Hi there! This is somewhat embarassing, but I've been trying this little thing for a while now, so I figured it's a good time to ask now: I would like to plot a velocity vector, e.g. representing a fluid flow, pointing at / acting on an object (let's say, a thin rectangle). But if I use quiver( ) to plot the vector, the vector would emanate from the object, and consequently point away from the object, not towards it. So, my little hack was this: Plot the vector using quiver; it'll emanate from the object and point away from the object; Negate the vector, so it emanates from the object, but now it points in the opposite direction; and Remove the arrowahead from the vector. Removing the arrowhead from the vector makes the line potentially look like it's pointing toward the object, which is what I want. But now, here's the hard part, Matlab doesn't appear to be able to add an arrowhead, so that the vector can point in the original direction, which is what I want, so that the vector looks like it's pointing at the object. Matlab appears to be able to only add an arrowhead in the direction that the vector values indicate. Basically, how can I add an arrowhead at the tail of the vector, but have it point in the opposite direction? Is there a better alternative? Thanks in advance,

Is it possible to plot a vector arrow using quiver( ), then, negate the vector so it points in the opposite direction, but then add arrowhead in original direction?

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Noob 2024-12-23

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https://ww2.mathworks.cn/matlabcentral/answers/2172437-is-it-possible-to-plot-a-vector-arrow-using-quiver-then-negate-the-vector-so-it-points-in-the-o

编辑： Cris LaPierre 2024-12-24

采纳的回答： Cris LaPierre

Hi there!

This is somewhat embarassing, but I've been trying this little thing for a while now, so I figured it's a good time to ask now:

I would like to plot a velocity vector, e.g. representing a fluid flow, pointing at / acting on an object (let's say, a thin rectangle).

But if I use quiver( ) to plot the vector, the vector would emanate from the object, and consequently point away from the object, not towards it.

So, my little hack was this:

Plot the vector using quiver; it'll emanate from the object and point away from the object;
Negate the vector, so it emanates from the object, but now it points in the opposite direction; and
Remove the arrowahead from the vector.

Removing the arrowhead from the vector makes the line potentially look like it's pointing toward the object, which is what I want.

But now, here's the hard part, Matlab doesn't appear to be able to add an arrowhead, so that the vector can point in the original direction, which is what I want, so that the vector looks like it's pointing at the object. Matlab appears to be able to only add an arrowhead in the direction that the vector values indicate.

Basically, how can I add an arrowhead at the tail of the vector, but have it point in the opposite direction?

Is there a better alternative?

Thanks in advance,

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dpb 2024-12-23

As always, it would be helpful if you would attach enough code/data to give us a working starting-off point...

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采纳的回答

Cris LaPierre 2024-12-23

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2172437-is-it-possible-to-plot-a-vector-arrow-using-quiver-then-negate-the-vector-so-it-points-in-the-o#answer_1556329

编辑：Cris LaPierre 2024-12-24

在 MATLAB Online 中打开

Perhaps you could share a working example?

When I negate U and V, the arrows point the opposite direction. I also modified the Alignment property so the arrows are generally in the same place.

tiledlayout(1,2)

nexttile

[X,Y] = meshgrid(0:6,0:6);

U = 0.25*X;

V = 0.5*Y;

quiver(X,Y,U,V,0)

title('Normal')

nexttile

[X,Y] = meshgrid(0:6,0:6);

U = 0.25*X;

V = 0.5*Y;

quiver(X,Y,-U,-V,0,'alignment','Head')

title('Negated')

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Noob 2024-12-23

编辑：Noob 2024-12-23

Hi Cris!

Yes, I am aware of this workflow. Here's my "issue":

My velocity vectors are supposed to represent fluid flow pointing up and to the left, pointing up at, say, a thin rectangle / winglike object. Now, if I use quiver, I don't know a good position to place these vectors. But scientific theory tells me where the fluid flow should act on the object, so I know that much. So, let's say the position on the object where the fluid flow acts on is just (0,0), the object's geometric centroid. I am plotting the fluid velocity vector at (0,0), using quiver( ). But, it'll emanate from (0,0) and point up and to the left, when what I really want, visually, is to have the flow be beneath the object, pointing up and to the left at it. So, my hack was to negate the vector, and remove the arrowhead. And the last step, which Matlab can't seem to do: Put an arrowhead at the tail of this negated vector, and have it point in the normal, unnegated direction -- i.e., point at the object. But an arrowhead on such a vector will necessarily point in the negated, new direction, it seems.

Does that make sense?

By the way, all of this is trivial to do in Adobe Illustrator, where I finish my Matlab plots. But, for working efficiently (running thousands of numerical experiments in Matlab), I am trying to avoid usage of Illustrator and am trying to automate all plotting within Matlab.

I look forward to your response.

Thanks!

Cris LaPierre 2024-12-24

Each quiver arrow is connected to an (X,Y) point. You provide those points as the first 2 inputs to the quiver function. Alignment determines which part of the arrow is touching that point. It can be 'Tail', which is the default, 'Center' or 'Head'

dpb 2024-12-24

Indeed...an extremely useful enhancement that solves the problem internally...one wonders why it wasn't an initial feature, but adding the enhancement certainly makes the function far more versatile...congrats to TMW!

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