dany/libwebp
1
0
Fork 0
mirror of https://github.com/webmproject/libwebp.git synced 2024-11-20 04:18:26 +01:00
Code Issues Actions Packages Projects Releases Wiki Activity

Fix the oscillating prediction problem at low quality

Browse Source 
For some exact resonance the over-quantization was exactly
compensating the under-quantization, leading to resonance
and strange patterns.

-> we special-handle the very flat blocks, hopefully for the
greater good (and not just the bad-resonance case).

For 'fast mode' (-m 3 or less), we just pay special attention
to the border of the image, where the oscillation / instability
usually starts. For the inner part of the image, since we're not
doing rd-opt, it's harder to fix anything.

Overall, on 'regular' images, the change is written the noise,
often leading to overall faster encoding (because of the short-cut).

BUG=webp:432

Change-Id: Ifaa8286499add80fd77daecf8e347abbff7c3a15
This commit is contained in:
Pascal Massimino 2019-07-03 08:40:41 -07:00
parent 312f74d010
commit 9d6988f44d
2 changed files with 35 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

15
src/dsp/quant.h
View File

@ -10,6 +10,8 @@
#ifndef WEBP_DSP_QUANT_H_
#define WEBP_DSP_QUANT_H_
#include <string.h>
#include "src/dsp/dsp.h"
#include "src/webp/types.h"
@ -67,4 +69,17 @@ static WEBP_INLINE int IsFlat(const int16_t* levels, int num_blocks,
#endif // defined(WEBP_USE_NEON) && !defined(WEBP_ANDROID_NEON) &&
// !defined(WEBP_HAVE_NEON_RTCD)
static WEBP_INLINE int IsFlatSource16(const uint8_t* src) {
const uint32_t v = src[0] * 0x01010101u;
int i;
for (i = 0; i < 16; ++i) {
if (memcmp(src + 0, &v, 4) || memcmp(src + 4, &v, 4) ||
memcmp(src + 8, &v, 4) || memcmp(src + 12, &v, 4)) {
return 0;
}
src += BPS;
}
return 1;
}
#endif // WEBP_DSP_QUANT_H_

26
src/enc/quant_enc.c
View File

@ -33,7 +33,7 @@
// number of non-zero coeffs below which we consider the block very flat
// (and apply a penalty to complex predictions)
#define FLATNESS_LIMIT_I16 10 // I16 mode
#define FLATNESS_LIMIT_I16 0 // I16 mode (special case)
#define FLATNESS_LIMIT_I4 3 // I4 mode
#define FLATNESS_LIMIT_UV 2 // UV mode
#define FLATNESS_PENALTY 140 // roughly ~1bit per block
@ -988,6 +988,7 @@ static void PickBestIntra16(VP8EncIterator* const it, VP8ModeScore* rd) {
VP8ModeScore* rd_cur = &rd_tmp;
VP8ModeScore* rd_best = rd;
int mode;
int is_flat = IsFlatSource16(it->yuv_in_ + Y_OFF_ENC);
rd->mode_i16 = -1;
for (mode = 0; mode < NUM_PRED_MODES; ++mode) {
@ -1003,10 +1004,14 @@ static void PickBestIntra16(VP8EncIterator* const it, VP8ModeScore* rd) {
tlambda ? MULT_8B(tlambda, VP8TDisto16x16(src, tmp_dst, kWeightY)) : 0;
rd_cur->H = VP8FixedCostsI16[mode];
rd_cur->R = VP8GetCostLuma16(it, rd_cur);
if (mode > 0 &&
IsFlat(rd_cur->y_ac_levels[0], kNumBlocks, FLATNESS_LIMIT_I16)) {
// penalty to avoid flat area to be mispredicted by complex mode
rd_cur->R += FLATNESS_PENALTY * kNumBlocks;
if (is_flat) {
// refine the first impression (which was in pixel space)
is_flat = IsFlat(rd_cur->y_ac_levels[0], kNumBlocks, FLATNESS_LIMIT_I16);
if (is_flat) {
// Block is very flat. We put emphasis on the distortion being very low!
rd_cur->D *= 2;
rd_cur->SD *= 2;
}
}
// Since we always examine Intra16 first, we can overwrite *rd directly.
@ -1087,7 +1092,8 @@ static int PickBestIntra4(VP8EncIterator* const it, VP8ModeScore* const rd) {
: 0;
rd_tmp.H = mode_costs[mode];
// Add flatness penalty
// Add flatness penalty, to avoid flat area to be mispredicted
// by a complex mode.
if (mode > 0 && IsFlat(tmp_levels, kNumBlocks, FLATNESS_LIMIT_I4)) {
rd_tmp.R = FLATNESS_PENALTY * kNumBlocks;
} else {
@ -1242,11 +1248,19 @@ static void RefineUsingDistortion(VP8EncIterator* const it,
if (mode > 0 && VP8FixedCostsI16[mode] > bit_limit) {
continue;
}
if (score < best_score) {
best_mode = mode;
best_score = score;
}
}
if (it->x_ == 0 || it->y_ == 0) {
// avoid starting a checkerboard resonance from the border. See bug #432.
if (IsFlatSource16(src)) {
best_mode = (it->x_ == 0) ? 0 : 2;
try_both_modes = 0; // stick to i16
}
}
VP8SetIntra16Mode(it, best_mode);
// we'll reconstruct later, if i16 mode actually gets selected
}