ans: Switch from uABS to rABS
This is in preparation for expanding the state range.
No discernible compression impact
ans_multioff@2017-01-25T20:58:18.756Z -> ans_multioff_rabs@2017-01-26T01:05:12.801Z
PSNR | PSNR Cb | PSNR Cr | PSNR HVS | SSIM | MS SSIM | CIEDE 2000
-0.0001 | -0.0001 | -0.0001 | -0.0001 | -0.0001 | -0.0001 | -0.0001
https://arewecompressedyet.com/?job=ans_multioff%402017-01-25T20%3A58%3A18.756Z&job=ans_multioff_rabs%402017-01-26T01%3A05%3A12.801Z
Change-Id: Ie1817991190f1de6d9c31e0c97f77efbd5869d35
diff --git a/aom_dsp/ansreader.h b/aom_dsp/ansreader.h
index 760c539..16fc3c8 100644
--- a/aom_dsp/ansreader.h
+++ b/aom_dsp/ansreader.h
@@ -11,8 +11,11 @@
#ifndef AOM_DSP_ANSREADER_H_
#define AOM_DSP_ANSREADER_H_
-// A uABS and rANS decoder implementation of Asymmetric Numeral Systems
+// An implementation of Asymmetric Numeral Systems
// http://arxiv.org/abs/1311.2540v2
+// Implements decoding of:
+// * rABS (range Asymmetric Binary Systems), a boolean coder
+// * rANS (range Asymmetric Numeral Systems), a multi-symbol coder
#include <assert.h>
#include "./aom_config.h"
@@ -57,43 +60,33 @@
return state;
}
-static INLINE int uabs_read(struct AnsDecoder *ans, AnsP8 p0) {
- AnsP8 p = ANS_P8_PRECISION - p0;
- int s;
- unsigned xp, sp;
- unsigned state;
+// Decode one rABS encoded boolean where the probability of the value being zero
+// is p0.
+static INLINE int rabs_read(struct AnsDecoder *ans, AnsP8 p0) {
#if ANS_MAX_SYMBOLS
if (ans->symbols_left-- == 0) {
ans_read_reinit(ans);
ans->symbols_left--;
}
#endif
- state = ans->state;
- sp = state * p;
- xp = sp / ANS_P8_PRECISION;
- s = (sp & 0xFF) >= p0;
- if (s)
- state = xp;
+ unsigned state = ans->state;
+ const unsigned quotient = state / ANS_P8_PRECISION;
+ const unsigned remainder = state % ANS_P8_PRECISION;
+ const int value = remainder >= p0;
+ if (value)
+ state = quotient * (ANS_P8_PRECISION - p0) + remainder - p0;
else
- state -= xp;
+ state = quotient * p0 + remainder;
ans->state = refill_state(ans, state);
- return s;
+ return value;
}
-static INLINE int uabs_read_bit(struct AnsDecoder *ans) {
- int s;
- unsigned state;
-#if ANS_MAX_SYMBOLS
- if (ans->symbols_left-- == 0) {
- ans_read_reinit(ans);
- ans->symbols_left--;
- }
-#endif
- state = ans->state;
- s = (int)(state & 1);
- state >>= 1;
- ans->state = refill_state(ans, state);
- return s;
+// Decode one rABS encoded boolean where the probability of the value being zero
+// is one half.
+static INLINE int rabs_read_bit(struct AnsDecoder *ans) {
+ // TODO(aconverse@google.com): Provide an optimized implementation of this
+ // routine.
+ return rabs_read(ans, 128);
}
struct rans_dec_sym {
