From 0e2feaf16371c2b6d2362d8b1323801fcd33d9e2 Mon Sep 17 00:00:00 2001
From: Mark Thompson <sw@jkqxz.net>
Date: Mon, 27 Nov 2023 12:32:17 +0000
Subject: [PATCH] Update uvlc definition to match implementations

The specified uvlc parsing process does not match that used by the
libaom reference implementation [1].  The specification encodes 2^32-1
as at least thirty-two zero bits followed by a one, while libaom uses
exactly thirty-two zero bits without a one.  Other implementations also
do not match the specification here ([2] matches the libaom behaviour,
[3] raises an error if it sees this case).

The difference is never encountered in conforming streams because the
one syntax element using the uvlc parsing process
(num_ticks_per_picture_minus_1) is not allowed to take the maximum
value of 2^32-1.  Given that this area is therefore not covered for
conformance purposes, it seems easier to update the text of the
specification to match the existing implementations than to do the
reverse.

[1] <https://aomedia.googlesource.com/aom/+/refs/heads/main/aom_dsp/bitreader_buffer.c>, line 63.

[2] <https://gitlab.com/AOMediaCodec/SVT-AV1/-/blob/master/Source/Lib/Decoder/Codec/EbDecBitstream.c?ref_type=heads#L68>

[3] <https://chromium.googlesource.com/codecs/libgav1/+/refs/heads/main/src/utils/raw_bit_reader.cc>, line 161.
---
 04.conventions.md | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/04.conventions.md b/04.conventions.md
index 4fc0a45d..4b284f39 100644
--- a/04.conventions.md
+++ b/04.conventions.md
@@ -345,14 +345,14 @@ The parsing process for this descriptor is specified below:
 | --------------------------------------------------------- | ---------------- |
 | uvlc() {                                                  | **Type**
 |     leadingZeros = 0                                      |
-|     while ( 1 ) {                                          |
+|     while ( leadingZeros \< 32 ) {                        |
 |         @@done                                            | f(1)
 |         if ( done )                                       |
 |             break                                         |
 |         leadingZeros++                                    |
 |     }                                                     |
-|     if ( leadingZeros >\= 32 ) {                           |
-|         return ( 1 \<\< 32 ) - 1                            |
+|     if ( leadingZeros \=\= 32 ) {                         |
+|         return ( 1 \<\< 32 ) - 1                          |
 |     }                                                     |
 |     @@value                                               | f(leadingZeros)
 |     return value + ( 1 \<\< leadingZeros ) - 1