I have edited the pixel value, how do i save it?

edit

I am able to edit and set the pixel value i want but i can't seems to save it.

When i cout before idct the value is corect but after idct the value is set to 0.

for (int i = 0; i < height-16; i += 16) 
{ 
    for (int j = 0; j < width-16; j+= 16) 
    {

            Mat block = dctImage(Rect(j, i, 8, 8)); //Draw rectangle box of 8x8
            vector<Mat> planes; 
            split(block, planes);                   //Split image

            vector<Mat> outplanes(planes.size());

            for (size_t k = 0; k < planes.size(); k++)
            {
                outplanes[k]=planes[k];
            }

            planes[0].convertTo(planes[0], CV_32FC1);   //Convert to float value
            dct(planes[0], outplanes[0]);               //Do DCT

            for (int x=2; x<4; x++)
            {
                for (int y=2; y<4; y++)
                {
                    if (inputTmp==0)
                    {
                        outplanes[0].at<float>(x,y) = avg + 3;
                        cout << outplanes[0].at<float>(x,y) << endl;
                    }

                    else
                    {
                        outplanes[0].at<float>(x,y) = avg - 3;
                        cout << outplanes[0].at<float>(x,y) << endl;
                    }
                }
            }

            for ( int i=0; i<8; i++)
            {
                for ( int j=0; j<8; j++)
                {
                    cout << "the dct value at position (" << i << "," << j << ") is :" << outplanes[0].at<float>(i,j) << endl; 
                }
            }

            //Do inverse DCT (Disable this function will make 8x8 pix visible
            idct(outplanes[0], outplanes[0]);           


            //Debug purpose
            dct(planes[0], outplanes[0]);               //Do DCT

            //Show pixel value after modifying
            for ( int i=0; i<8; i++)
            {
                for ( int j=0; j<8; j++)
                {
                    cout << "the dct value at position (" << i << "," << j << ") is :" << outplanes[0].at<float>(i,j) << endl; 
                }
            }

            //Convert to 1 channel
            outplanes[0].convertTo(outplanes[0], CV_8UC1);

            //Merge image
            merge(outplanes, block); 

        } 
    }

Did i do anything wrongly. Please advise on this.