code snips for http://www.fpgarelated.com/showarticle/578.php, see link for description.

fpga578_snip1.c

pix_t median(pix_t window[N])
{
    pix_t t[N], z[N];
    int ii, k, stage;
  
    // copy locally
    for(ii=0; ii<N; ii++) z[ii] = window[ii];
  
    for(stage=1; stage<=N; stage++){
        k = (stage%2==1) ? 0 : 1;
        for(ii=k; ii<N-1; ii++){
          t[ii] = MIN(z[ii], z[ii+1]);
          t[ii+1] = MAX(z[ii], z[ii+1]);
          z[ii] = t[ii];
          z[ii+1] = t[ii+1];
        }
      }

    return z[N/2];
}

fpga578_snip2.py

def median(x):
    N = len(x)

    def compare_stage(z, stage, N):
        t, k = copy(z), 0 if (stage%2) else 1
        for ii in range(k, N-1, 2):
            t[ii] = min(z[ii], z[ii+1])
            t[ii+1] = max(z[ii], z[ii+1])
        return t

    z = x
    for stage in range(N):
        z = compare_stage(z, stage, N)

    return z[N//2], z

fpga578_snip3.py

def compare(clock, reset, x, z, stage=0):
    N, K = len(z), 0 if stage%2 else 1
    B = 0 if K == 1 else N-1

    @always_seq(clock.posedge, reset=reset)
    def beh():
        z[B].next = x[B]  # pass-thru
        for ii in range(K, N-1, 2):
            z[ii].next = x[ii] if x[ii] < x[ii+1] else x[ii+1]
            z[ii+1].next = x[ii] if x[ii] > x[ii+1] else x[ii+1]

    return beh

fpga578_snip4.py

def median_distributed(clock, reset, wi, wo, med):
    N, st = len(wi), wi[0]
    z = [wi] + [[Signal(st.val) for _ in range(N)]
                for stage in range(N-1)] + [wo]

    cmp_inst = [compare(clock, reset, z[ii], z[ii+1], ii)
                for ii in range(N)]

    mid = N//2

    @always_comb
    def beh_assign():
        med.next = wo[mid]

    return cmp_inst, beh_assign