How can I split this function into smaller functions without sacrificing its functionality?

Question

I got the code for MJPEG decoding from here and I am trying to split the code for IDCT into smaller functions.

The IDCT function in the original code is as follows:

Answer 1

The function IDCT() declares the array Y[] which transfers data across all for-loops. In your refactored code every function declares its own Y[] array. The same error you did with the Yc[] array. Make the arrays global and see if the code runs.

Edit 2017_08-28

Give Yc[] an extra dimension:

void IDCTforY(int32_t *input, uint8_t *output) 
{
  int32_t Y[64];
  int32_t k, l;
  int32_t Yc[8][8];

  for (l = 0; l < 8; l++) 
  {
      for (k = 0; k < 8; k++)
          Yc[l][k] = Y(k, l);
      idct_1d(Yc[l]);
   }

   //Running the loop for de-scaling separately....

   for (l = 0; l < 8; l++) 
   {
       for (k = 0; k < 8; k++) 
       {   
           int32_t r = 128 + DESCALE(Yc[l][k], S_BITS + 3);
           r = r > 0 ? (r < 255 ? r : 255) : 0;
           X(k, l) = r;
       }
   }
}

Edit 2017-08-29

I cannot explain the optical effect, but you broke the data flow. The original code was like this:

for (l = 0; l < 8; l++) 
{
    int32_t Yc[8];

    Fill(Yc);

    idct_1d(Yc);

    Descale_and_WriteOut(Yc);
}

You made of it:

int32_t Yc[8];
for (l = 0; l < 8; l++) 
{
    Fill(Yc);
    idct_1d(Yc);
}
for (l = 0; l < 8; l++) 
{
  Descale_and_WriteOut(Yc);
}

You see, that only the result of the last iteration of the input-and-process-loop is passed to the output-loop. I gave every l-iteration own memory in Yc[][].