Use a predictable RNG when adding noise to low-bitrate images.

The recovery code assumes that encoding a given frame from the
same data will always give the same output; it always encodes frame 0
again to set things up properly and if that frame is a different size
to the previous run everything breaks.

1 files changed, 42 insertions, 0 deletions

diff --git a/src/lib/rng.cc b/src/lib/rng.cc
new file mode 100644
index 000000000..3fc41725d
--- /dev/null
+++ b/src/lib/rng.cc
@@ -0,0 +1,42 @@
+/*
+    Copyright (C) 2020 Carl Hetherington <cth@carlh.net>
+
+    This file is part of DCP-o-matic.
+
+    DCP-o-matic is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or
+    (at your option) any later version.
+
+    DCP-o-matic is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with DCP-o-matic.  If not, see <http://www.gnu.org/licenses/>.
+
+*/
+
+
+#include "rng.h"
+#include <stdint.h>
+
+
+using namespace dcpomatic;
+
+
+RNG::RNG (int32_t seed)
+	: _state (static_cast<uint32_t>(seed))
+{
+
+}
+
+
+int32_t RNG::get ()
+{
+	uint32_t const b = ((_state >> 0) ^ (_state >> 1) ^ (_state >> 2) ^ (_state >> 7));
+	_state = (_state >> 1) | (b << 23);
+	return static_cast<int32_t>(_state);
+}
+