MATLAB: What is the meaning of BW = ~ BW

code

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?

Best Answer

~ 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.
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