invcmap.c

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	  gcdp = gdp = rdp, gcrgbp = grgbp = rrgbp, first = 1;
	  g <= max;
	  g++, gdp += gstride, gcdp += gstride, grgbp += gstride, gcrgbp += gstride,
	  gdist += gxx, gcdist += gxx, gxx += txsqr, first = 0 )
    {
	if ( blueloop( first ) )
	{
	    if ( !detect )
	    {
		/* Remember here and associated data! */
		if ( g > here )
		{
		    here = g;
		    rdp = gcdp;
		    rrgbp = gcrgbp;
		    rdist = gcdist;
		    ginc = gxx;
#ifdef MINMAX_TRACK
		    thismin = here;
#endif
		}
		detect = 1;
	    }
	}
	else if ( detect )
	{
#ifdef MINMAX_TRACK
	    thismax = g - 1;
#endif
	    break;
	}
    }
    
    /* Basic loop down. */
    for ( g = here - 1, gxx = ginc - txsqr, gcdist = gdist = rdist - gxx,
	  gcdp = gdp = rdp - gstride, gcrgbp = grgbp = rrgbp - gstride,
	  first = 1;
	  g >= min;
	  g--, gdp -= gstride, gcdp -= gstride, grgbp -= gstride, gcrgbp -= gstride,
	  gxx -= txsqr, gdist -= gxx, gcdist -= gxx, first = 0 )
    {
	if ( blueloop( first ) )
	{
	    if ( !detect )
	    {
		/* Remember here! */
		here = g;
		rdp = gcdp;
		rrgbp = gcrgbp;
		rdist = gcdist;
		ginc = gxx;
#ifdef MINMAX_TRACK
		thismax = here;
#endif
		detect = 1;
	    }
	}
	else if ( detect )
	{
#ifdef MINMAX_TRACK
	    thismin = g + 1;
#endif
	    break;
	}
    }
    
#ifdef MINMAX_TRACK
    /* If we saw something, update the edge trackers.  For now, only
     * tracks edges that are "shrinking" (min increasing, max
     * decreasing.
     */
    if ( detect )
    {
	if ( thismax < prevmax )
	    max = thismax;

	prevmax = thismax;

	if ( thismin > prevmin )
	    min = thismin;

	prevmin = thismin;
    }
#endif

    return detect;
}

/* blueloop -- loop up and down from blue center. */
int32_T
blueloop( int32_T restart )
{
    int32_T detect;
    register uint32_T *dp;
    register int32_T *rgbp;
    register int32_T bdist, bxx;
    register int32_T b, i = cindex;
    register int32_T txsqr = xsqr + xsqr;
    register int32_T lim;
    static int32_T here, min, max;
#ifdef MINMAX_TRACK
    static int32_T prevmin, prevmax;
    int32_T thismin, thismax;
#endif /* MINMAX_TRACK */
    static int32_T binc;

    if ( restart )
    {
	here = bcenter;
	min = 0;
	max = colormax - 1;
	binc = cbinc;
#ifdef MINMAX_TRACK
	prevmin = colormax;
	prevmax = 0;
#endif /* MINMAX_TRACK */
    }

    detect = 0;
#ifdef MINMAX_TRACK
    thismin = min;
    thismax = max;
#endif

    /* Basic loop up. */
    /* First loop just finds first applicable cell. */
    for ( b = here, bdist = gdist, bxx = binc, dp = gdp, rgbp = grgbp, lim = max;
	  b <= lim;
	  b++, dp++, rgbp++,
	  bdist += bxx, bxx += txsqr )
    {
#ifdef INSTRUMENT_IT
	outercount++;
#endif
	if ( *dp > bdist )
	{
	    /* Remember new 'here' and associated data! */
	    if ( b > here )
	    {
		here = b;
		gdp = dp;
		grgbp = rgbp;
		gdist = bdist;
		binc = bxx;
#ifdef MINMAX_TRACK
		thismin = here;
#endif
	    }
	    detect = 1;
#ifdef INSTRUMENT_IT
	    outercount--;
#endif
	    break;
	}
    }
    /* Second loop fills in a run of closer cells. */
    for ( ;
	  b <= lim;
	  b++, dp++, rgbp++,
	  bdist += bxx, bxx += txsqr )
    {
#ifdef INSTRUMENT_IT
	outercount++;
#endif
	if ( *dp > bdist )
	{
#ifdef INSTRUMENT_IT
	    innercount++;
#endif
	    *dp = bdist;
	    *rgbp = i;
	}
	else
	{
#ifdef MINMAX_TRACK
	    thismax = b - 1;
#endif
	    break;
	}
    }
    
    /* Basic loop down. */
    /* Do initializations here, since the 'find' loop might not get
     * executed. 
     */
    lim = min;
    b = here - 1;
    bxx = binc - txsqr;
    bdist = gdist - bxx;
    dp = gdp - 1;
    rgbp = grgbp - 1;
    /* The 'find' loop is executed only if we didn't already find
     * something.
     */
    if ( !detect )
	for ( ;
	      b >= lim;
	      b--, dp--, rgbp--,
	      bxx -= txsqr, bdist -= bxx )
	{
#ifdef INSTRUMENT_IT
	    outercount++;
#endif
	    if ( *dp > bdist )
	    {
		/* Remember here! */
		/* No test for b against here necessary because b <
		 * here by definition.
		 */
		here = b;
		gdp = dp;
		grgbp = rgbp;
		gdist = bdist;
		binc = bxx;
#ifdef MINMAX_TRACK
		thismax = here;
#endif
		detect = 1;
#ifdef INSTRUMENT_IT
		outercount--;
#endif
		break;
	    }
	}
    /* The 'update' loop. */
    for ( ;
	  b >= lim;
	  b--, dp--, rgbp--,
	  bxx -= txsqr, bdist -= bxx )
    {
#ifdef INSTRUMENT_IT
	outercount++;
#endif
	if ( *dp > bdist )
	{
#ifdef INSTRUMENT_IT
	    innercount++;
#endif
	    *dp = bdist;
	    *rgbp = i;
	}
	else
	{
#ifdef MINMAX_TRACK
	    thismin = b + 1;
#endif
	    break;
	}
    }


	/* If we saw something, update the edge trackers. */
#ifdef MINMAX_TRACK
    if ( detect )
    {
	/* Only tracks edges that are "shrinking" (min increasing, max
	 * decreasing.
	 */
	if ( thismax < prevmax )
	    max = thismax;

	if ( thismin > prevmin )
	    min = thismin;
    
	/* Remember the min and max values. */
	prevmax = thismax;
	prevmin = thismin;
    }
#endif /* MINMAX_TRACK */

    return detect;
}

#if 0	/* inv_cmap_1 not used by ditherc.c */

/*****************************************************************
 * TAG( inv_cmap_1 )
 *
 * Compute an inverse colormap efficiently.
 * Inputs:
 * 	colors:		Number of colors in the forward colormap.
 * 	colormap:	The forward colormap.
 * 	bits:		Number of quantization bits.  The inverse
 * 			colormap will have (2^bits)^3 entries.
 * 	dist_buf:	An array of (2^bits)^3 long integers to be
 * 			used as scratch space.
 * Outputs:
 * 	rgbmap:		The output inverse colormap.  The entry
 * 			rgbmap[(r<<(2*bits)) + (g<<bits) + b]
 * 			is the colormap entry that is closest to the
 * 			(quantized) color (r,g,b).
 * Assumptions:
 * 	Quantization is performed by right shift (low order bits are
 * 	truncated).  Thus, the distance to a quantized color is
 * 	actually measured to the color at the center of the cell
 * 	(i.e., to r+.5, g+.5, b+.5, if (r,g,b) is a quantized color).
 * Algorithm:
 * 	Uses a "distance buffer" algorithm:
 * 	The distance from each representative in the forward color map
 * 	to each point in the rgb space is computed.  If it is less
 * 	than the distance currently stored in dist_buf, then the
 * 	corresponding entry in rgbmap is replaced with the current
 * 	representative (and the dist_buf entry is replaced with the
 * 	new distance).
 *
 * 	The distance computation uses an efficient incremental formulation.
 *
 * 	Right now, distances are computed for all entries in the rgb
 * 	space.  Thus, the complexity of the algorithm is O(K N^3),
 * 	where K = colors, and N = 2^bits.
 */
void
inv_cmap_1( colors, colormap, bits, dist_buf, rgbmap )
int colors, bits;
unsigned char *colormap[3], *rgbmap;
unsigned long *dist_buf;
{
    register unsigned long *dp;
    register unsigned char *rgbp;
    register long bdist, bxx;
    register int b, i;
    int nbits = 8 - bits;
    register int colormax = 1 << bits;
    register long xsqr = 1 << (2 * nbits);
    int x = 1 << nbits;
    int rinc, ginc, binc, r, g;
    long rdist, gdist, rxx, gxx;

    for ( i = 0; i < colors; i++ )
    {
	/*
	 * Distance formula is
	 * (red - map[0])^2 + (green - map[1])^2 + (blue - map[2])^2
	 *
	 * Because of quantization, we will measure from the center of
	 * each quantized "cube", so blue distance is
	 * 	(blue + x/2 - map[2])^2,
	 * where x = 2^(8 - bits).
	 * The step size is x, so the blue increment is
	 * 	2*x*blue - 2*x*map[2] + 2*x^2
	 *
	 * Now, b in the code below is actually blue/x, so our
	 * increment will be 2*x*x*b + (2*x^2 - 2*x*map[2]).  For
	 * efficiency, we will maintain this quantity in a separate variable
	 * that will be updated incrementally by adding 2*x^2 each time.
	 */
	rdist = colormap[0][i] - x/2;
	gdist = colormap[1][i] - x/2;
	bdist = colormap[2][i] - x/2;
	rdist = rdist*rdist + gdist*gdist + bdist*bdist;

	rinc = 2 * (xsqr - (colormap[0][i] << nbits));
	ginc = 2 * (xsqr - (colormap[1][i] << nbits));
	binc = 2 * (xsqr - (colormap[2][i] << nbits));
	dp = dist_buf;
	rgbp = rgbmap;
	for ( r = 0, rxx = rinc;
	      r < colormax;
	      rdist += rxx, r++, rxx += xsqr + xsqr )
	    for ( g = 0, gdist = rdist, gxx = ginc;
		  g < colormax;
		  gdist += gxx, g++, gxx += xsqr + xsqr )
		for ( b = 0, bdist = gdist, bxx = binc;
		      b < colormax;
		      bdist += bxx, b++, dp++, rgbp++,
		      bxx += xsqr + xsqr )
		{
#ifdef INSTRUMENT_IT
		    outercount++;
#endif
		    if ( i == 0 || *dp > bdist )
		    {
#ifdef INSTRUMENT_IT
			innercount++;
#endif
			*dp = bdist;
			*rgbp = i;
		    }
		}
    }
#ifdef INSTRUMENT_IT
    printf( "K = %d, N = %d, outer count = %ld, inner count = %ld\n",
	     colors, colormax, outercount, innercount );
#endif
}

#endif	/* inv_cmap_1 not used in ditherc.c */