you to set colors differently for each line rather than creating each quiver object individuallywhich is what my approach is doing and is therefore much slower.

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