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C++ API

Function File: [axes l moments] = principalaxes (shape)

Calculates the principal axes of a shape.

Returns a matrix axes where each row corresponds to one of the principal axes of the shape. l is the second moment of area around the correspoding principal axis. axes is order from lower to higher l.

shape can be defined by a polygon or by a piece-wise smooth shape.

See also: inertiamoment masscenter

Demonstration 1

The following code

 t = linspace(0,2*pi,64).';
 shape = [cos(t)-0.3*cos(3*t) sin(t)](1:end-1,:);
 shapeR = shape*rotv([0 0 1],pi/4)(1:2,1:2);
 [PAr l] = principalaxes(shapeR);
 [PA l] = principalaxes(shape);

 figure (1)
 clf
 plot(shape(:,1),shape(:,2),'-k');
 line([0 PA(1,1)],[0 PA(1,2)],'color','r');
 line([0 PA(2,1)],[0 PA(2,2)],'color','b');

 hold on

 plot(shapeR(:,1)+3,shapeR(:,2),'-k');
 line([3 PAr(1,1)+3],[0 PAr(1,2)],'color','r');
 line([3 PAr(2,1)+3],[0 PAr(2,2)],'color','b');

 axis equal
 axis square

Produces the following figure

Figure 1