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ANS: Switch from PDFs to CDFs.

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Make the RANS implementation operate on cumulative distribution
functions rather than individual probability distribution functions.
CDFs have shown themselves more flexible to work with.

Reduces decoding memory usage from scaling O(num_distributions *
symbol_resolution) to O(num_distributions).

No bitstream change. This is an purely implementation change.

Change-Id: I4e18d3a0a3d37a36a61487c3d778f9d088b0b374
This commit is contained in:
Alex Converse
2016-02-16 13:41:01 -08:00
committed by Pascal Massimino
parent 339ef0ce7a
commit 6bbbe31656
10 changed files with 50 additions and 48 deletions
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45
vp10/common/ans.h
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@@ -241,23 +241,16 @@ struct rans_dec_sym {
AnsP8 cum_prob; // not-inclusive
};
typedef struct rans_dec_sym rans_dec_lut[ans_p8_precision];
// This is now just a boring cdf. It starts with an explict zero.
// TODO(aconverse): Remove starting zero.
typedef uint16_t rans_dec_lut[16];
static INLINE void rans_build_dec_tab(const AnsP8 token_probs[],
rans_dec_lut dec_tab) {
int val = 0;
int cum_prob = 0;
int sym_end = token_probs[0];
static INLINE void rans_build_cdf_from_pdf(const AnsP8 token_probs[],
rans_dec_lut cdf_tab) {
int i;
for (i = 0; i < 256; ++i) {
if (i == sym_end) {
++val;
cum_prob = sym_end;
sym_end += token_probs[val];
}
dec_tab[i].val = val;
dec_tab[i].prob = token_probs[val];
dec_tab[i].cum_prob = cum_prob;
cdf_tab[0] = 0;
for (i = 1; cdf_tab[i - 1] < ans_p8_precision; ++i) {
cdf_tab[i] = cdf_tab[i - 1] + token_probs[i - 1];
}
}
@@ -275,20 +268,32 @@ static INLINE void rans_write(struct AnsCoder *ans,
(ans->state / p) * ans_p8_precision + ans->state % p + sym->cum_prob;
}
static INLINE void fetch_sym(struct rans_dec_sym *out, const rans_dec_lut cdf,
AnsP8 rem) {
int i = 0;
// TODO(skal): if critical, could be a binary search.
// Or, better, an O(1) alias-table.
while (rem >= cdf[i]) {
++i;
}
out->val = i - 1;
out->prob = cdf[i] - cdf[i - 1];
out->cum_prob = cdf[i - 1];
}
static INLINE int rans_read(struct AnsDecoder *ans,
const rans_dec_lut tab) {
unsigned rem;
unsigned quo;
int val;
struct rans_dec_sym sym;
if (ans->state < l_base && ans->buf_offset > 0) {
ans->state = ans->state * io_base + ans->buf[--ans->buf_offset];
}
quo = ans->state / ans_p8_precision;
rem = ans->state % ans_p8_precision;
val = tab[rem].val;
ans->state = quo * tab[rem].prob + rem - tab[rem].cum_prob;
return val;
fetch_sym(&sym, tab, rem);
ans->state = quo * sym.prob + rem - sym.cum_prob;
return sym.val;
}
static INLINE int ans_read_init(struct AnsDecoder *const ans,