Speeding up the computation of SSD of 3x3 patches

edit

Hi,

As part of a bigger application, I need to compute the following code:

    ax2 += (int)(25 + 0.5);
    ay2 += (int)(25 + 0.5);

    bx2 += (int)(25 + 0.5);
    by2 += (int)(25 + 0.5);

    cx2 += (int)(25 + 0.5);
    cy2 += (int)(25 + 0.5);

        for (int ix = -1; ix <= 1; ix++){
            for (int iy = -1; iy <= 1; iy++){
                suma += (grayImage.at<uchar>(ay2 + iy, ax2 + ix) - grayImage.at<uchar>(by2 + iy, bx2 + ix))* grayImage.at<uchar>(ay2 + iy, ax2 + ix) - grayImage.at<uchar>(by2 + iy, bx2 + ix));
    }
    }

It basically computes the sum of squared difference of two 3X3 patches.

It runs extremely slow. Is there any way of speeding it up?

EDIT:

I changed to the following version:

for (int ix = -1; ix <= 1; ix++){
        for (int iy = -1; iy <= 1; iy++){
            double difa = grayImage.at<uchar>(ay2 + iy, ax2 + ix) - grayImage.at<uchar>(by2 + iy, bx2 + ix);
            suma += (difa)*(difa);
        }
}

And it runs faster, but is there any way to improve it further?

EDIT: Thanks for the answer, I'm not using the following code:

//int iy = -1;
     Mi_a = grayImage.ptr<uchar>(ay2 - 1);
     Mi_b = grayImage.ptr<uchar>(by2 - 1);
     Mi_c = grayImage.ptr<uchar>(cy2 - 1);

    difa = Mi_a[ax2 - 1] - Mi_b[bx2 - 1];
    suma += (difa)*(difa);
    difc = Mi_c[cx2 - 1] - Mi_b[bx2 - 1];
    sumc += (difc)*(difc);
    difa = Mi_a[ax2 + 0] - Mi_b[bx2 + 0];
    suma += (difa)*(difa);
    difc = Mi_c[cx2 + 0] - Mi_b[bx2 + 0];
    sumc += (difc)*(difc);
    difa = Mi_a[ax2 + 1] - Mi_b[bx2 + 1];
    suma += (difa)*(difa);
    difc = Mi_c[cx2 + 1] - Mi_b[bx2 + 1];
    sumc += (difc)*(difc);



    //int iy=0;
    Mi_a = grayImage.ptr<uchar>(ay2 + 0);
    Mi_b = grayImage.ptr<uchar>(by2 + 0);
    Mi_c = grayImage.ptr<uchar>(cy2 + 0);

    difa = Mi_a[ax2 - 1] - Mi_b[bx2 - 1];
    suma += (difa)*(difa);
    difc = Mi_c[cx2 - 1] - Mi_b[bx2 - 1];
    sumc += (difc)*(difc);
    difa = Mi_a[ax2 + 0] - Mi_b[bx2 + 0];
    suma += (difa)*(difa);
    difc = Mi_c[cx2 + 0] - Mi_b[bx2 + 0];
    sumc += (difc)*(difc);
    difa = Mi_a[ax2 + 1] - Mi_b[bx2 + 1];
    suma += (difa)*(difa);
    difc = Mi_c[cx2 + 1] - Mi_b[bx2 + 1];
    sumc += (difc)*(difc);


    //int iy=1
    Mi_a = grayImage.ptr<uchar>(ay2 + 1);
    Mi_b = grayImage.ptr<uchar>(by2 + 1);
    Mi_c = grayImage.ptr<uchar>(cy2 + 1);

    difa = Mi_a[ax2 - 1] - Mi_b[bx2 - 1];
    suma += (difa)*(difa);
    difc = Mi_c[cx2 - 1] - Mi_b[bx2 - 1];
    sumc += (difc)*(difc);
    difa = Mi_a[ax2 + 0] - Mi_b[bx2 + 0];
    suma += (difa)*(difa);
    difc = Mi_c[cx2 + 0] - Mi_b[bx2 + 0];
    sumc += (difc)*(difc);
    difa = Mi_a[ax2 + 1] - Mi_b[bx2 + 1];
    suma += (difa)*(difa);
    difc = Mi_c[cx2 + 1] - Mi_b[bx2 + 1];
    sumc += (difc)*(difc);

Is there any way to speed it up even further?

Thanks,

Gil.

P.S. it's part of an algorithm that I intend to contribute to OpenCV, once it will be published.