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

### Daniel Gray (view profile)

on 19 Jun 2017
Latest activity Commented on by Image Analyst

on 21 Jun 2017

### KSSV (view profile)

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.

### KSSV (view profile)

on 19 Jun 2017

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

Daniel Gray

### Daniel Gray (view profile)

on 19 Jun 2017
You are amazing! Thank you

on 19 Jun 2017
If you want an actual equilateral triangle then Andrei Bobrov's answer will create that. This triangle is not actually equilateral.

### Andrei Bobrov (view profile)

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)

### Image Analyst (view profile)

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

### Andrei Bobrov (view profile)

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]);
imagesc(a)

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

### Daniel Gray (view profile)

on 19 Jun 2017
Thank you, Is there a way to get an equilateral triangle in the centre?
Sophia Andaloro

### Sophia Andaloro (view profile)

on 21 Jun 2017
I'm wondering the same thing. Let me know if you figure it out.
Image Analyst

### Image Analyst (view profile)

on 21 Jun 2017
Like I said, in my answer, poly2mask() will do it, subject to quantization of course.