/*---------------------------------------------------------------------------
med3x3.asm  - perform three-by-three median filtering
-----------------------------------------------------------------------------
Description:

A median filter replaces a given pixel with the median value of its
neighbors.  Unlike a 3x3 convolution, which can destroy some spatial
resolution, the median filter is useful for removing isolated lines or pixels
while preserving spatial resolution.  It performs well on images containing
binary noise, but poorly when the noise is Gaussian.  It also does not work
well when the number of noise pixels in the 3x3 window is greater than 4.

This implementation is based on magic described in Andrew Glassner's
"Graphics Gems", cited below in the "References" section.

-----------------------------------------------------------------------------
Program Characteristics:

Calling Registers:
    i0          index to data values
    m0          column offset (usually 1)
    m1          row offset (usually [#pixels/row - 2])
    m2          offset to next 3x3 block (usually -[2*m1 + 1])
        
Altered Registers:
    r0-r6       tmp values

Return Values:
    r0          median value

Computation Time:   56 cycles
-----------------------------------------------------------------------------
Author:     Jim Donahue, Analog Devices DSP Division
References: Glassner, Andrew S. (1990), "Graphics Gems", Academic Press, Inc.
            p.171,711
            Jain, Anil K. (1989) "Fundamentals of Digital Image Processing",
            Prentice Hall, p.246-247
Created:    27-MAR-91
----------------------------------------------------------------------------*/
#define s2(a,b)          comp(a,b); if gt b = pass a, a = b;

#define mnmx3(a,b,c)     s2(b,c); s2(a,c); s2(a,b)

#define mnmx4(a,b,c,d)   s2(a,b); s2(c,d); s2(a,c); s2(b,d)

#define mnmx5(a,b,c,d,e) s2(a,b); s2(c,d); s2(a,c); s2(a,e); \
                         s2(d,e); s2(b,e)

#define mnmx6(a,b,c,d,e,f)  s2(a,d); s2(b,e); s2(c,f); \
                            s2(a,b); s2(a,c); s2(e,f); s2(d,f)

.SEGMENT /pm    pm_code;
.GLOBAL med3x3;
med3x3:     r1=dm(i0,m0);
            r2=dm(i0,m0);
            r3=dm(i0,m1);
            r4=dm(i0,m0);
            r5=dm(i0,m0);
            r6=dm(i0,m1);
strt:
            mnmx6(r1, r2, r3, r4, r5, r6);
mm6:
            r1 = dm(i0,m0);
            mnmx5(r1, r2, r3, r4, r5);
mm5:
            r1 = dm(i0,m0);
            mnmx4(r1, r2, r3, r4);
mm4:
            r1 = dm(i0,m2);
            mnmx3(r1, r2, r3);
mm3:
            rts (db);
                r0 = r2;
                nop;
.ENDSEG;