how do I calculate the area of the black pixels?

I have a pixture and i want to calculate the surface of the black area in the middle. I know the size of the small square's (10x10mm). and the distance between them is 18cm.

I have a script that outputs the amount of black pixels and white pixels but i would like to to output the surface area in milimeters.

Is this possible to do? I dont have much experience with matlab so does anyone have any tips on how to do it if it is possible

black pixels: 873454

white pixels: 2640296

Answer 1

[Walter Roberson]

filename = 'https://www.mathworks.com/matlabcentral/answers/uploaded_files/665000/image.jpeg';
img = imread(filename);
BW = imbinarize(rgb2gray(img));
imshow(BW)

format long g
props = regionprops(~BW, 'Area');
sA = sort([props.Area])

sA = 1×5

930 997 1051 1087 92901

mA = mean(sA(1:end-1))

mA = 
                   1016.25

relA = sA(end)/mA

relA = 
          91.4154981549815

small_area = 10*10;
absA = relA * small_area

absA = 
          9141.54981549815

The small black squares are not all the same area, so I took the mean() area as being representative.

The large square is 91.4 times larger than the (mean) smaller square

The smaller square is 10x10 = 100 mm^2 so the larger one is 9141.5 mm^2

[SvenvdB]

Thank you so much.

Can you explain te code so i can understand it and learn from it.

[Walter Roberson]

You read the file. It is a JPEG file, and like 99.9% of .jpeg files, it is RGB even if it does not look like RGB.

So the first thing you need to do is convert the RGB to binary. Then the code displays the binary image.

After that, regionprops() is called to find the blobs and extract statistics about the blobs -- in particular the Area statistic. But regionprops() works on blobs that are non-zero, and here we need it to work on the blobs that are 0 (black), so ask regionprops() to work on ~BW with the ~ part converting black to white and white to black.

Once the areas of all of the blobs are extracted, sort the areas. The big blob will have the highest area (by far). Average the ones other than the last to find out how big they are (on average)

You know the small ones are supposed to be 100 mm^2. You can calculate the area of the big one by taking the number of pixels in it divide by the (average) number of pixels in the smaller ones, multiply by the known mm^2 of the small ones, to get the mm^2 of the large one.

Whether you should use the mean() or min() or max() of the size of the smaller blobs is open to discussion. You say that they are all 100 mm^2 but they all occupy different number of pixels, with about 10% difference between the size of the largest and smallest.

[SvenvdB]

On the normal jpg they are al the same size. but the picture above i took with a camera and thats why the are different. thanks for the explaination, helped alot.