Sum of RGB components

I'm trying to sum the components of RGB of a point. Like it follows in the code. For example, if I have RGB = [230 100 150],

I want to get for RGB(1) + RGB(2) + RGB(3) the value of 230 + 100 + 150 = 480. The problem is that it returns 255. I know that 255 is the maximum value in the RGB system, but how could I fix the code so I can get the right sum?

b = squeeze(a(c, d, :));       % Value in RGB of a point of a image. 'C' and 'd' are the  
                               % coordinates of the point
if ((b(1) + b(2) + b(3)) > 725)
    K = 2.00;

elseif ((b(1) > b(2)) && (674.5 < (b(1) + b(2) + b(3))) && ((b(1) + b(2) + b(3)) <= 725))
    K = 2.10;

Answer 1

[Stephen23]

Currently the numeric class is uint8, which only supports maximum value of 2^8-1 = 255. You need to convert the numeric class to single, double or uint16 (or some other integer).

What you are currently doing:

>> X = uint8([230 100 150]);
>> X(1)+X(2)+X(3)
ans = 255

Option one use sum, which converts internally to double:

>> sum(X)
ans =  480

Option two convert explicitly to uint16 or double:

>> Y = uint16(X);
>> Y(1)+Y(2)+Y(3)
ans = 480

[Pedro Minsk]

It's worked! I used the option one.

Thank you so much.

Answer 2

[Image Analyst]

I get the idea that you are going to do this on all pixels in the image and then threshold. There are vectorized ways, like

% Sum R+G+B for all pixels in the image:
sumImage = sum(double(rgbImage), 3);
% Get a mask where the sum is more than 725.
brightPixelsMask = sumImage > 725
% and so on.

[Pedro Minsk]

In this case it would be just for 1 pixel. It's the pixel that represents a city in a map.

But I'll keep this tip in mind. I didn't know about this command. I'm sure it will be helpful in the future!

Thank you very much!

[Stephen23]

Note that sum converts to double, so there is no need to call double:

sumImage = sum(rgbImage,3);

[Image Analyst]

You're right. I wonder why conv() or conv2() don't do that.