gstreamer/gst/dvbsubenc/libimagequant/nearest.c

#include "libimagequant.h"
#include "pam.h"
#include "nearest.h"
#include "mempool.h"
#include <stdlib.h>

struct sorttmp
{
  float radius;
  unsigned int index;
};

struct head
{
  // colors less than radius away from vantage_point color will have best match in candidates
  f_pixel vantage_point;
  float radius;
  unsigned int num_candidates;
  f_pixel *candidates_color;
  unsigned short *candidates_index;
};

struct nearest_map
{
  const colormap *map;
  float nearest_other_color_dist[256];
  mempool mempool;
  struct head heads[];
};

static float
distance_from_nearest_other_color (const colormap * map, const unsigned int i)
{
  float second_best = MAX_DIFF;
  for (unsigned int j = 0; j < map->colors; j++) {
    float diff;

    if (i == j)
      continue;

    diff = colordifference (map->palette[i].acolor, map->palette[j].acolor);
    if (diff <= second_best) {
      second_best = diff;
    }
  }
  return second_best;
}

static int
compareradius (const void *ap, const void *bp)
{
  float a = ((const struct sorttmp *) ap)->radius;
  float b = ((const struct sorttmp *) bp)->radius;
  return a > b ? 1 : (a < b ? -1 : 0);
}

static struct head
build_head (f_pixel px, const colormap * map, unsigned int num_candidates,
    mempool * m, float error_margin, bool skip_index[], unsigned int *skipped)
{
  struct sorttmp *colors = g_alloca (sizeof (struct sorttmp) * map->colors);
  unsigned int colorsused, i;
  struct head h;

  colorsused = 0;

  for (i = 0; i < map->colors; i++) {
    if (skip_index[i])
      continue;                 // colors in skip_index have been eliminated already in previous heads
    colors[colorsused].index = i;
    colors[colorsused].radius = colordifference (px, map->palette[i].acolor);
    colorsused++;
  }

  qsort (colors, colorsused, sizeof (colors[0]), compareradius);
  assert (colorsused < 2 || colors[0].radius <= colors[1].radius);      // closest first

  num_candidates = MIN (colorsused, num_candidates);

  h.candidates_color =
      mempool_alloc (m, num_candidates * sizeof (h.candidates_color[0]), 0);
  h.candidates_index =
      mempool_alloc (m, num_candidates * sizeof (h.candidates_index[0]), 0);
  h.vantage_point = px;
  h.num_candidates = num_candidates;

  for (i = 0; i < num_candidates; i++) {
    h.candidates_color[i] = map->palette[colors[i].index].acolor;
    h.candidates_index[i] = colors[i].index;
  }
  // if all colors within this radius are included in candidates, then there cannot be any other better match
  // farther away from the vantage point than half of the radius. Due to alpha channel must assume pessimistic radius.
  h.radius = min_colordifference (px, h.candidates_color[num_candidates - 1]) / 4.0f;   // /4 = half of radius, but radius is squared

  for (i = 0; i < num_candidates; i++) {
    // divide again as that's matching certain subset within radius-limited subset
    // - 1/256 is a tolerance for miscalculation (seems like colordifference isn't exact)
    if (colors[i].radius < h.radius / 4.f - error_margin) {
      skip_index[colors[i].index] = true;
      (*skipped)++;
    }
  }
  return h;
}

static colormap *
get_subset_palette (const colormap * map)
{
  unsigned int subset_size, i;
  colormap *subset_palette;

  if (map->subset_palette) {
    return map->subset_palette;
  }

  subset_size = (map->colors + 3) / 4;
  subset_palette = pam_colormap (subset_size, map->malloc, map->free);

  for (i = 0; i < subset_size; i++) {
    subset_palette->palette[i] = map->palette[i];
  }

  return subset_palette;
}

LIQ_PRIVATE struct nearest_map *
nearest_init (const colormap * map, bool fast)
{
  colormap *subset_palette = get_subset_palette (map);
  const unsigned int num_vantage_points =
      map->colors > 16 ? MIN (map->colors / (fast ? 4 : 3),
      subset_palette->colors) : 0;
  const unsigned long heads_size = sizeof (struct head) * (num_vantage_points + 1);     // +1 is fallback head

  const unsigned long mempool_size =
      (sizeof (f_pixel) +
      sizeof (unsigned int)) * subset_palette->colors * map->colors / 5 +
      (1 << 14);
  mempool m = NULL;
  struct nearest_map *centroids = mempool_create (&m,
      sizeof (*centroids) + heads_size /* heads array is appended to it */ ,
      mempool_size, map->malloc, map->free);
  unsigned int skipped;
  const float error_margin = fast ? 0 : 8.f / 256.f / 256.f;
  unsigned int h, i, j;
  bool *skip_index;

  centroids->mempool = m;

  for (i = 0; i < map->colors; i++) {
    const float dist = distance_from_nearest_other_color (map, i);
    centroids->nearest_other_color_dist[i] = dist / 4.f;        // half of squared distance
  }

  centroids->map = map;

  skipped = 0;
  assert (map->colors > 0);

  skip_index = g_alloca (sizeof (bool) * map->colors);

  for (j = 0; j < map->colors; j++)
    skip_index[j] = false;

  // floats and colordifference calculations are not perfect
  for (h = 0; h < num_vantage_points; h++) {
    unsigned int num_candiadtes =
        1 + (map->colors - skipped) / ((1 + num_vantage_points - h) / 2);

    centroids->heads[h] =
        build_head (subset_palette->palette[h].acolor, map, num_candiadtes,
        &centroids->mempool, error_margin, skip_index, &skipped);
    if (centroids->heads[h].num_candidates == 0) {
      break;
    }
  }

  // assumption that there is no better color within radius of vantage point color
  // holds true only for colors within convex hull formed by palette colors.
  // The fallback must contain all colors, since there are too many edge cases to cover.
  if (!fast)
    for (j = 0; j < map->colors; j++) {
      skip_index[j] = false;
    }

  centroids->heads[h] = build_head ((f_pixel) {
      0, 0, 0, 0}
      , map, map->colors, &centroids->mempool, error_margin,
      skip_index, &skipped);
  centroids->heads[h].radius = MAX_DIFF;

  // get_subset_palette could have created a copy
  if (subset_palette != map->subset_palette) {
    pam_freecolormap (subset_palette);
  }

  return centroids;
}

LIQ_PRIVATE unsigned int
nearest_search (const struct nearest_map *centroids, const f_pixel px,
    int likely_colormap_index, const float min_opaque_val, float *diff)
{
  const bool iebug = px.a > min_opaque_val;
  const struct head *const heads = centroids->heads;
  float guess_diff;
  unsigned int i;

  assert (likely_colormap_index < centroids->map->colors);

  guess_diff =
      colordifference (centroids->map->palette[likely_colormap_index].acolor,
      px);
  if (guess_diff < centroids->nearest_other_color_dist[likely_colormap_index]) {
    if (diff)
      *diff = guess_diff;
    return likely_colormap_index;
  }

  for (i = 0; /* last head will always be selected */ ; i++) {
    float vantage_point_dist = colordifference (px, heads[i].vantage_point);

    if (vantage_point_dist <= heads[i].radius) {
      unsigned int ind = 0;
      float dist;

      assert (heads[i].num_candidates);

      dist = colordifference (px, heads[i].candidates_color[0]);

      /* penalty for making holes in IE */
      if (iebug && heads[i].candidates_color[0].a < 1) {
        dist += 1.f / 1024.f;
      }

      for (unsigned int j = 1; j < heads[i].num_candidates; j++) {
        float newdist = colordifference (px, heads[i].candidates_color[j]);

        /* penalty for making holes in IE */
        if (iebug && heads[i].candidates_color[j].a < 1) {
          newdist += 1.f / 1024.f;
        }

        if (newdist < dist) {
          dist = newdist;
          ind = j;
        }
      }
      if (diff)
        *diff = dist;
      return heads[i].candidates_index[ind];
    }
  }
}

LIQ_PRIVATE void
nearest_free (struct nearest_map *centroids)
{
  mempool_destroy (centroids->mempool);
}