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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.

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4 Answers

Answer by KSSV
on 19 Jun 2017
 Accepted Answer

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

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

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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]));
imagesc(a)

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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).

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+1
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);
imagesc(a)

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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.

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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.

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