How to calculate gradient energy of an image by sobel mask?

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C++

Hi all.

I have an image by Sobel mask, and I want to calculate 'gradient energy'.

I was trying to using this code:

int w = 474;
int h = 477;
int sum=0;
BYTE *ptr; //byte array of image data.

//...

for (int j=0; j<h; j++)
    for (int i=0; i<w; i++)
        sum += ptr[w*j+i];

But this code is so slow. (if image size is big)

So I'm trying to use SSE Operators.

__m128i energy;
__m128i current_1; // image data

//...
for (int j=0; j<h; j++)
{
    for (int i=0; i<w; i+=8)
    {
     //...
        energy = _mm_add_epi8(energy,current_1);
     //...
    }
}

But it happend overflow ..

How can I avoid overflow and calculate gradient energy?

Thank you :)

What I have tried:

I tried to use code as above. But it happned overflow in __m128i.