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Code Issues Releases Activity

avfilter/vf_thumbnail: unroll and use multiple histograms

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This naive hist[p[x]]++ loop suffers badly when there are large regions of
identical values in the image, because of store-to-load forwarding delay.

Splitting up the histogram into four "parallel" histograms and processing
them one at a time speeds things up significantly, about 40% on my end.
This commit is contained in:
Niklas Haas
2025-07-12 13:30:07 +02:00
parent e44a1aaeec
commit bc8d06d541
1 changed files with 57 additions and 14 deletions
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71
libavfilter/vf_thumbnail.c
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@ -27,6 +27,7 @@
* @see http://notbrainsurgery.livejournal.com/29773.html
*/
#include "libavutil/intreadwrite.h"
#include "libavutil/mem.h"
#include "libavutil/opt.h"
#include "libavutil/pixdesc.h"
@ -145,6 +146,59 @@ static AVFrame *get_best_frame(AVFilterContext *ctx)
return picref;
}
static void get_hist8(int *hist, const uint8_t *p, ptrdiff_t stride,
ptrdiff_t width, ptrdiff_t height)
{
int shist[4][256] = {0};
const int width4 = width & ~3;
while (height--) {
for (int x = 0; x < width4; x += 4) {
const uint32_t v = AV_RN32(&p[x]);
shist[0][(uint8_t) (v >> 0)]++;
shist[1][(uint8_t) (v >> 8)]++;
shist[2][(uint8_t) (v >> 16)]++;
shist[3][(uint8_t) (v >> 24)]++;
}
/* handle tail */
for (int x = width4; x < width; x++)
hist[p[x]]++;
p += stride;
}
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 256; j++)
hist[j] += shist[i][j];
}
}
static void get_hist16(int *hist, const uint8_t *p, ptrdiff_t stride,
ptrdiff_t width, ptrdiff_t height, int shift)
{
int shist[4][256] = {0};
const int width4 = width & ~3;
while (height--) {
const uint16_t *p16 = (const uint16_t *) p;
for (int x = 0; x < width4; x += 4) {
const uint64_t v = AV_RN64(&p16[x]);
shist[0][(uint8_t) (v >> (shift + 0))]++;
shist[1][(uint8_t) (v >> (shift + 16))]++;
shist[2][(uint8_t) (v >> (shift + 32))]++;
shist[3][(uint8_t) (v >> (shift + 48))]++;
}
/* handle tail */
for (int x = width4; x < width; x++)
hist[p16[x]]++;
p += stride;
}
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 256; j++)
hist[j] += shist[i][j];
}
}
static int do_slice(AVFilterContext *ctx, void *arg, int jobnr, int nb_jobs)
{
ThumbContext *s = ctx->priv;
@ -204,22 +258,11 @@ static int do_slice(AVFilterContext *ctx, void *arg, int jobnr, int nb_jobs)
const ptrdiff_t linesize = frame->linesize[plane];
const int planewidth = s->planewidth[plane];
int *hhist = hist + 256 * plane;
if (s->bitdepth > 8) {
const uint16_t *p16 = (const uint16_t *) p;
const int shift = s->bitdepth - 8;
for (int j = slice_start; j < slice_end; j++) {
for (int i = 0; i < planewidth; i++)
hhist[(p16[i] >> shift) & 0xFF]++;
p16 += linesize >> 1;
}
get_hist16(hhist, p, linesize, planewidth, slice_end - slice_start,
s->bitdepth - 8);
} else {
for (int j = slice_start; j < slice_end; j++) {
for (int i = 0; i < planewidth; i++)
hhist[p[i]]++;
p += linesize;
}
get_hist8(hhist, p, linesize, planewidth, slice_end - slice_start);
}
}
break;