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Smarter LZ77 for uniform regions.

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No need to find backward references for pixels in uniform regions
by looking at all pixels.
Only pixels at the same distance from the end need to be compared to.

Change-Id: I4f187e965f0667d3a929775726a412f7e69f6473
This commit is contained in:
Vincent Rabaud 2016-08-25 09:17:02 +02:00
parent 6585075f8a
commit 85cd5d061c
1 changed files with 65 additions and 7 deletions
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70
src/enc/backward_references.c
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@ -249,13 +249,20 @@ int VP8LHashChainFill(VP8LHashChain* const p, int quality,
const int iter_min = iter_max - quality / 10;
const uint32_t window_size = GetWindowSizeForHashChain(quality, xsize);
int pos;
int argb_comp;
uint32_t base_position;
int32_t* hash_to_first_index;
// Temporarily use the p->offset_length_ as a hash chain.
int32_t* chain = (int32_t*)p->offset_length_;
assert(size > 0);
assert(p->size_ != 0);
assert(p->offset_length_ != NULL);
if (size <= 2) {
p->offset_length_[0] = p->offset_length_[size - 1] = 0;
return 1;
}
hash_to_first_index =
(int32_t*)WebPSafeMalloc(HASH_SIZE, sizeof(*hash_to_first_index));
if (hash_to_first_index == NULL) return 0;
@ -263,19 +270,61 @@ int VP8LHashChainFill(VP8LHashChain* const p, int quality,
// Set the int32_t array to -1.
memset(hash_to_first_index, 0xff, HASH_SIZE * sizeof(*hash_to_first_index));
// Fill the chain linking pixels with the same hash.
for (pos = 0; pos < size - 1; ++pos) {
const uint32_t hash_code = GetPixPairHash64(argb + pos);
chain[pos] = hash_to_first_index[hash_code];
hash_to_first_index[hash_code] = pos;
argb_comp = (argb[0] == argb[1]);
for (pos = 0; pos < size - 2;) {
uint32_t hash_code;
const int argb_comp_next = (argb[pos + 1] == argb[pos + 2]);
if (argb_comp && argb_comp_next) {
// Consecutive pixels with the same color will share the same hash.
// We therefore use a different hash: the color and its repetition
// length.
uint32_t tmp[2];
uint32_t len = 1;
tmp[0] = argb[pos];
// Figure out how far the pixels are the same.
// The last pixel has a different 64 bit hash, as its next pixel does
// not have the same color, so we just need to get to the last pixel equal
// to its follower.
while (pos + (int)len + 2 < size && argb[pos + len + 2] == argb[pos]) {
++len;
}
if (len > MAX_LENGTH) {
// Skip the pixels that match for distance=1 and length>MAX_LENGTH
// because they are linked to their predecessor and we automatically
// check that in the main for loop below. Skipping means setting no
// predecessor in the chain, hence -1.
memset(chain + pos, 0xff, (len - MAX_LENGTH) * sizeof(*chain));
pos += len - MAX_LENGTH;
len = MAX_LENGTH;
}
// Process the rest of the hash chain.
while (len) {
tmp[1] = len--;
hash_code = GetPixPairHash64(tmp);
chain[pos] = hash_to_first_index[hash_code];
hash_to_first_index[hash_code] = pos++;
}
argb_comp = 0;
} else {
// Just move one pixel forward.
hash_code = GetPixPairHash64(argb + pos);
chain[pos] = hash_to_first_index[hash_code];
hash_to_first_index[hash_code] = pos++;
argb_comp = argb_comp_next;
}
}
// Process the penultimate pixel.
chain[pos] = hash_to_first_index[GetPixPairHash64(argb + pos)];
WebPSafeFree(hash_to_first_index);
// Find the best match interval at each pixel, defined by an offset to the
// pixel and a length. The right-most pixel cannot match anything to the right
// (hence a best length of 0) and the left-most pixel nothing to the left
// (hence an offset of 0).
assert(size > 2);
p->offset_length_[0] = p->offset_length_[size - 1] = 0;
for (base_position = size - 2 < 0 ? 0 : size - 2; base_position > 0;) {
for (base_position = size - 2; base_position > 0;) {
const int max_len = MaxFindCopyLength(size - 1 - base_position);
const uint32_t* const argb_start = argb + base_position;
int iter = iter_max;
@ -288,14 +337,23 @@ int VP8LHashChainFill(VP8LHashChain* const p, int quality,
pos = chain[base_position];
if (!low_effort) {
int curr_length;
// Heuristic: use the comparison with the above line as an initialization.
if (base_position >= (uint32_t)xsize) {
const int curr_length = FindMatchLength(argb_start - xsize, argb_start,
curr_length = FindMatchLength(argb_start - xsize, argb_start,
best_length, max_len);
if (curr_length > best_length) {
best_length = curr_length;
best_distance = xsize;
}
--iter;
}
// Heuristic: compare to the previous pixel.
curr_length =
FindMatchLength(argb_start - 1, argb_start, best_length, max_len);
if (curr_length > best_length) {
best_length = curr_length;
best_distance = 1;
}
--iter;
// Skip the for loop if we already have the maximum.