how do I calculate the area of the black pixels?

2 次查看（过去 30 天）
SvenvdB
SvenvdB 2021-6-25
评论： SvenvdB 2021-6-25
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

请先登录，再回答此问题。

采纳的回答

Walter Roberson
Walter Roberson 2021-6-25
Ran in:
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
  3 个评论
显示 1更早的评论
Walter Roberson
Walter Roberson 2021-6-25
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
SvenvdB 2021-6-25
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.

请先登录，再进行评论。

更多回答（0 个）

类别

在 Help Center 和 File Exchange 中查找有关 Get Started with MATLAB 的更多信息

标签

产品


版本

R2021a

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by