bugGNU Octave - Bugs: bug #62081, quiver: arrowheads damage plot (to...


bug #62081: quiver: arrowheads damage plot (to large)

Submitter:  None
Submitted:  Fri 18 Feb 2022 06:51:01 PM UTC
Category:  Plotting Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Feature Request
Status:  Works For Me Assigned to:  None
Originator Name:  Manfred Quasten Originator Email:  -email is unavailable-
Open/Closed:  * Closed Release:  * 5.2.0
Operating System:  * Microsoft Windows
* Mandatory Fields

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Wed 01 Jun 2022 06:31:26 PM UTC, comment #2: 

also comparing in Octave 6.4.0 and Matlab 2022a, I get the same plots from your richtungsfeld function, imaged attached.

Noticing the report release tag, maybe this issue was unique to v 5.2.0? absent any other followup since Feb, closing report. can reopen if new information arises.

Nicholas Jankowski <nrjank>
Project Member
Fri 18 Feb 2022 10:58:03 PM UTC, comment #1: 

Have you tried this in Matlab?

I tried the code in richtungsfeld.m in both Octave and Matlab and they produce the same plots.  It appears that this is what is supposed to happen.

If I go to review the source code at https://de.wikipedia.org/wiki/Richtungsfeld

it is not the same as the attached m-file.

I copied the code that is on de.wikipedia.org and organized it into one file which is attached and shown below


function richtungsfeld(dgl)
% dgl ist die erste Ableitung von y nach x und ist i.A. eine Funktion von x und y
% Ausschnitt und Abstand zwischen den Vektoren
y = -5:.5:5; x = -5:.5:5;
for y_n = 1:length(y)
 for x_n = 1:length(x)
   len = sqrt( dgl(y(y_n), x(x_n))^2 + 1 ); % Länge des Vektors für Normierung
   dx(y_n,x_n) = 1 / len;                   % Länge des Vektors entlang der Abszisse
   dy(y_n,x_n) = dgl(y(y_n), x(x_n)) / len; % Länge des Vektors entlang der Ordinate
h=quiver(x, y, dx, dy,0.5,"r","linewidth",1); % Vektoren zeichnen
set (h, "maxheadsize", 0.1);
xlabel ("x");
print('field.svg', '-dsvg')  % Plot als svg-Datei exportieren
% Ende des Files

dgl = @(y, x) y-x   % Funktionsdefinition

It produces the plot shown on the Wikipedia page.

(file #52890)

Rik <rik5>
Project Administrator
Fri 18 Feb 2022 06:51:01 PM UTC, original submission:  

plotting  a direction field of an ode the plot is damaged by the arrow head when ordinate scale is smal.

without line 14 (richtungsfeld.m)  the direction field from

z' = 1/0.00159979*sqrt(1+z^2)

is not useable. Switching the arrowheads off, it looks good.  The arrow heads will damage it.  In this case it make no difference to skip it because z' is monotonous increasing on R, and no information will be lost.  It have taken some time to  found that the script works and that the size of the arrow heads will be the problem. The size of the arrowheads should scale automaticly, the default is too large and I was not able to resize it (MaxHeadSize property).



(Note: upload size limit is set to 16384 kB, after insertion of the required escape characters.)

Attach Files:

Attached Files
file #53265:  Picture1.png added by nrjank (80KiB - image/png - matlab/octave plot comparison - no noticeable difference)
file #52890:  tst_arrow_sz.m added by rik5 (800B - text/x-matlab)
file #52883:  richtungsfeld.m added by None (618B - application/octet-stream)
file #52884:  direction-field-modern-art.png added by None (75KiB - image/png)
file #52886:  math.txt added by None (560B - text/plain)


Depends on the following items: None found

Items that depend on this one: None found


Carbon-Copy List
  • -email is unavailable- added by nrjank (Updated the item)
  • -email is unavailable- added by rik5 (Updated the item)
  • -email is unavailable- added by None (Submitted the item)

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    Follow 9 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2022-06-01 nrjank Attached File- Added Picture1.png, #53265
        StatusNeed Info Works For Me
        Open/ClosedOpen Closed
    2022-02-18 rik5 StatusNone Need Info
    2022-02-18 rik5 Attached File- Added tst_arrow_sz.m, #52890
    2022-02-18 None Attached File- Added richtungsfeld.m, #52883
        Attached File- Added direction-field-modern-art.png, #52884
        Attached File- Added direction-field-without-arrowheads.png, #52885
        Attached File- Added math.txt, #52886

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