Asked by Daniel Gray
on 19 Jun 2017

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.

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)

Daniel Gray
on 19 Jun 2017

You are amazing! Thank you

Adam
on 19 Jun 2017

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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).

Andrei Bobrov
on 19 Jun 2017

+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.

Daniel Gray
on 19 Jun 2017

Thank you, Is there a way to get an equilateral triangle in the centre?

Sophia Andaloro
on 21 Jun 2017

I'm wondering the same thing. Let me know if you figure it out.

Image Analyst
on 21 Jun 2017

Like I said, in my answer, poly2mask() will do it, subject to quantization of course.

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