ring (annulis) patch?

Dear friends,

How do I make a 2D ring (annulus) patch? Is it possible to make it as a single patch with a single object handle?

See annulus http://en.wikipedia.org/wiki/Ring_(geometric)

I want plot several 2D objects in a figure. The objects behind the ring should be visible trough the hole, and I want to control the properties of the ring, e.g. with respect to transperancy, color and line style.

Sincerely, Peter.

Answer 1

[Matt Fig]

This might serve your purpose. The function creates an annulus object and allows you to set the linestyle and edgecolor while preserving the correct look of the annulus.

function [P] = annulus(r,R,xf,Xf,yf,Yf)
% Creates a annulus patch object and returns the handle.  Input arguments 
% are the inner radius, outer radius, inner x offset, outer x offset, inner
% y offset and outer Y offset.  Changes to the edgecolor and linestyle are
% allowed, and will preserve the correct look of the annulus

t = linspace(0,2*pi,200);
x = xf + r*cos(t);
y = yf + r*sin(t);
X = Xf + R*cos(t);
Y = Yf + R*sin(t);
P = patch([x X],[y Y],[1,.5,.5],'linestyle','non','facealph',.5);
L(1) = line(x,y,'color','k');
L(2) = line(X,Y,'color','k');
axis equal
plistener(P,'edgecolor',@edgec) % listeners for changes to props.
plistener(P,'linestyle',@lnstl)

      function [] = plistener(ax,prp,func)
          % Sets the properties. From proplistener by Yair Altman.
          psetact = 'PropertyPostSet';
          hC = handle(ax);
          hSrc = hC.findprop(prp);
          hl = handle.listener(hC, hSrc, psetact, {func,ax});
          p = findprop(hC, 'Listeners__');
          if isempty(p)
              p = schema.prop(hC, 'Listeners__', 'handle vector');
              set(p,'AccessFlags.Serialize', 'off', ...
                  'AccessFlags.Copy', 'off',...
                  'FactoryValue', [], 'Visible', 'off');
          end
          hC.Listeners__ = hC.Listeners__(ishandle(hC.Listeners__));
          hC.Listeners__ = [hC.Listeners__; hl];
      end
      function [] = edgec(varargin)
          St = get(varargin{3},'edgecolor');
          set(L,'color',St)
      end
      function [] = lnstl(varargin)
          St = get(varargin{3},'linestyle');
          set(varargin{3},'linestyle','none')
          set(L,'linestyle',St)
      end
end

[Walter Roberson]

Yikes!

[Matt Fig]

Yikes indeed! Yet it works, at least on 2006b and 2007b...

[Matt Tearle]

MATLAB: it's a programming environment *and* an obsessive disorder. Order your copy today!

[Walter Roberson]

@Matt Fig: I wonder if linkprop() might have been easier?

@Matt Tearle: Shouldn't that be *copies*, not *copy* ? ;-)

[Peter]

Dear Matt Fig, thanks a lot – the function is brilliant! And it taught me a lot. Sincerely, Peter.

[Matt Fig]

@Walter, LINKPROP will not work as I understand it. This problem calls for the properties to NOT be identical.

Answer 2

[Matt Tearle]

How's this? The patch is a single object, but you have to add the edge lines separately in this approach (otherwise you see the "branch cut" in the annulus).

% Make a plot to add the ring to
x = rand(100,1);
y = rand(100,1);
plot(x,y,'o')

% Make inner and outer boundaries
t = linspace(0,2*pi);
rin = 0.1;
rout = 0.25;
xin = 0.5 + rin*cos(t);
xout = 0.5 + rout*cos(t);
yin = 0.5 + rin*sin(t);
yout = 0.5 + rout*sin(t);

% Make patch
hp = patch([xout,xin],[yout,yin],'g','linestyle','none','facealpha',0.25);
hl1 = line(xin,yin,'color','k');
hl2 = line(xout,yout,'color','k');

[Peter]

Dear Matt Tearle, thank you for your answer - it looks beautiful! This is very close to what I asked for. I guess it is impossible to make pure hollow patches … Thanks, Peter.

[Walter Roberson]

Try putting a NaN between xout and xin, and between yout and yin.

[Peter]

Dear Walter Roberson, thank you for taking your time. The NaN trick is clever! However, if I put in NaNs before and between the x and y terms ([nan,xin,nan,xout] and [nan,yin,nan,yout]), I get a single patch without face color. Do not know why the face disappeared, but I could use this patch instead of the two lines that was proposed by Matt Tearle (two handles is better than three). Sincerely, Peter.

[Walter Roberson]

I suspect this is due to the face color interpolation method that is set, especially with the leading nan (which you do not need, but a trailing nan might not hurt.) I cannot test out my speculations at the moment as I do not have graphic access right now.

[Matt Tearle]

Yes, I thought of trying it with NaN, but that seems to confuse patch -- I think it's unable to work out where the "inside" of the region is. But I feel like a total idiot for not using the same trick for the boundary line.

Anyway, Peter, what's still missing/lacking from this approach? What do you mean by "pure hollow patches"?

[Peter]

Matt Tearle, I was naive thinking that it is possible to make any geometric shape with a singe patch. From yours and others answers I conclude that it is not that simple (though I wish it was). Thank you, Peter.

Answer 3

[Sean de Wolski]

2d or 3d?

For the 2d case just subtract circles. Learn how to make circles here: http://matlab.wikia.com/wiki/FAQ#How_do_I_create_a_circle.3F

You could use the same logic for the 3d "donut" using a formula found on wikipedia.

[Peter]

Thank you for your answer! I want plot several 2D objects in a figure. The objects behind the ring should be visible trough the hole. Peter.