MATLAB Central Newsreader - hide vectors in quiver plot completely

hide vectors in quiver plot completely

Fab Braun — Wed, 04 Nov 2009 20:09:02 -0500

hi all!

i'm dealing around with showing the optical flow of an image in a quiver plot.
so far no problem, except I haven't found a solution yet to completely remove zero valued vectors from the quiver plot.

the following i read somewhere:
Vectors(Vectors == 0) = nan;

bud this doesn't work 100%...because there is still a small point left everywhere i want to set these vectors to zero.

any other ideas?

thanks in advance!
fabian

Re: hide vectors in quiver plot completely

Mohd Kharbat — Fri, 06 Nov 2009 00:29:19 -0500

Hello Fabian

One way to do it is not to include the zero vectors in the first place. You can scan through the flow field and exclude those of zero value from being plotted, like this:

function quiverTest(u,v)
% Initialise arrays
xx=[]; yy=[]; uu=[]; vv=[];
% Scan through all elements
for i=1:size(u,1)
    for j=1:size(u,2)
        if sqrt(u(i,j).^2+v(i,j).^2)~=0 % If it's a zero vector, ignore it, otherwise add it to the array
            xx=[xx i];
            yy=[yy j];
            uu=[uu u(i,j)];
            vv=[vv v(i,j)];
        end
    end
end
% Quiver and reverse the Y axis.
quiver(xx,yy,uu,vv);
set(gca,'YDir','reverse');

Hope this helps,
M

"Fab Braun" <f1braun@hsr.ch> wrote in message <hcsn0u$s36$1@fred.mathworks.com>...
> hi all!
>
> i'm dealing around with showing the optical flow of an image in a quiver plot.
> so far no problem, except I haven't found a solution yet to completely remove zero valued vectors from the quiver plot.
>
> the following i read somewhere:
> Vectors(Vectors == 0) = nan;
>
> bud this doesn't work 100%...because there is still a small point left everywhere i want to set these vectors to zero.
>
> any other ideas?
>
> thanks in advance!
> fabian

Re: hide vectors in quiver plot completely

Fab Braun — Fri, 06 Nov 2009 10:25:05 -0500

Hi Mohd,

Great, Thanks a lot!
That was exactly what i was looking for...

i implemented it a bit in a different way than you suggested - no for loops --> not that slow!

% get all the valid flow vectors - might add some additional conditions here
ValidIndex = (abs(FlowVectors) ~= 0);

% plot only valid flow vectors
quiver(GridX(ValidIndex), GridY(ValidIndex), ...
real(FlowVectors(ValidIndex)), -imag(FlowVectors(ValidIndex)));

than you very much!
fabian