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

Question

Alberto Acri 2023-9-26

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https://ww2.mathworks.cn/matlabcentral/answers/2026017-use-the-patch-function-with-different-results-using-different-matrices

评论： Walter Roberson 2023-9-26

采纳的回答： Matt J

在 MATLAB Online 中打开

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'?

load V

axes1 = axes('Parent',figure);

patch(V(:,1), V(:,2),V(:,3),'k');

view(axes1,[45.7625707544407 8.27114462456761]);

axis equal

%%

load trace

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 2023-9-26

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 2023-9-26

在 MATLAB Online 中打开

trace.mat

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:

load trace

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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Answer 1

Matt J 2023-9-26

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https://ww2.mathworks.cn/matlabcentral/answers/2026017-use-the-patch-function-with-different-results-using-different-matrices#answer_1318912

编辑：Matt J 2023-9-26

在 MATLAB Online 中打开

trace.mat

If you are trying to recover the boundary from a surface triangulation, then,

load trace

[~,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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Answer 2

Walter Roberson 2023-9-26

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https://ww2.mathworks.cn/matlabcentral/answers/2026017-use-the-patch-function-with-different-results-using-different-matrices#answer_1318897

在 MATLAB Online 中打开

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.

load V

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');

%%

load trace

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 2023-9-26

Note that if you know you have scattered coordinates, then see also boundary instead of the sorting method I used.

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