Adding elements of a matrix based on an if condition

In this code, the value of rblush at the end of the loop shows 255 but it should be a really large number.

could somebody tell me how to rectify this mistake?

red(x,y) is a matrix of all the red pixels of an image. I want to get the sum of all the elements of the matrix greater 150. Thanks!

for x=1:rows
        for y=1:columns
            rd_pixel= red(x,y);
            if(rd_pixel>=150)
                n = n+1;
                rtot = sum(rd_pixel);
            end
        end
    end
    rmean= rtot/n;

Answer 1

[Guillaume]

There is no rblush in your code.

Note that sum(scalar_value) is simply scalar_value. The code you've written does not work at all, since each time you encounter a pixel above the treshold you simply copy its value in rtot instead of adding it to rtot.

I strongly suspect you're working with uint8 values. Note that the range of uint8 is 0-255. If you add two uint8 numbers and their theoretical sum is greater than 255, matlab will give you 255. Converting to double would avoid this problem, but even better getting rid of the loop and using simple logical indexing with mean would work just as well.

rmean = mean(red(red >= 150));  %mean will convert the pixels to double.

[Aishwarya Jayachandran]

would rmean = mean(red(red >= 150)); take the sum of all the red matrix values i need based on the if condition?

initially i used sum(double(red(:))) but i was told it takes the sum of all the values in the matrix if the condition is met

Thanks!

[Guillaume]

red(red >= 150) is a vector containing all the elements of red which are greater than or equal to 150. If you take the mean of that, you get the mean of all the elements >= 150. It's what you tried to do.

sum(red(:))  %double(xxx) not needed

will return the sum of all of red

sum(red(red >= 150))

would return the sum of red whose value are >= 150. You could then divide by nnz(red >= 150) to get the mean, but it's simpler to call mean directly.

Note:

>> x = uint8([200 100])
x =
  1×2 uint8 row vector
   200   100

>> x(1) + x(2)  %sum is greater than 255, but since it's uint8 returns 255
ans =
  uint8
   255
   
>> sum(x)  %sum automatically convert to double, so works properly
ans =
   300

So avoid summing uint8 in a loop. Convert to double beforehand. sum and mean do it automatically.

[Aishwarya Jayachandran]

Thank you so much!

i would like to apply the same concept in this code

for x=1:rows
        for y=1:columns
            red_pixel= red(x,y);
            if(red_pixel>=200)
                if(red_pixel> rmean)
                blush= blush+1;
                rblush =sum(rblush,red_pixel);
                end
            end 
        end
    end

here are two conditions are here. How do i implement this?

[Aishwarya Jayachandran]

Ive modified the code as such

for x=1:rows
        for y=1:columns
            
            if(red(x,y)>rmean)
                rblush= sum(red(red >= 200));
                blush= blush+1;
                
               
            end 
        end
    end

Is this correct? Is there anyway to make this take less time?

[Guillaume]

No, you need to learn logical indexing. You shouldn't have any loop.

mask = red >= 200 & red > rmean;  %which pixels to take into account
rblush = sum(red(mask));  %use mask to compute the mask
blush = nnz(mask);  %number of true pixels in the mask

is all that is needed.

Note that

for x = ...
    result = something 
end

will always overwrite result on each step of the loop. When the loop finishes, you're left with just the result from the last iteration, so clearly your loop was never going to work.