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Remove the starting zero from ANS CDFs.

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This brings it in line with the Daala CDFs and will make it easier to
share code.

Change-Id: Idfd2d2b33c3b9b2c4e72ce72fb3d8039013448b9
(cherry picked from aom/master commit af98507ca928afe33e9f88fdd2ca168379528d6a)
This commit is contained in:
Alex Converse
2016-08-22 12:44:16 -07:00
parent a1ac972867
commit e9f70f8f10
5 changed files with 23 additions and 18 deletions
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3
aom_dsp/ans.h
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@@ -36,8 +36,7 @@ typedef uint16_t AnsP10;
#define IO_BASE 256
// Range I = { L_BASE, L_BASE + 1, ..., L_BASE * IO_BASE - 1 }
// This is now just a boring cdf. It starts with an explicit zero.
// TODO(aconverse): Remove starting zero.
// This is now just a boring cdf.
typedef uint16_t rans_lut[16];
void aom_rans_build_cdf_from_pdf(const AnsP10 token_probs[], rans_lut cdf_tab);

13
aom_dsp/ansreader.h
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@@ -68,15 +68,16 @@ struct rans_dec_sym {
static INLINE void fetch_sym(struct rans_dec_sym *out, const rans_lut cdf,
AnsP10 rem) {
int i = 0;
int i;
AnsP10 cum_prob = 0, top_prob;
// TODO(skal): if critical, could be a binary search.
// Or, better, an O(1) alias-table.
while (rem >= cdf[i]) {
++i;
for (i = 0; rem >= (top_prob = cdf[i]); ++i) {
cum_prob = top_prob;
}
out->val = i - 1;
out->prob = (AnsP10)(cdf[i] - cdf[i - 1]);
out->cum_prob = (AnsP10)cdf[i - 1];
out->val = i;
out->prob = top_prob - cum_prob;
out->cum_prob = cum_prob;
}
static INLINE int rans_read(struct AnsDecoder *ans, const rans_lut tab) {

4
aom_dsp/bitwriter.h
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@@ -92,8 +92,8 @@ static INLINE void aom_write_symbol(aom_writer *w, int symb,
struct rans_sym s;
(void)nsymbs;
assert(cdf);
s.cum_prob = cdf[symb];
s.prob = cdf[symb + 1] - s.cum_prob;
s.cum_prob = symb > 0 ? cdf[symb - 1] : 0;
s.prob = cdf[symb] - s.cum_prob;
buf_rans_write(w, &s);
#else
(void)w;

15
av1/common/entropy.c
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@@ -2801,6 +2801,14 @@ void av1_model_to_full_probs(const aom_prob *model, aom_prob *full) {
}
#if CONFIG_ANS
static void build_token_cdfs(const aom_prob *pdf_model, rans_lut cdf) {
int i, sum = 0;
assert(pdf_model[2] != 0);
for (i = 0; i < ENTROPY_TOKENS - 2; ++i) {
cdf[i] = sum += av1_pareto8_token_probs[pdf_model[2] - 1][i];
}
}
void av1_coef_pareto_cdfs(FRAME_CONTEXT *fc) {
TX_SIZE t;
int i, j, k, l;
@@ -2809,11 +2817,8 @@ void av1_coef_pareto_cdfs(FRAME_CONTEXT *fc) {
for (j = 0; j < REF_TYPES; ++j)
for (k = 0; k < COEF_BANDS; ++k)
for (l = 0; l < BAND_COEFF_CONTEXTS(k); ++l) {
const aom_prob *const tree_probs = fc->coef_probs[t][i][j][k][l];
aom_prob pivot = tree_probs[PIVOT_NODE];
assert(pivot != 0);
aom_rans_build_cdf_from_pdf(av1_pareto8_token_probs[pivot - 1],
fc->coef_cdfs[t][i][j][k][l]);
build_token_cdfs(fc->coef_probs[t][i][j][k][l],
fc->coef_cdfs[t][i][j][k][l]);
}
}
#endif // CONFIG_ANS

6
test/ans_test.cc
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@@ -98,9 +98,9 @@ std::vector<int> ans_encode_build_vals(const rans_sym *tab, int iters) {
}
void rans_build_dec_tab(const struct rans_sym sym_tab[], rans_lut dec_tab) {
dec_tab[0] = 0;
for (int i = 1; dec_tab[i - 1] < RANS_PRECISION; ++i) {
dec_tab[i] = dec_tab[i - 1] + sym_tab[i - 1].prob;
unsigned int sum = 0;
for (int i = 0; sum < RANS_PRECISION; ++i) {
dec_tab[i] = sum += sym_tab[i].prob;
}
}