what is the meaning of BW = ~ BW?

function [shape] = Classify(ImageFile)

if ndims(imread(ImageFile))>2

RGB = imread(ImageFile);

GRAY = rgb2gray(RGB);

else
    GRAY=imread(ImageFile);
    %RGB=GRAY;

end

threshold = graythresh(GRAY);
BW = im2bw(GRAY, threshold);

%Invert the Binary Image
BW = ~ BW;
[~,L] = bwboundaries(BW, 'noholes');

STATS = regionprops(L,'all'); 

for i=1:length(STATS)

if norm([STATS(i).Extrema(2,1), STATS(i).Extrema(2,2)]-[STATS(i).Extrema(1,1), STATS(i).Extrema(1,2)])<5 %TR,TL

      %can be triangle hexagon pentagon
      if norm([STATS(i).Extrema(4,1), STATS(i).Extrema(4,2)]-[STATS(i).Extrema(3,1), STATS(i).Extrema(3,2)])<5 %RT,RB
      %can be pentagon triangle
          if  norm([STATS(i).Extrema(4,1), STATS(i).Extrema(4,2)]-[STATS(i).Extrema(5,1), STATS(i).Extrema(5,2)])<5 %RT,RB,BR = Triangle
              shape(i,1)=1; % Triangle
          else
              shape(i,1)=3; % Pentagon
          end
      else
           shape(i,1) = 4; % hexagon
      end
  elseif (STATS(i).MajorAxisLength/STATS(i).MinorAxisLength<1.1 && STATS(i).Eccentricity <0.5 || (STATS(i).Solidity/STATS(i).Extent)==1)&(STATS(i).Area<0.1*numel(GRAY))
      shape(i,1)=2; % Rectangle

else
    shape(i,1)=0;
end

end

if ~isempty(find(shape==2))
    disp('Rectangle found');
else
    disp('No rectangle in the image')
end

return

what is the meaning of

BW = ~ BW;

this line?

[Shaila parvin]

what is the meaning of this line?

BW = im2bw(GRAY, threshold);

Answer 1

[Walter Roberson]

~ is logical negation in this context. If you selected one area into BW, then ~BW is everything else that was not that area. bwthresh selects areas with grey scale above a certain level; ~BW would then be everywhere with grey scale below that level. Useful if you have black letters on a light background for example.

Answer 2

[John Doe]

It gives you back the inverted binary matrix BW. That means, all 1's turns to 0's, and vice versa.

In plain text:

BW = not BW.

Suppose the binary image is represented as:

BW =

     1     1     1     1
     1     0     1     0
     0     0     0     1
     1     1     1     0

Then BW = ~ BW gives:

BW =

     0     0     0     0
     0     1     0     1
     1     1     1     0
     0     0     0     1

Hope this answers your question =)

- Robert

[Shaila parvin]

what is the meaning of this line?

BW = im2bw(GRAY, threshold);