Insufficient number of outputs

I'm using this code to capture the cap of a bottle. It's working if the cap is on the bottle, but it's not working when i remove the cap because by doing imcrop on the cap there is not a white area in the figure and therefore no level detection. What should I do to fix my error? Below is my code:

a=imread('emptybottle.png');
b=rgb2gray(a);
f1 = imcrop(b,[400 0 200 130]); 
imhist(f1);  
f2=im2bw(f1,0.78); 
f3=~f2; 
imshow(f3);
 f4=bwareaopen(f3,250); 
mask=strel('diamond',5);  
f5=imopen(f4,mask); 
imshow(f5);
g=bwareaopen(f5,8000); % remove the cap
imshow(g); 
f6=regionprops(g) % show the properties of the white area
level = f6.Area  % saves the number of pixels inside the white area
level  

[Walter Roberson]

f6=regionprops(g)

That is not guaranteed to return exactly one region. You should be testing numel to check you got exactly one.

[elie lattouf]

what do you mean ? I don't understand.

[Walter Roberson]

if numel(f6) ~= 1
  error("expected exactly 1 region, got %d", numel(f6))
end

[elie lattouf]

how to fix it in the full code ?

Answer 1

[Walter Roberson]

a=imread('emptybottle.png');
b=rgb2gray(a);
f1 = imcrop(b,[400 0 200 130]); 
imhist(f1);  
f2=im2bw(f1,0.78); 
f3=~f2; 
imshow(f3);
 f4=bwareaopen(f3,250); 
mask=strel('diamond',5);  
f5=imopen(f4,mask); 
imshow(f5);
g=bwareaopen(f5,8000); % remove the cap
imshow(g); 
f6=regionprops(g) % show the properties of the white area
if numel(f6) ~= 1
      error("expected exactly 1 region, got %d", numel(f6));
end
level = f6.Area  % saves the number of pixels inside the white area
level