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avfilter/palettegen: define the best axis to cut using the squared error 

This is following the results from personal research¹.

¹: https://github.com/ubitux/research/tree/main/color-quantization#results
This commit is contained in:
Clément Bœsch 2022-12-27 18:25:18 +01:00
parent 21f7814238
commit 86bced7c65
3 changed files with 27 additions and 19 deletions
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42
libavfilter/vf_palettegen.c
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@ -147,31 +147,39 @@ static av_always_inline int diff(const uint32_t a, const uint32_t b)
static void compute_box_stats(PaletteGenContext *s, struct range_box *box)
{
int rr, gr, br;
int avg[3];
int64_t er2[3] = {0};
/* compute the box weight (sum all the weights of the colors in the
* range) and its boundings */
uint8_t min[3] = {0xff, 0xff, 0xff};
uint8_t max[3] = {0x00, 0x00, 0x00};
/* Compute average color */
uint64_t sr = 0, sg = 0, sb = 0;
box->weight = 0;
for (int i = box->start; i < box->start + box->len; i++) {
const struct color_ref *ref = s->refs[i];
const uint32_t rgb = ref->color;
const uint8_t r = rgb >> 16 & 0xff, g = rgb >> 8 & 0xff, b = rgb & 0xff;
min[0] = FFMIN(r, min[0]), max[0] = FFMAX(r, max[0]);
min[1] = FFMIN(g, min[1]), max[1] = FFMAX(g, max[1]);
min[2] = FFMIN(b, min[2]), max[2] = FFMAX(b, max[2]);
sr += (ref->color >> 16 & 0xff) * ref->count;
sg += (ref->color >> 8 & 0xff) * ref->count;
sb += (ref->color & 0xff) * ref->count;
box->weight += ref->count;
}
avg[0] = sr / box->weight;
avg[1] = sg / box->weight;
avg[2] = sb / box->weight;
/* define the axis to sort by according to the widest range of colors */
rr = max[0] - min[0];
gr = max[1] - min[1];
br = max[2] - min[2];
/* Compute squared error of each color channel */
for (int i = box->start; i < box->start + box->len; i++) {
const struct color_ref *ref = s->refs[i];
const int64_t dr = (int)(ref->color >> 16 & 0xff) - avg[0];
const int64_t dg = (int)(ref->color >> 8 & 0xff) - avg[1];
const int64_t db = (int)(ref->color & 0xff) - avg[2];
er2[0] += dr * dr * ref->count;
er2[1] += dg * dg * ref->count;
er2[2] += db * db * ref->count;
}
/* Define the best axis candidate for cutting the box */
box->major_axis = 1; // pick green by default (the color the eye is the most sensitive to)
if (br >= rr && br >= gr) box->major_axis = 2;
if (rr >= gr && rr >= br) box->major_axis = 0;
if (gr >= rr && gr >= br) box->major_axis = 1; // prefer green again
if (er2[2] >= er2[0] && er2[2] >= er2[1]) box->major_axis = 2;
if (er2[0] >= er2[1] && er2[0] >= er2[2]) box->major_axis = 0;
if (er2[1] >= er2[0] && er2[1] >= er2[2]) box->major_axis = 1; // prefer green again
}
/**

2
tests/ref/fate/filter-palettegen-1
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@ -3,4 +3,4 @@
#codec_id 0: rawvideo
#dimensions 0: 16x16
#sar 0: 1/1
0, 0, 0, 1, 1024, 0x3395ef5a
0, 0, 0, 1, 1024, 0x394ee723

2
tests/ref/fate/filter-palettegen-2
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@ -3,4 +3,4 @@
#codec_id 0: rawvideo
#dimensions 0: 16x16
#sar 0: 1/1
0, 0, 0, 1, 1024, 0x23e072c8
0, 0, 0, 1, 1024, 0xc54d773d