[FFmpeg-devel] [PATCH 1/3] lavc/h274: fix PRNG definition

From: Niklas Haas <ffmpeg@haasn.xyz>
To: ffmpeg-devel@ffmpeg.org
Cc: Niklas Haas <git@haasn.dev>
Subject: [FFmpeg-devel] [PATCH 1/3] lavc/h274: fix PRNG definition
Date: Wed, 27 Sep 2023 15:56:15 +0200
Message-ID: <20230927135617.33952-1-ffmpeg@haasn.xyz> (raw)

From: Niklas Haas <git@haasn.dev>

The spec specifies x^31 + x^3 + 1 as the polynomial, but the diagram in
Figure 1-1 omits the +1 offset. The initial implementation was based on
the diagram, but this is wrong (produces subtly incorrect results).
---
 libavcodec/h274.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libavcodec/h274.c b/libavcodec/h274.c
index a69f9411429..fc111cdb50d 100644
--- a/libavcodec/h274.c
+++ b/libavcodec/h274.c
@@ -38,7 +38,7 @@ static void prng_shift(uint32_t *state)
 {
     // Primitive polynomial x^31 + x^3 + 1 (modulo 2)
     uint32_t x = *state;
-    uint8_t feedback = (x >> 2) ^ (x >> 30);
+    uint8_t feedback = 1u ^ (x >> 2) ^ (x >> 30);
     *state = (x << 1) | (feedback & 1u);
 }
 
-- 
2.42.0

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