MATLAB Answers


If I start with a matrix of zeros, how can I easily create a triangle of ones in that matrix?

Asked by Daniel Gray on 19 Jun 2017
Latest activity Commented on by Image Analyst
on 21 Jun 2017
Accepted Answer by KSSV
I want to be able to get a .mat file that will create a filled triangle when I use imagesc. I have been told the easiest way is to start with a matrix full of zeros and then create a triangle of ones inside of that, but I can not figure out how to do that, other than manually which would take a while.


Sign in to comment.

4 Answers

Answer by KSSV
on 19 Jun 2017
 Accepted Answer

N = 100 ;
T1 = triu(ones(N),1) ;
T2 = fliplr(T1) ;
T = [T2 T1] ;


If you want an actual equilateral triangle then Andrei Bobrov's answer will create that. This triangle is not actually equilateral.

Sign in to comment.

Answer by Andrei Bobrov
on 19 Jun 2017

N = 1000;
T = tril(ones(N)) ;
a = round(imresize([flip(T,2),T],[round(N/2*tan(pi/3)) N]));


Sign in to comment.

Answer by Image Analyst
on 19 Jun 2017

If you have the Image Processing Toolbox, it's a simple one liner:
triangleMatrix = poly2mask(x, y, rows, columns);
where x and y are 3 element arrays of the rows and columns where you want the vertices, and rows and columns are the overall size of your output matrix. Be careful not to mix up x and y with row and column - remember (x,y) = (column,row), not (row, column).

  1 Comment

N = 1000;
k = 1/sqrt(3);
c = round(.5*N*[1-k,4/3;1,1/3;1+k,4/3]);
a = poly2mask(c(:,1),c(:,2),N,N);

Sign in to comment.

Answer by Adam
on 19 Jun 2017

It depends where you want the triangle. For example
res = tril( ones(100) );
will produce a triangle in the lower left of the matrix.


Thank you, Is there a way to get an equilateral triangle in the centre?
I'm wondering the same thing. Let me know if you figure it out.
Like I said, in my answer, poly2mask() will do it, subject to quantization of course.

Sign in to comment.