Union() operation adds hole to polyshape

I have attached a file containing a vector of 11 polyshape objects

>> load('polyshape_vector.mat')
>> whos pgon
  Name      Size            Bytes  Class        Attributes

  pgon      1x11             3080  polyshape

When these polyshapes are plotted, they form an agglomeration of triangles which, to the eye at least, has 2 holes

>> plot(pgon)

However, when I plot the union of this polyshape vector, a 3rd hole forms (this is in R2018a). It defies my intuition that taking the union of a collection of polygons could introduce more holes. Can anyone think why this might occur?

   >> plot(union(pgon))

[Walter Roberson]

is it possible that one has the opposite orientation of the other, clockwise vs counterclockwise?

[Catalytic]

Quite strange. But whatever it is, it looks like it got fixed in later releases. I do not see it in 2019a

[Matt J]

@Walter,

I was also wondering about whether ordering of the vertices mattered in some way, but no, all the vertices are in clockwise order for all the polygons.

[Jyotsna Talluri]

Seems to be a bug.I have brought this to the notice of our developers.They will look into it and investigate further

[Matt J]

Thank you!

Answer 1

[Matt J]

https://www.mathworks.com/matlabcentral/answers/518591-union-operation-adds-hole-to-polyshape#comment_834234