# use the 'patch' function with different results (using different matrices)

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Alberto Acri il 26 Set 2023
Commentato: Walter Roberson il 26 Set 2023
Hi! I have two matrices: 'V' and 'trace'.
Using patches with V generates a completely black surface on the figure and I'm fine with it.
When I use the 'trace' matrix the result is different. How come? Is there a way to display 'trace' in the image in the same way as 'V'?
axes1 = axes('Parent',figure);
patch(V(:,1), V(:,2),V(:,3),'k');
view(axes1,[45.7625707544407 8.27114462456761]);
axis equal
%%
axes2 = axes('Parent',figure);
patch(trace(:,1), trace(:,2),trace(:,3),'r');
view(axes2,[31.7625655339418 23.9174311926605]);
axis equal
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Alberto Acri il 26 Set 2023
The matrices are different. I expected that using the 'trace' matrix I would get a black geometry display similar to the one I got with the V matrix!
Walter Roberson il 26 Set 2023
Why would you expect that?
Remember that for patch(), points are considered to be ordered
If you sort the original points by angle, and plot the sort order:
axes3 = axes('Parent', figure);
tc = mean(trace,1);
ang = atan2(trace(:,2) - tc(2), trace(:,1) - tc(1));
[~, order1] = sort(ang);
[~, order2] = sort(order1);
plot(order2)
adjacent points on input bounce back and forth in angle order -- and not even alternately.
figure();
plot(order2(1:50))

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### Risposta accettata

Matt J il 26 Set 2023
Modificato: Matt J il 26 Set 2023
If you are trying to recover the boundary from a surface triangulation, then,
[~,P]=freeBoundary(delaunayTriangulation(trace(:,1:2)));
Warning: Duplicate data points have been detected and removed.
The Triangulation indices are defined with respect to the unique set of points in delaunayTriangulation.
[~,loc]=ismember(P,trace(:,1:2),'rows');
V=num2cell([P,trace(loc,3)],1);
patch(V{:},'k');
view([31.7625655339418 23.9174311926605]);
axis equal
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### Più risposte (1)

Walter Roberson il 26 Set 2023
In your first patch() call you ask for 'k' -- black -- for the faces. The edge colors default to black as well.
In the second patch() call you ask for 'r' -- red -- for the faces. The edge colors default to black.
The V matrix has coordinates for a simple circle. The trace matrix has much more complex coordinates, going back and forth.
If you were to sort the coordinates then you could get a circle as well. But remember that the coordinates for patch() are ordered so those represent completely different graphing tasks.
axes1 = axes('Parent',figure);
patch(axes1, V(:,1), V(:,2), V(:,3), 'facecolor', 'r', 'facealpha', 0.5, 'edgecolor', 'k');
view(axes1,[45.7625707544407 8.27114462456761]);
axis(axes1, 'equal');
%%
axes2 = axes('Parent',figure);
patch(axes2, trace(:,1), trace(:,2), trace(:,3), 'facecolor', 'r', 'facealpha', 0.5, 'edgecolor', 'k');
view(axes2,[31.7625655339418 23.9174311926605]);
axis(axes2, 'equal');
axes3 = axes('Parent', figure);
tc = mean(trace,1);
ang = atan2(trace(:,2) - tc(2), trace(:,1) - tc(1));
[~, order] = sort(ang);
patch(axes3, trace(order,1), trace(order,2), trace(order,3), 'facecolor', 'r', 'facealpha', 0.5, 'edgecolor', 'k');
view(axes3,[31.7625655339418 23.9174311926605]);
axis(axes3, 'equal')
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Walter Roberson il 26 Set 2023
Note that if you know you have scattered coordinates, then see also boundary instead of the sorting method I used.

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