Learn how easy it is to sync an existing GitHub or Google Code repo to a SourceForge project!

Diff of /inst/inertia_moment_poly2d.m [000000] .. [b345c0] Maximize Restore

Switch to side-by-side view

--- a
+++ b/inst/inertia_moment_poly2d.m
@@ -0,0 +1,63 @@
+%% Copyright (c) 2011 Juan Pablo Carbajal <carbajal@ifi.uzh.ch>
+%%
+%%    This program is free software: you can redistribute it and/or modify
+%%    it under the terms of the GNU General Public License as published by
+%%    the Free Software Foundation, either version 3 of the License, or
+%%    any later version.
+%%
+%%    This program is distributed in the hope that it will be useful,
+%%    but WITHOUT ANY WARRANTY; without even the implied warranty of
+%%    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+%%    GNU General Public License for more details.
+%%
+%%    You should have received a copy of the GNU General Public License
+%%    along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+%% -*- texinfo -*-
+%% @deftypefn {Function File} @var{I} = inertia_moment_poly2d (@var{p}, @var{m}, @var{offset})
+%% Calculates the moment of inertia of a 2D star-shaped polygon.
+%%
+%% The polygon is described in @var{p}, where each row is a different vertex.
+%% @var{m} is the total mass of the polygon, assumed uniformly distributed.
+%% The optinal argument @var{offset} is an origin translation vector. All vertex
+%% are transformed to the reference frame with origin at @var{offset} respect to
+%% the center of mass.
+%%
+%% This expression assumes that the polygon is star-shaped. The position of the
+%% vertices is assumed to be given from the center of mass of the polygon.
+%% To change a general polygon to this description you can use:
+%% @code{P = P - repmat(center_mass_poly2d(P),size(P,1))}.
+%%
+%% @seealso{}
+%% @end deftypefn
+
+function I = inertia_moment_poly2d(poly,mass,offset=[0 0])
+  numVerts = size(poly,1);
+  R = repmat(offset,numVerts,1);
+
+  V = poly - R;
+  Vnext = poly([2:numVerts 1],:) - R;
+
+  %% Area of the parallelograms
+  A = sqrt(sumsq(cross([Vnext zeros(numVerts,1)],[V zeros(numVerts,1)],2),2));
+  %% Distance between points
+  B = dot(V,V,2) + dot(V,Vnext,2) + dot(Vnext,Vnext,2);
+
+  C = sum(A.*B);
+
+  I = mass*C/(6*sum(A));
+end
+
+%!demo
+%!
+%! % The same triangle respect to one of its vertices described with two polygons
+%!  P = [0 0; 1 0; 0 1];
+%!  P2=[0 0; 0.1 0; 0.2 0; 0.25 0; 1 0; 0 1];
+%!
+%! % Now described from the center of mass of the triangle
+%! Pc = P - repmat(center_mass_poly2d(P), 3, 1);
+%! inertia_moment_poly2d(Pc,1)
+%!
+%! Pc = P2 -repmat(center_mass_poly2d(P2), size(P2,1), 1);
+%! inertia_moment_poly2d(Pc,1)
+