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Speed-up uniform-region processing.

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Change-Id: I9a88d0ac97c31d19323c9505ebe21f375d2e96b8
This commit is contained in:
Vincent Rabaud 2016-06-21 15:11:19 +02:00
parent 23e29cb1e3
commit 7d58d1b7b9
1 changed files with 102 additions and 39 deletions
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133
src/enc/backward_references.c
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@ -653,6 +653,7 @@ typedef struct {
size_t cache_intervals_size_;
double cost_cache_[MAX_LENGTH]; // Contains the GetLengthCost(cost_model, k).
double min_cost_cache_; // The minimum value in cost_cache_[1:].
double max_cost_cache_; // The maximum value in cost_cache_[1:].
float* costs_;
uint16_t* dist_array_;
// Most of the time, we only need few intervals -> use a free-list, to avoid
@ -662,6 +663,10 @@ typedef struct {
// These are regularly malloc'd remains. This list can't grow larger than than
// size COST_CACHE_INTERVAL_SIZE_MAX - COST_MANAGER_MAX_FREE_LIST, note.
CostInterval* recycled_intervals_;
// Buffer used in BackwardReferencesHashChainDistanceOnly to store the ends
// of the intervals that can have impacted the cost at a pixel.
int* interval_ends_;
int interval_ends_size_;
} CostManager;
static int IsCostCacheIntervalWritable(int start, int end) {
@ -710,6 +715,7 @@ static void CostManagerClear(CostManager* const manager) {
WebPSafeFree(manager->costs_);
WebPSafeFree(manager->cache_intervals_);
WebPSafeFree(manager->interval_ends_);
// Clear the interval lists.
DeleteIntervalList(manager, manager->head_);
@ -731,6 +737,9 @@ static int CostManagerInit(CostManager* const manager,
// Empirically, differences between intervals is usually of more than 1.
const double min_cost_diff = 0.1;
manager->costs_ = NULL;
manager->cache_intervals_ = NULL;
manager->interval_ends_ = NULL;
manager->head_ = NULL;
manager->recycled_intervals_ = NULL;
manager->count_ = 0;
@ -750,8 +759,11 @@ static int CostManagerInit(CostManager* const manager,
// Compute the minimum of cost_cache_.
if (i == 1) {
manager->min_cost_cache_ = manager->cost_cache_[1];
manager->max_cost_cache_ = manager->cost_cache_[1];
} else if (manager->cost_cache_[i] < manager->min_cost_cache_) {
manager->min_cost_cache_ = manager->cost_cache_[i];
} else if (manager->cost_cache_[i] > manager->max_cost_cache_) {
manager->max_cost_cache_ = manager->cost_cache_[i];
}
}
@ -826,6 +838,54 @@ static int CostManagerInit(CostManager* const manager,
// Set the initial costs_ high for every pixel as we will keep the minimum.
for (i = 0; i < pix_count; ++i) manager->costs_[i] = 1e38f;
// The cost at pixel is influenced by the cost intervals from previous pixels.
// Let us take the specific case where the offset is the same (which actually
// happens a lot in case of uniform regions).
// pixel i contributes to j>i a cost of: offset cost + cost_cache_[j-i]
// pixel i+1 contributes to j>i a cost of: 2*offset cost + cost_cache_[j-i-1]
// pixel i+2 contributes to j>i a cost of: 3*offset cost + cost_cache_[j-i-2]
// and so on.
// A pixel i influences the following length(j) < MAX_LENGTH pixels. What is
// the value of j such that pixel i + j cannot influence any of those pixels?
// This value is such that:
// max of cost_cache_ < j*offset cost + min of cost_cache_
// (pixel i + j 's cost cannot beat the worst cost given by pixel i).
// This value will be used to optimize the cost computation in
// BackwardReferencesHashChainDistanceOnly.
{
// The offset cost is computed in GetDistanceCost and has a minimum value of
// the minimum in cost_model->distance_. The case where the offset cost is 0
// will be dealt with differently later so we are only interested in the
// minimum non-zero offset cost.
double offset_cost_min = 0.;
int size;
for (i = 0; i < NUM_DISTANCE_CODES; ++i) {
if (cost_model->distance_[i] != 0) {
if (offset_cost_min == 0.) {
offset_cost_min = cost_model->distance_[i];
} else if (cost_model->distance_[i] < offset_cost_min) {
offset_cost_min = cost_model->distance_[i];
}
}
}
// In case all the cost_model->distance_ is 0, the next non-zero cost we
// can have is from the extra bit in GetDistanceCost, hence 1.
if (offset_cost_min < 1.) offset_cost_min = 1.;
size = 1 + (int)ceil((manager->max_cost_cache_ - manager->min_cost_cache_) /
offset_cost_min);
// Empirically, we usually end up with a value below 100.
if (size > MAX_LENGTH) size = MAX_LENGTH;
manager->interval_ends_ =
(int*)WebPSafeMalloc(size, sizeof(*manager->interval_ends_));
if (manager->interval_ends_ == NULL) {
CostManagerClear(manager);
return 0;
}
manager->interval_ends_size_ = size;
}
return 1;
}
@ -1158,53 +1218,56 @@ static int BackwardReferencesHashChainDistanceOnly(
const int code = DistanceToPlaneCode(xsize, offset);
const double offset_cost = GetDistanceCost(cost_model, code);
const int first_i = i;
int j_max;
int j_max = 0, interval_ends_index = 0;
const int is_offset_zero = (offset_cost == 0.);
if (offset_cost == 0.) {
if (!is_offset_zero) {
j_max = (int)ceil(
(cost_manager->max_cost_cache_ - cost_manager->min_cost_cache_) /
offset_cost);
if (j_max < 1) {
j_max = 1;
} else {
j_max = (int)ceil(cost_manager->min_cost_cache_ / offset_cost);
assert(j_max >= 1);
} else if (j_max > cost_manager->interval_ends_size_ - 1) {
// This could only happen in the case of MAX_LENGTH.
j_max = cost_manager->interval_ends_size_ - 1;
}
} // else j_max is unused anyway.
// With the values we currently use for the model, offset_cost is
// between 10 and 20 and j_max ends up at 2 or 3.
if (j_max > MAX_LENGTH - 1) j_max = MAX_LENGTH - 1;
// The cost to get to pixel i+k is:
// prev_cost + best weight of path going from i to l.
// For the last cost, if the path were composed of several segments
// (like [i,j), [j,k), [k,l) ), its cost would be:
// offset cost + manager->cost_cache_[j-i] +
// offset cost + manager->cost_cache_[k-j] +
// offset cost + manager->cost_cache_[l-j]
// As the offset_cost is close to the values in cost_cache_ (both are the
// cost to store a number, whether it's an offset or a length), the best
// paths are therefore the ones involving one hop.
// Instead of considering all contributions from a pixel i by calling:
// PushInterval(cost_manager, prev_cost + offset_cost, i, len);
// we optimize these contributions in case offset_cost stays the same for
// consecutive pixels. This describes a set of pixels similar to a
// previous set (e.g. constant color regions).
for (; i < pix_count - 1; ++i) {
int offset_next, len_next;
double distance_cost;
prev_cost = cost_manager->costs_[i - 1];
distance_cost = prev_cost + offset_cost;
if (len != MAX_LENGTH || i == first_i) {
if (is_offset_zero) {
// No optimization can be made so we just push all of the
// contributions from i.
PushInterval(cost_manager, prev_cost, i, len);
} else {
// j_max is chosen as the smallest j such that:
// max of cost_cache_ < j*offset cost + min of cost_cache_
// Therefore, the pixel influenced by i-j_max, cannot be influenced
// by i. Only the costs after the end of what i contributed need to be
// updated. cost_manager->interval_ends_ is a circular buffer that
// stores those ends.
const double distance_cost = prev_cost + offset_cost;
int j = cost_manager->interval_ends_[interval_ends_index];
if (i - first_i <= j_max ||
!IsCostCacheIntervalWritable(j, i + len)) {
PushInterval(cost_manager, distance_cost, i, len);
} else {
int j;
// The pixel at i + MAX_LENGTH - 1 is influenced by all the pixels
// before at i + MAX_LENGTH - j. Then again, the distance cost
// provided by those is increased by offset_cost*j. Therefore, only
// the pixels providing a length cost inferior to offset_cost*j can be
// considered.
// As we know this cost is at least cost_manager->min_cost_cache_:
// therefore only the first j_max pixels from i influence
// i + MAX_LENGTH - 1.
// Reciprocally, pixel i only influences the last j_max pixels.
for (j = 1; j <= j_max; ++j) {
UpdateCost(cost_manager, i + MAX_LENGTH - j, i, distance_cost);
for (; j < i + len; ++j) {
UpdateCost(cost_manager, j, i, distance_cost);
}
}
// Store the new end in the circular buffer.
assert(interval_ends_index < cost_manager->interval_ends_size_);
cost_manager->interval_ends_[interval_ends_index] = i + len;
if (++interval_ends_index > j_max) interval_ends_index = 0;
}
// Check whether i is the last pixel to consider, as it is handled
// differently.