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.

4 files changed, 101 insertions, 8 deletions

diff --git a/src/lib/dcp_video.cc b/src/lib/dcp_video.cc
index 6bbd0d423..1f9f76771 100644
--- a/src/lib/dcp_video.cc
+++ b/src/lib/dcp_video.cc
@@ -1,5 +1,5 @@
 /*
-    Copyright (C) 2012-2016 Carl Hetherington <cth@carlh.net>
+    Copyright (C) 2012-2021 Carl Hetherington <cth@carlh.net>
 
     This file is part of DCP-o-matic.
 
@@ -18,6 +18,7 @@
 
 */
 
+
 /** @file  src/dcp_video_frame.cc
  *  @brief A single frame of video destined for a DCP.
  *
@@ -28,17 +29,19 @@
  *  of images that require encoding.
  */
 
-#include "dcp_video.h"
+
+#include "compose.hpp"
 #include "config.h"
-#include "exceptions.h"
-#include "encode_server_description.h"
+#include "cross.h"
+#include "dcp_video.h"
+#include "dcpomatic_log.h"
 #include "dcpomatic_socket.h"
+#include "encode_server_description.h"
+#include "exceptions.h"
 #include "image.h"
 #include "log.h"
-#include "dcpomatic_log.h"
-#include "cross.h"
 #include "player_video.h"
-#include "compose.hpp"
+#include "rng.h"
 #include "warnings.h"
 #include <libcxml/cxml.h>
 #include <dcp/raw_convert.h>
@@ -143,6 +146,8 @@ DCPVideo::encode_locally ()
 			break;
 		}
 
+		LOG_GENERAL (N_("Frame %1 encoded size was small (%2); adding noise at level %3"), _index, enc.size(), noise_amount);
+
 		/* The JPEG2000 is too low-bitrate for some decoders <cough>DSS200</cough> so add some noise
 		 * and try again.  This is slow but hopefully won't happen too often.  We have to do
 		 * convert_to_xyz() again because compress_j2k() corrupts its xyz parameter.
@@ -151,10 +156,11 @@ DCPVideo::encode_locally ()
 		xyz = convert_to_xyz (_frame, boost::bind(&Log::dcp_log, dcpomatic_log.get(), _1, _2));
 		auto size = xyz->size ();
 		auto pixels = size.width * size.height;
+		dcpomatic::RNG rng(42);
 		for (auto c = 0; c < 3; ++c) {
 			auto p = xyz->data(c);
 			for (auto i = 0; i < pixels; ++i) {
-				*p = std::min(4095, std::max(0, *p + rand() % noise_amount));
+				*p = std::min(4095, std::max(0, *p + (rng.get() % noise_amount)));
 				++p;
 			}
 		}
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);
+}
+
diff --git a/src/lib/rng.h b/src/lib/rng.h
new file mode 100644
index 000000000..f3349ce9b
--- /dev/null
+++ b/src/lib/rng.h
@@ -0,0 +1,44 @@
+/*
+    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 <stdint.h>
+
+
+namespace dcpomatic {
+
+
+/** @class  RNG
+ *  @brief  Linear-feedback random number generator
+ */
+class RNG
+{
+public:
+	RNG (int32_t seed);
+
+	int32_t get ();
+
+private:
+	uint32_t _state;
+};
+
+
+}
+
diff --git a/src/lib/wscript b/src/lib/wscript
index bff3382e0..1b45152a4 100644
--- a/src/lib/wscript
+++ b/src/lib/wscript
@@ -144,6 +144,7 @@ sources = """
           render_text.cc
           resampler.cc
           rgba.cc
+          rng.cc
           scoped_temporary.cc
           scp_uploader.cc
           screen.cc