Issues on DCT transform: results are incorrect

Question

I am implementing DCT transform using this formula:

But the results are incorrect. For some 8 by 8 matrix,

 0  0  0  0  0  0  0  0 
 210  210  210  210  210  210  210  210 
 255  255  255  255  255  255  255  255 
 210  210  210  210  210  210  210  210 
 0  0  0  0  0  0  0  0 
 210  210  210  210  210  210  210  210 
 255  255  255  255  255  255  255  255 
 210  210  210  210  210  210  210  210

The results I got after passing the data to dct transform function are:

1350.000000    0.000000    -0.000000    0.000000    0.000000    0.000000    -0.000000    -0.000000  
  -250.897627    -0.000000    0.000000    -0.000000    -0.000000    -0.000000    0.000000    0.000000  
  -0.000000    0.000000    0.000000    -0.000000    0.000000    -0.000000    0.000000    -0.000000  
  -461.931139    -0.000000    0.000000    -0.000000    -0.000000    -0.000000    0.000000    0.000000  
  -510.000000    0.000000    0.000000    -0.000000    -0.000000    -0.000000    0.000000    0.000000  
  156.770200    0.000000    -0.000000    0.000000    0.000000    0.000000    -0.000000    -0.000000  
  -0.000000    -0.000000    -0.000000    -0.000000    -0.000000    0.000000    0.000000    -0.000000  
  -260.946562    -0.000000    0.000000    -0.000000    -0.000000    -0.000000    0.000000    0.000000  

(Only 1st column has non-zero values)

The problem is that I was told that the correct results should be only non-zero values at upper left corner of the matrix. And I am not sure where might be wrong in my code. Can anyone help me? Thanks.

Here is my DCT code:

static double C(int val){
   if(val == 0)
    return 1.0 / sqrt(2.0);
   else
    return 1.0;
}

void dctTransform(int matrix[8][8], double dctMatrix[8][8]){
    int u, v, x, y;
    double temp; 

    for(u=0; u<8; u++)
      for(v=0; v<8; v++){

         temp = 0;
         dctMatrix[u][v] = 0;

         for(x=0;x<8;x++){
           for(y=0;y<8;y++){
             temp += matrix[y][x]*cos(((2*x+1)*u*M_PI) / 16)*cos(((2*y+1)*v*M_PI) / 16);
            }
         }
         dctMatrix[u][v] = C(u) * C(v) * 0.25 * temp;
      }  
}

Answer 1

Aren't you supposed to first change the input values from 0..255 to -128..127? Haven't you also switched x and y in matrix[y][x]?

If I subtract 128 from the input values to center them around zero, and then switch x and y, your code at least gives me the correct values according to the example in Wikipedia.