imagequant/nearest.c

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/*
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** © 2009-2015 by Kornel Lesiński.
** © 1989, 1991 by Jef Poskanzer.
** © 1997, 2000, 2002 by Greg Roelofs; based on an idea by Stefan Schneider.
**
** See COPYRIGHT file for license.
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*/
#include "libimagequant.h"
#include "pam.h"
#include "nearest.h"
#include "mempool.h"
#include <stdlib.h>
typedef struct vp_sort_tmp {
float distance_squared;
unsigned int idx;
} vp_sort_tmp;
typedef struct vp_search_tmp {
float distance;
unsigned int idx;
int exclude;
} vp_search_tmp;
typedef struct vp_node {
struct vp_node *near, *far;
f_pixel vantage_point;
float radius;
unsigned int idx;
} vp_node;
struct nearest_map {
vp_node *root;
const colormap_item *palette;
float nearest_other_color_dist[256];
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mempoolptr mempool;
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};
static void vp_search_node(const vp_node *node, const f_pixel *const needle, vp_search_tmp *const best_candidate);
static int vp_compare_distance(const void *ap, const void *bp) {
float a = ((const vp_sort_tmp*)ap)->distance_squared;
float b = ((const vp_sort_tmp*)bp)->distance_squared;
return a > b ? 1 : -1;
}
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static void vp_sort_indexes_by_distance(const f_pixel vantage_point, vp_sort_tmp indexes[], int num_indexes, const colormap_item items[]) {
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for(int i=0; i < num_indexes; i++) {
indexes[i].distance_squared = colordifference(vantage_point, items[indexes[i].idx].acolor);
}
qsort(indexes, num_indexes, sizeof(indexes[0]), vp_compare_distance);
}
/*
* Usually it should pick farthest point, but picking most popular point seems to make search quicker anyway
*/
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static int vp_find_best_vantage_point_index(vp_sort_tmp indexes[], int num_indexes, const colormap_item items[]) {
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int best = 0;
float best_popularity = items[indexes[0].idx].popularity;
for(int i = 1; i < num_indexes; i++) {
if (items[indexes[i].idx].popularity > best_popularity) {
best_popularity = items[indexes[i].idx].popularity;
best = i;
}
}
return best;
}
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static vp_node *vp_create_node(mempoolptr *m, vp_sort_tmp indexes[], int num_indexes, const colormap_item items[]) {
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if (num_indexes <= 0) {
return NULL;
}
vp_node *node = mempool_alloc(m, sizeof(node[0]), 0);
if (num_indexes == 1) {
*node = (vp_node){
.vantage_point = items[indexes[0].idx].acolor,
.idx = indexes[0].idx,
.radius = MAX_DIFF,
};
return node;
}
const int ref = vp_find_best_vantage_point_index(indexes, num_indexes, items);
const int ref_idx = indexes[ref].idx;
// Removes the `ref_idx` item from remaining items, because it's included in the current node
num_indexes -= 1;
indexes[ref] = indexes[num_indexes];
vp_sort_indexes_by_distance(items[ref_idx].acolor, indexes, num_indexes, items);
// Remaining items are split by the median distance
const int half_idx = num_indexes/2;
*node = (vp_node){
.vantage_point = items[ref_idx].acolor,
.idx = ref_idx,
.radius = sqrtf(indexes[half_idx].distance_squared),
};
node->near = vp_create_node(m, indexes, half_idx, items);
node->far = vp_create_node(m, &indexes[half_idx], num_indexes - half_idx, items);
return node;
}
LIQ_PRIVATE struct nearest_map *nearest_init(const colormap *map) {
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mempoolptr m = NULL;
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struct nearest_map *handle = mempool_create(&m, sizeof(handle[0]), sizeof(handle[0]) + sizeof(vp_node)*map->colors+16, map->malloc, map->free);
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LIQ_ARRAY(vp_sort_tmp, indexes, map->colors);
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for(unsigned int i=0; i < map->colors; i++) {
indexes[i].idx = i;
}
vp_node *root = vp_create_node(&m, indexes, map->colors, map->palette);
*handle = (struct nearest_map){
.root = root,
.palette = map->palette,
.mempool = m,
};
for(unsigned int i=0; i < map->colors; i++) {
vp_search_tmp best = {
.distance = MAX_DIFF,
.exclude = i,
};
vp_search_node(root, &map->palette[i].acolor, &best);
handle->nearest_other_color_dist[i] = best.distance * best.distance / 4.0; // half of squared distance
}
return handle;
}
static void vp_search_node(const vp_node *node, const f_pixel *const needle, vp_search_tmp *const best_candidate) {
do {
const float distance = sqrtf(colordifference(node->vantage_point, *needle));
if (distance < best_candidate->distance && best_candidate->exclude != node->idx) {
best_candidate->distance = distance;
best_candidate->idx = node->idx;
}
// Recurse towards most likely candidate first to narrow best candidate's distance as soon as possible
if (distance < node->radius) {
if (node->near) {
vp_search_node(node->near, needle, best_candidate);
}
// The best node (final answer) may be just ouside the radius, but not farther than
// the best distance we know so far. The vp_search_node above should have narrowed
// best_candidate->distance, so this path is rarely taken.
if (node->far && distance >= node->radius - best_candidate->distance) {
node = node->far; // Fast tail recursion
} else {
break;
}
} else {
if (node->far) {
vp_search_node(node->far, needle, best_candidate);
}
if (node->near && distance <= node->radius + best_candidate->distance) {
node = node->near; // Fast tail recursion
} else {
break;
}
}
} while(true);
}
LIQ_PRIVATE unsigned int nearest_search(const struct nearest_map *handle, const f_pixel *px, const int likely_colormap_index, float *diff) {
const float guess_diff = colordifference(handle->palette[likely_colormap_index].acolor, *px);
if (guess_diff < handle->nearest_other_color_dist[likely_colormap_index]) {
if (diff) *diff = guess_diff;
return likely_colormap_index;
}
vp_search_tmp best_candidate = {
.distance = sqrtf(guess_diff),
.idx = likely_colormap_index,
.exclude = -1,
};
vp_search_node(handle->root, px, &best_candidate);
if (diff) {
*diff = best_candidate.distance * best_candidate.distance;
}
return best_candidate.idx;
}
LIQ_PRIVATE void nearest_free(struct nearest_map *centroids)
{
mempool_destroy(centroids->mempool);
}