/*
 *  libpipi       Proper image processing implementation library
 *  Copyright (c) 2004-2008 Sam Hocevar <sam@zoy.org>
 *                All Rights Reserved
 *
 *  $Id$
 *
 *  This library is free software. It comes without any warranty, to
 *  the extent permitted by applicable law. You can redistribute it
 *  and/or modify it under the terms of the Do What The Fuck You Want
 *  To Public License, Version 2, as published by Sam Hocevar. See
 *  http://sam.zoy.org/wtfpl/COPYING for more details.
 */

/*
 * floydsteinberg.c: Floyd-Steinberg dithering functions
 */

#include "config.h"
#include "common.h"

#include "pipi.h"
#include "pipi_internals.h"

pipi_image_t *pipi_dither_floydsteinberg(pipi_image_t *img, pipi_scan_t scan)
{
    pipi_image_t *dst;
    pipi_pixels_t *dstp;
    float *dstdata;
    int x, y, w, h;

    w = img->w;
    h = img->h;

    dst = pipi_copy(img);
    dstp = pipi_getpixels(dst, PIPI_PIXELS_Y_F);
    dstdata = (float *)dstp->pixels;

    for(y = 0; y < h; y++)
    {
        int reverse = (y & 1) && (scan == PIPI_SCAN_SERPENTINE);

        for(x = 0; x < w; x++)
        {
            float p, q, e;
            int x2 = reverse ? w - 1 - x : x;
            int s = reverse ? -1 : 1;

            p = dstdata[y * w + x2];
            q = p < 0.5 ? 0. : 1.;
            dstdata[y * w + x2] = q;

            /* FIXME: according to our 2008 paper, [7 4 5 0] is a better
             * error diffusion kernel for serpentine scan than [7 3 5 1]. */
            e = (p - q) / 16;
            if(x < w - 1)
                dstdata[y * w + x2 + s] += e * 7;
            if(y < h - 1)
            {
                if(x > 0)
                    dstdata[(y + 1) * w + x2 - s] += e * 3;
                dstdata[(y + 1) * w + x2] += e * 5;
                if(x < w - 1)
                    dstdata[(y + 1) * w + x2 + s] += e;
            }
        }
    }

    return dst;
}