PROBLEM WITH QUIVER ARROW SIZE

Hello,

I am encountering a problem with arrows aspect ratio:

clf;
hold on;
load('Problem with Quiver.mat'); % load A and B , which are both  5743 X 1 double arrays

scatter(A, B, 1,'.'); % '1' is the marker size
DiffA=  diff(A); 
DiffB=  diff(B); 
quiver(A(1:1000:end-1),B(1:1000:end-1),DiffA(1:1000:end), DiffB(1:1000:end)); % step of 1000 to reduce the number of arrows plotted
title('Silly arrow size !')

%quiver(A(1:1000:end-1),B(1:1000:end-1),DiffA(1:1000:end), DiffB(1:1000:end),'MaxHeadSize', 1.e-4);
%title('Better arrow size, but silly aspect ratio for arrows (does not look as arrows, unless zoomed-in!')


% BELOW: 1st plot (no markerSize specif): the arrows are much too large.

% BELOW: MarkerSize specified: But the aspect ratio of the arrows is not
% correct! (the "arrow heads" apear as near-vertical lines, because their aspect ratio is not correct.
%  An arrow "look" is only obtained locally if appropriately zooming-in horizontally. 

I have NOT found the correct Quiver parameter set that would allow a reasonable display of the arrows on my graph.

HELP !

[Pai-Feng Teng]

i am on the same boat with the same problem.I tried both quiver and annotation for marking out the direction of the loop

It seems like the quiver is nearly useless if the x-axis and y-axris aspect ratio is too high. It can be used if the aspect ratio of the x-axis and y-axis are close

I tried annotation arrowhead but it is just invisible

Answer 1

[Adam Danz]

The problem

The quality of arrows produced by quiver is poor in highly skewed data aspect ratios.

In many cases equating the data aspect ratio using axis equal will create better looking arrows but if the data aspect ratio is sufficiently skewed, it will trade one problem for another, tiny or compressed arrows.

For a demonstration, here's @francois heslot's quiver plot with and without setting axis equal

load('Problem with Quiver.mat'); 
DiffA=  diff(A); 
DiffB=  diff(B); 

figure()
tiledlayout(2,1)
ax1 = nexttile();
quiver(A(1:1000:end-1),B(1:1000:end-1),DiffA(1:1000:end), DiffB(1:1000:end)); 
title('Default')

ax2 = nexttile();
copyobj(ax1.Children, ax2)
box on 
axis equal
title('"axis equal"')

I recently explained what's going on in a comment in another thread but I'll recap here. If you look at the distribution of U and V values in quiver(x,y,u,v) which are the horizontal and vertical components of the vectors, you'll see

1. both are small, centered around 0
2. the vertical components tend to be much smaller than the horizontal components, by 4 orders of magnitude

figure
tiledlayout(2,1)
nexttile
histogram(DiffA(1:1000:end))
title('Horizontal component')

nexttile
histogram(DiffB(1:1000:end))
title('Vertical component')

Solution space

There isn't a quick and easy solution to this, assuming axis equal isn't suitable.

If you're OK with removing or replacing the arrowheads, you could turn the arrowheads off and place a marker at the tail or head of the arrows. Unlike arrowheads, markers do not use data aspect ratio.

Placing markers at the head require MATLAB R2022a or later.

In this demo, I chose a 'o' marker.

figure
tiledlayout(2,1)
nexttile()
quiver(A(1:1000:end-1),B(1:1000:end-1),DiffA(1:1000:end), DiffB(1:1000:end), 'o',...
    'ShowArrowHead','off'); 
title('Marker at the tail')

nexttile()
quiver(A(1:1000:end-1),B(1:1000:end-1),DiffA(1:1000:end), DiffB(1:1000:end), 'o',...
    'ShowArrowHead','off', ...
    'Alignment', 'head');  % R2022a or later
title('Marker at the head')