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Provide one entry point for backward references.

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The new method VP8LGetBackwardReferences hides internal
heuristics used for choosing RLE or LZ77 based refs.
- Tuned VP8LHashChainFindCopy for better compression at higher Q.
- Refactored code.
- Removed the unused method VP8LVerifyBackwardReferences.

Change-Id: Ibb7bb072bab5a49a001577a20d88226f52e6c663
This commit is contained in:
Vikas Arora 2012-05-09 12:11:55 +05:30
parent c4ccab6463
commit e7167a2b95
3 changed files with 158 additions and 235 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

270
src/enc/backward_references.c
View File

@ -33,7 +33,7 @@ static const uint8_t plane_to_code_lut[128] = {
static const int kMinLength = 2;
int VP8LDistanceToPlaneCode(int xsize, int dist) {
static int DistanceToPlaneCode(int xsize, int dist) {
const int yoffset = dist / xsize;
const int xoffset = dist - yoffset * xsize;
if (xoffset <= 8 && yoffset < 8) {
@ -62,14 +62,11 @@ static const int kWindowSize = (1 << 20) - 120; // A window with 1M pixels
// special codes for short
// distances.
static WEBP_INLINE uint64_t GetHash64(uint64_t num) {
num *= kHashMultiplier;
num >>= 64 - HASH_BITS;
return num;
}
static WEBP_INLINE uint64_t GetPixPair(const uint32_t* const argb) {
return ((uint64_t)(argb[1]) << 32) | argb[0];
static WEBP_INLINE uint64_t GetPixPairHash64(const uint32_t* const argb) {
uint64_t key = ((uint64_t)(argb[1]) << 32) | argb[0];
key *= kHashMultiplier;
key >>= 64 - HASH_BITS;
return key;
}
typedef struct {
@ -104,36 +101,37 @@ static void VP8LHashChainClear(VP8LHashChain* const p) {
static void VP8LHashChainInsert(VP8LHashChain* const p,
const uint32_t* const argb, int32_t pos) {
// Insertion of two pixels at a time.
const uint64_t key = GetPixPair(argb);
const uint64_t hash_code = GetHash64(key);
const uint64_t hash_code = GetPixPairHash64(argb);
p->chain_[pos] = p->hash_to_first_index_[hash_code];
p->hash_to_first_index_[hash_code] = pos;
}
static int VP8LHashChainFindCopy(
static int HashChainFindCopy(
const VP8LHashChain* const p, int quality, int index, int xsize,
const uint32_t* const argb, int maxlen, int* const distance_ptr,
int* const length_ptr) {
const uint64_t next_two_pixels = GetPixPair(&argb[index]);
const uint64_t hash_code = GetHash64(next_two_pixels);
const uint64_t hash_code = GetPixPairHash64(&argb[index]);
int prev_length = 0;
int64_t best_val = 0;
int give_up = 10 + (quality >> 1);
const int iter_min_mult = (quality < 50) ? 2 : (quality <= 75) ? 4 : 8;
const int iter_min = -quality * iter_min_mult;
int iter_cnt = 10 + (quality >> 1);
const int min_pos = (index > kWindowSize) ? index - kWindowSize : 0;
int32_t pos;
int64_t val;
int best_length = 0;
int best_distance = 0;
assert(xsize > 0);
for (pos = p->hash_to_first_index_[hash_code];
pos >= min_pos;
pos = p->chain_[pos]) {
int curr_length;
if (give_up < 0) {
if (give_up < -quality * 2 || best_val >= 0xff0000) {
if (iter_cnt < 0) {
if (iter_cnt < iter_min || best_val >= 0xff0000) {
break;
}
}
--give_up;
--iter_cnt;
if (best_length != 0 &&
argb[pos + best_length - 1] != argb[index + best_length - 1]) {
continue;
@ -187,7 +185,7 @@ static WEBP_INLINE void PushBackCopy(VP8LBackwardRefs* const refs, int length) {
}
}
void VP8LBackwardReferencesRle(
static void BackwardReferencesRle(
int xsize, int ysize, const uint32_t* const argb,
VP8LBackwardRefs* const refs) {
const int pix_count = xsize * ysize;
@ -206,20 +204,23 @@ void VP8LBackwardReferencesRle(
PushBackCopy(refs, match_len);
}
// Returns 1 when successful.
int VP8LBackwardReferencesHashChain(
int xsize, int ysize, int use_color_cache, const uint32_t* const argb,
static int BackwardReferencesHashChain(
int xsize, int ysize, const uint32_t* const argb,
int cache_bits, int quality, VP8LBackwardRefs* const refs) {
int i;
int ok = 0;
int cc_init = 0;
const int use_color_cache = (cache_bits > 0);
const int pix_count = xsize * ysize;
VP8LHashChain* hash_chain = (VP8LHashChain*)malloc(sizeof(*hash_chain));
VP8LColorCache hashers;
if (hash_chain == NULL ||
!VP8LColorCacheInit(&hashers, cache_bits) ||
if (hash_chain == NULL) return 0;
if (!(cc_init = VP8LColorCacheInit(&hashers, cache_bits)) ||
!VP8LHashChainInit(hash_chain, pix_count)) {
goto Error;
}
refs->size = 0;
for (i = 0; i < pix_count; ) {
// Alternative#1: Code the pixels starting at 'i' using backward reference.
@ -230,8 +231,8 @@ int VP8LBackwardReferencesHashChain(
if (maxlen > kMaxLength) {
maxlen = kMaxLength;
}
VP8LHashChainFindCopy(hash_chain, quality, i, xsize, argb, maxlen,
&offset, &len);
HashChainFindCopy(hash_chain, quality, i, xsize, argb, maxlen,
&offset, &len);
}
if (len >= kMinLength) {
// Alternative#2: Insert the pixel at 'i' as literal, and code the
@ -245,8 +246,8 @@ int VP8LBackwardReferencesHashChain(
if (maxlen > kMaxLength) {
maxlen = kMaxLength;
}
VP8LHashChainFindCopy(hash_chain, quality,
i + 1, xsize, argb, maxlen, &offset2, &len2);
HashChainFindCopy(hash_chain, quality,
i + 1, xsize, argb, maxlen, &offset2, &len2);
if (len2 > len + 1) {
// Alternative#2 is a better match. So push pixel at 'i' as literal.
if (use_color_cache && VP8LColorCacheContains(&hashers, argb[i])) {
@ -292,9 +293,9 @@ int VP8LBackwardReferencesHashChain(
}
ok = 1;
Error:
if (cc_init) VP8LColorCacheClear(&hashers);
VP8LHashChainClear(hash_chain);
free(hash_chain);
VP8LColorCacheClear(&hashers);
return ok;
}
@ -309,9 +310,13 @@ typedef struct {
int cache_bits_;
} CostModel;
static int BackwardReferencesTraceBackwards(
int xsize, int ysize, int recursive_cost_model,
const uint32_t* const argb, int cache_bits, VP8LBackwardRefs* const refs);
static int CostModelBuild(CostModel* const p, int xsize, int ysize,
int recursion_level, int use_color_cache,
const uint32_t* const argb, int cache_bits) {
int recursion_level, const uint32_t* const argb,
int cache_bits) {
int ok = 0;
VP8LHistogram histo;
VP8LBackwardRefs refs;
@ -320,15 +325,14 @@ static int CostModelBuild(CostModel* const p, int xsize, int ysize,
p->cache_bits_ = cache_bits;
if (recursion_level > 0) {
if (!VP8LBackwardReferencesTraceBackwards(xsize, ysize, recursion_level - 1,
use_color_cache, argb, cache_bits,
&refs)) {
if (!BackwardReferencesTraceBackwards(xsize, ysize, recursion_level - 1,
argb, cache_bits, &refs)) {
goto Error;
}
} else {
const int quality = 100;
if (!VP8LBackwardReferencesHashChain(xsize, ysize, use_color_cache, argb,
cache_bits, quality, &refs)) {
if (!BackwardReferencesHashChain(xsize, ysize, argb, cache_bits, quality,
&refs)) {
goto Error;
}
}
@ -378,26 +382,28 @@ static WEBP_INLINE double GetDistanceCost(const CostModel* const p,
}
static int BackwardReferencesHashChainDistanceOnly(
int xsize, int ysize, int recursive_cost_model, int use_color_cache,
const uint32_t* const argb, int cache_bits, uint32_t* const dist_array) {
int xsize, int ysize, int recursive_cost_model, const uint32_t* const argb,
int cache_bits, uint32_t* const dist_array) {
int i;
int ok = 0;
int cc_init = 0;
const int quality = 100;
const int pix_count = xsize * ysize;
const int use_color_cache = (cache_bits > 0);
double* cost = (double*)malloc(pix_count * sizeof(*cost));
int i;
CostModel* cost_model = (CostModel*)malloc(sizeof(*cost_model));
VP8LColorCache hashers;
VP8LHashChain* hash_chain = (VP8LHashChain*)malloc(sizeof(*hash_chain));
int ok = 0;
VP8LColorCache hashers;
if (cost == NULL ||
cost_model == NULL ||
hash_chain == NULL ||
!VP8LColorCacheInit(&hashers, cache_bits)) {
!(cc_init = VP8LColorCacheInit(&hashers, cache_bits)) ||
!VP8LHashChainInit(hash_chain, pix_count)) {
goto Error;
}
VP8LHashChainInit(hash_chain, pix_count);
CostModelBuild(cost_model, xsize, ysize, recursive_cost_model,
use_color_cache, argb, cache_bits);
CostModelBuild(cost_model, xsize, ysize, recursive_cost_model, argb,
cache_bits);
for (i = 0; i < pix_count; ++i) {
cost[i] = 1e100;
}
@ -418,11 +424,11 @@ static int BackwardReferencesHashChainDistanceOnly(
if (maxlen > pix_count - i) {
maxlen = pix_count - i;
}
VP8LHashChainFindCopy(hash_chain, quality, i, xsize, argb, maxlen,
&offset, &len);
HashChainFindCopy(hash_chain, quality, i, xsize, argb, maxlen,
&offset, &len);
}
if (len >= kMinLength) {
const int code = VP8LDistanceToPlaneCode(xsize, offset);
const int code = DistanceToPlaneCode(xsize, offset);
const double distance_cost =
prev_cost + GetDistanceCost(cost_model, code);
int k;
@ -483,11 +489,11 @@ static int BackwardReferencesHashChainDistanceOnly(
// through cheaper means already.
ok = 1;
Error:
if (hash_chain) VP8LHashChainClear(hash_chain);
if (cc_init) VP8LColorCacheClear(&hashers);
VP8LHashChainClear(hash_chain);
free(hash_chain);
free(cost_model);
free(cost);
VP8LColorCacheClear(&hashers);
return ok;
}
@ -516,31 +522,35 @@ static void TraceBackwards(
}
static int BackwardReferencesHashChainFollowChosenPath(
int xsize, int ysize, int use_color_cache, const uint32_t* const argb,
int cache_bits, const uint32_t* const chosen_path, int chosen_path_size,
int xsize, int ysize, const uint32_t* const argb, int cache_bits,
const uint32_t* const chosen_path, int chosen_path_size,
VP8LBackwardRefs* const refs) {
const int quality = 100;
const int pix_count = xsize * ysize;
const int use_color_cache = (cache_bits > 0);
int size = 0;
int i = 0;
int k;
int ix;
int ok = 0;
VP8LColorCache hashers;
int cc_init = 0;
VP8LHashChain* hash_chain = (VP8LHashChain*)malloc(sizeof(*hash_chain));
VP8LHashChainInit(hash_chain, pix_count);
VP8LColorCache hashers;
if (hash_chain == NULL ||
!VP8LColorCacheInit(&hashers, cache_bits)) {
!(cc_init = VP8LColorCacheInit(&hashers, cache_bits)) ||
!VP8LHashChainInit(hash_chain, pix_count)) {
goto Error;
}
refs->size = 0;
for (ix = 0; ix < chosen_path_size; ++ix, ++size) {
int offset = 0;
int len = 0;
int maxlen = chosen_path[ix];
if (maxlen != 1) {
VP8LHashChainFindCopy(hash_chain, quality,
i, xsize, argb, maxlen, &offset, &len);
HashChainFindCopy(hash_chain, quality,
i, xsize, argb, maxlen, &offset, &len);
assert(len == maxlen);
refs->refs[size] = PixOrCopyCreateCopy(offset, len);
for (k = 0; k < len; ++k) {
@ -570,18 +580,16 @@ static int BackwardReferencesHashChainFollowChosenPath(
refs->size = size;
ok = 1;
Error:
if (cc_init) VP8LColorCacheClear(&hashers);
VP8LHashChainClear(hash_chain);
if (hash_chain) {
free(hash_chain);
}
VP8LColorCacheClear(&hashers);
free(hash_chain);
return ok;
}
// Returns 1 on success.
int VP8LBackwardReferencesTraceBackwards(
int xsize, int ysize, int recursive_cost_model, int use_color_cache,
const uint32_t* const argb, int cache_bits, VP8LBackwardRefs* const refs) {
static int BackwardReferencesTraceBackwards(
int xsize, int ysize, int recursive_cost_model, const uint32_t* const argb,
int cache_bits, VP8LBackwardRefs* const refs) {
int ok = 0;
const int dist_array_size = xsize * ysize;
uint32_t* chosen_path = NULL;
@ -592,16 +600,14 @@ int VP8LBackwardReferencesTraceBackwards(
goto Error;
}
if (!BackwardReferencesHashChainDistanceOnly(
xsize, ysize, recursive_cost_model, use_color_cache, argb, cache_bits,
dist_array)) {
xsize, ysize, recursive_cost_model, argb, cache_bits, dist_array)) {
free(dist_array);
goto Error;
}
TraceBackwards(dist_array, dist_array_size, &chosen_path, &chosen_path_size);
free(dist_array);
if (!BackwardReferencesHashChainFollowChosenPath(
xsize, ysize, use_color_cache, argb, cache_bits,
chosen_path, chosen_path_size, refs)) {
xsize, ysize, argb, cache_bits, chosen_path, chosen_path_size, refs)) {
goto Error;
}
ok = 1;
@ -610,67 +616,92 @@ int VP8LBackwardReferencesTraceBackwards(
return ok;
}
void VP8LBackwardReferences2DLocality(int xsize, VP8LBackwardRefs* const refs) {
static void BackwardReferences2DLocality(int xsize,
VP8LBackwardRefs* const refs) {
int i;
for (i = 0; i < refs->size; ++i) {
if (PixOrCopyIsCopy(&refs->refs[i])) {
const int dist = refs->refs[i].argb_or_distance;
const int transformed_dist = VP8LDistanceToPlaneCode(xsize, dist);
const int transformed_dist = DistanceToPlaneCode(xsize, dist);
refs->refs[i].argb_or_distance = transformed_dist;
}
}
}
int VP8LVerifyBackwardReferences(
const uint32_t* const argb, int xsize, int ysize, int cache_bits,
const VP8LBackwardRefs* const refs) {
int num_pixels = 0;
int i;
VP8LColorCache hashers;
VP8LColorCacheInit(&hashers, cache_bits);
for (i = 0; i < refs->size; ++i) {
const PixOrCopy token = refs->refs[i];
if (PixOrCopyIsLiteral(&token)) {
if (argb[num_pixels] != PixOrCopyArgb(&token)) {
VP8LColorCacheClear(&hashers);
return 0;
}
VP8LColorCacheInsert(&hashers, argb[num_pixels]);
++num_pixels;
} else if (PixOrCopyIsCacheIdx(&token)) {
const uint32_t cache_entry =
VP8LColorCacheLookup(&hashers, PixOrCopyCacheIdx(&token));
if (argb[num_pixels] != cache_entry) {
VP8LColorCacheClear(&hashers);
return 0;
}
VP8LColorCacheInsert(&hashers, argb[num_pixels]);
++num_pixels;
} else if (PixOrCopyIsCopy(&token)) {
int k;
if (PixOrCopyDistance(&token) == 0) {
VP8LColorCacheClear(&hashers);
return 0;
}
for (k = 0; k < token.len; ++k) {
if (argb[num_pixels] != argb[num_pixels - PixOrCopyDistance(&token)]) {
VP8LColorCacheClear(&hashers);
return 0;
}
VP8LColorCacheInsert(&hashers, argb[num_pixels]);
++num_pixels;
}
}
int VP8LGetBackwardReferences(int width, int height,
const uint32_t* const argb,
int quality, int cache_bits, int use_2d_locality,
VP8LBackwardRefs* const best) {
int ok = 0;
int lz77_is_useful;
VP8LBackwardRefs refs_rle, refs_lz77;
const int num_pix = width * height;
VP8LBackwardRefsAlloc(&refs_rle, num_pix);
VP8LBackwardRefsAlloc(&refs_lz77, num_pix);
VP8LInitBackwardRefs(best);
if (refs_rle.refs == NULL || refs_lz77.refs == NULL) {
Error1:
VP8LClearBackwardRefs(&refs_rle);
VP8LClearBackwardRefs(&refs_lz77);
goto End;
}
if (!BackwardReferencesHashChain(width, height, argb, cache_bits, quality,
&refs_lz77)) {
goto End;
}
// Backward Reference using RLE only.
BackwardReferencesRle(width, height, argb, &refs_rle);
{
const int pix_count = xsize * ysize;
if (num_pixels != pix_count) {
VP8LColorCacheClear(&hashers);
return 0;
}
double bit_cost_lz77, bit_cost_rle;
VP8LHistogram* const histo = (VP8LHistogram*)malloc(sizeof(*histo));
if (histo == NULL) goto Error1;
// Evaluate lz77 coding
VP8LHistogramCreate(histo, &refs_lz77, cache_bits);
bit_cost_lz77 = VP8LHistogramEstimateBits(histo);
// Evaluate RLE coding
VP8LHistogramCreate(histo, &refs_rle, cache_bits);
bit_cost_rle = VP8LHistogramEstimateBits(histo);
// Decide if LZ77 is useful.
lz77_is_useful = (bit_cost_lz77 < bit_cost_rle);
free(histo);
}
VP8LColorCacheClear(&hashers);
return 1;
// Choose appropriate backward reference.
if (lz77_is_useful) {
// TraceBackwards is costly. Run it for higher qualities.
const int try_lz77_trace_backwards = (quality >= 75);
*best = refs_lz77; // default guess: lz77 is better
VP8LClearBackwardRefs(&refs_rle);
if (try_lz77_trace_backwards) {
const int recursion_level = (num_pix < 320 * 200) ? 1 : 0;
VP8LBackwardRefs refs_trace;
if (!VP8LBackwardRefsAlloc(&refs_trace, num_pix)) {
goto End;
}
if (BackwardReferencesTraceBackwards(
width, height, recursion_level, argb, cache_bits, &refs_trace)) {
VP8LClearBackwardRefs(&refs_lz77);
*best = refs_trace;
}
}
} else {
VP8LClearBackwardRefs(&refs_lz77);
*best = refs_rle;
}
if (use_2d_locality) { // Use backward reference with 2D locality.
BackwardReferences2DLocality(width, best);
}
ok = 1;
End:
if (!ok) {
VP8LClearBackwardRefs(best);
}
return ok;
}
// Returns 1 on success.
@ -723,8 +754,7 @@ int VP8LCalculateEstimateForCacheSize(
static const double kSmallPenaltyForLargeCache = 4.0;
static const int quality = 30;
if (!VP8LBackwardRefsAlloc(&refs, xsize * ysize) ||
!VP8LBackwardReferencesHashChain(xsize, ysize, 0, argb, 0, quality,
&refs)) {
!BackwardReferencesHashChain(xsize, ysize, argb, 0, quality, &refs)) {
goto Error;
}
for (cache_bits = 0; cache_bits <= kColorCacheBitsMax; ++cache_bits) {

38
src/enc/backward_references.h
View File

@ -206,38 +206,12 @@ static WEBP_INLINE int VP8LBackwardRefsAlloc(VP8LBackwardRefs* const refs,
return 1;
}
// Ridiculously simple backward references for images where it is unlikely
// that there are large backward references (photos).
void VP8LBackwardReferencesRle(
int xsize, int ysize, const uint32_t* const argb,
VP8LBackwardRefs* const refs);
// This is a simple fast function for obtaining backward references
// based on simple heuristics. Returns 1 on success.
int VP8LBackwardReferencesHashChain(
int xsize, int ysize, int use_color_cache, const uint32_t* const argb,
int cache_bits, int quality, VP8LBackwardRefs* const refs);
// This method looks for a shortest path through the backward reference
// network based on a cost model generated by a first round of compression.
// Returns 1 on success.
int VP8LBackwardReferencesTraceBackwards(
int xsize, int ysize, int recursive_cost_model, int use_color_cache,
const uint32_t* const argb, int cache_bits, VP8LBackwardRefs* const refs);
// Convert backward references that are of linear distance along
// the image scan lines to have a 2d locality indexing where
// smaller values are used for backward references that are close by.
void VP8LBackwardReferences2DLocality(int xsize, VP8LBackwardRefs* const refs);
// Internals of locality transform exposed for testing use.
int VP8LDistanceToPlaneCode(int xsize, int distance);
// Returns true if the given backward references actually produce
// the image given in tuple (argb, xsize, ysize).
int VP8LVerifyBackwardReferences(
const uint32_t* const argb, int xsize, int ysize, int cache_bits,
const VP8LBackwardRefs* const refs);
// Evaluates best possible backward references for specified quality.
// Further optimize for 2D locality if use_2d_locality flag is set.
int VP8LGetBackwardReferences(int width, int height,
const uint32_t* const argb,
int quality, int cache_bits, int use_2d_locality,
VP8LBackwardRefs* const best);
// Produce an estimate for a good color cache size for the image.
int VP8LCalculateEstimateForCacheSize(

85
src/enc/vp8l.c
View File

@ -157,86 +157,6 @@ static int VP8LEncAnalyze(VP8LEncoder* const enc) {
// -----------------------------------------------------------------------------
static int GetBackwardReferences(int width, int height,
const uint32_t* const argb,
int quality, int use_color_cache,
int cache_bits, int use_2d_locality,
VP8LBackwardRefs* const best) {
int ok = 0;
int lz77_is_useful;
VP8LBackwardRefs refs_rle, refs_lz77;
const int num_pix = width * height;
VP8LBackwardRefsAlloc(&refs_rle, num_pix);
VP8LBackwardRefsAlloc(&refs_lz77, num_pix);
VP8LInitBackwardRefs(best);
if (refs_rle.refs == NULL || refs_lz77.refs == NULL) {
Error1:
VP8LClearBackwardRefs(&refs_rle);
VP8LClearBackwardRefs(&refs_lz77);
goto End;
}
if (!VP8LBackwardReferencesHashChain(width, height, use_color_cache,
argb, cache_bits, quality,
&refs_lz77)) {
goto End;
}
// Backward Reference using RLE only.
VP8LBackwardReferencesRle(width, height, argb, &refs_rle);
{
double bit_cost_lz77, bit_cost_rle;
VP8LHistogram* const histo = (VP8LHistogram*)malloc(sizeof(*histo));
if (histo == NULL) goto Error1;
// Evaluate lz77 coding
VP8LHistogramCreate(histo, &refs_lz77, cache_bits);
bit_cost_lz77 = VP8LHistogramEstimateBits(histo);
// Evaluate RLE coding
VP8LHistogramCreate(histo, &refs_rle, cache_bits);
bit_cost_rle = VP8LHistogramEstimateBits(histo);
// Decide if LZ77 is useful.
lz77_is_useful = (bit_cost_lz77 < bit_cost_rle);
free(histo);
}
// Choose appropriate backward reference.
if (lz77_is_useful) {
// TraceBackwards is costly. Run it for higher qualities.
const int try_lz77_trace_backwards = (quality >= 75);
*best = refs_lz77; // default guess: lz77 is better
VP8LClearBackwardRefs(&refs_rle);
if (try_lz77_trace_backwards) {
const int recursion_level = (num_pix < 320 * 200) ? 1 : 0;
VP8LBackwardRefs refs_trace;
if (!VP8LBackwardRefsAlloc(&refs_trace, num_pix)) {
goto End;
}
if (VP8LBackwardReferencesTraceBackwards(width, height, recursion_level,
use_color_cache,
argb, cache_bits,
&refs_trace)) {
VP8LClearBackwardRefs(&refs_lz77);
*best = refs_trace;
}
}
} else {
VP8LClearBackwardRefs(&refs_lz77);
*best = refs_rle;
}
if (use_2d_locality) { // Use backward reference with 2D locality.
VP8LBackwardReferences2DLocality(width, best);
}
ok = 1;
End:
if (!ok) {
VP8LClearBackwardRefs(best);
}
return ok;
}
// Heuristics for selecting the stride ranges to collapse.
static int ValuesShouldBeCollapsedToStrideAverage(int a, int b) {
return abs(a - b) < 4;
@ -684,9 +604,8 @@ static int EncodeImageInternal(VP8LBitWriter* const bw,
if (histogram_image == NULL || histogram_symbols == NULL) goto Error;
// Calculate backward references from ARGB image.
if (!GetBackwardReferences(width, height, argb, quality,
use_color_cache, cache_bits, use_2d_locality,
&refs)) {
if (!VP8LGetBackwardReferences(width, height, argb, quality, cache_bits,
use_2d_locality, &refs)) {
goto Error;
}
// Build histogram image & symbols from backward references.