MATLAB: Question about indexing of logic matrix

Question

Hi:

I read through the blog presented in link:

https://blogs.mathworks.com/graphics/2015/07/22/implicit-surface-intersections/

there is an operation using command:

mask(hel.faces)

Where hel.faces is a m*3 matrix, and mask is a n*1 logic matrix. I could not understand how could this operation happens because it can not reproduce in my side. my test commands are:

A=rand(20,3);
B=rand(20,1);
C=B>0.5;
C(A)

could anyone give me some suggestions?

Thanks!

Yu

Answer 1

Your mistake is on this line:

A=rand(20,3);

You correctly wrote that "hel.faces is a m*3 matrix", but you seem to have missed that its values are all indices, i.e. positive integers, with values from 1 to numel(mask):

>> numel(mask)
ans =
      121368
>> max(hel.faces(:))
ans =
      121368
>> min(hel.faces(:))
ans =
     1
>> find(mod(hel.faces(:),1)) % only integers
ans =
   Empty matrix: 0-by-1

That is why it can be used as an index into another array (e.g. mask): because it it contains indices.

But you generated A with random numbers between 0 and 1, which are not indices (indices must be positive integers). Once you generate A with positive integers, with values from 1 to numel(B), then your code will work:

>> B = rand(20,1);
>> A = randi([1,20],20,3);
>> C = B>0.5;
>> C(A)
ans =
     0     1     0
     1     1     0
     0     0     0
     1     0     0
     0     1     1
     0     1     1
     1     1     0
     1     1     0
     0     1     0
     1     0     0
     1     0     0
     1     0     0
     0     0     0
     1     1     1
     1     0     1
     0     1     0
     0     1     1
     0     0     0
     1     1     1
     1     1     1