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

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 1

[KSSV]

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

[Daniel Gray]

You are amazing! Thank you

[Adam]

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

Answer 2

[Andrei Bobrov]

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

Answer 3

[Image Analyst]

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]

+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)

Answer 4

[Adam]

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]

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

[Sophia Andaloro]

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

[Image Analyst]

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