Question

What is the sort algorithm with fewest number of operations? I need to implement it in HLSL as part of a pixel shader effect v2.0 for WPF, so it needs to have a really small number of operations, considering Pixel Shader’s limitations. I need to sort 9 values, specifically the current pixel and its neighbors.

Answer 1

You either want to use Insertion sort or Radix sort. Here are some C++ implementations:

Radix Sort

void radix (int byte, long N, long *source, long *dest)
{
  long count[256];
  long index[256];
  memset (count, 0, sizeof (count));
  for ( int i=0; i<N; i++ )
    count[((source[i])>>(byte*8))&0xff]++;

  index[0]=0;
  for ( i=1; i<256; i++ )
    index[i]=index[i-1]+count[i-1];
  for ( i=0; i<N; i++ )
    dest[index[((source[i])>>(byte*8))&0xff]++] = source[i];
}

You need to call radix() four times, as it only works on one column.

Insertion Sort

void insertSort(int a[], int length)
{    
    int i, j, value;
    for(i = 1; i < length; i++)
    {
        value = a[i];
        for (j = i - 1; j >= 0 && a[j] > value; j--)
            a[j + 1] = a[j];
        a[j + 1] = value;
    }
}