From 24ee9f8cf3736c5cd946eb6df8bd3189f8a62ce0 Mon Sep 17 00:00:00 2001
From: Carl Hetherington <cth@carlh.net>
Date: Tue, 14 Oct 2025 21:14:04 +0200
Subject: Fix MPEG2 DCP gamma (#3026).

---
 src/lib/colour_conversion.cc | 14 ++++++++++++++
 src/lib/mpeg2_encoder.cc     | 20 +++++++++++++++++++-
 2 files changed, 33 insertions(+), 1 deletion(-)

(limited to 'src/lib')

diff --git a/src/lib/colour_conversion.cc b/src/lib/colour_conversion.cc
index b5d6cd4c5..5dc633f60 100644
--- a/src/lib/colour_conversion.cc
+++ b/src/lib/colour_conversion.cc
@@ -138,6 +138,12 @@ ColourConversion::ColourConversion(cxml::NodePtr node, int version)
 	} else {
 		_out_j2k = make_shared<dcp::IdentityTransferFunction>();
 	}
+
+	if (auto mpeg2_gamma = node->optional_number_child<double>("OutputMPEG2Gamma")) {
+		_out_mpeg2 = make_shared<dcp::GammaTransferFunction>(*mpeg2_gamma);
+	} else {
+		_out_mpeg2 = make_shared<dcp::IdentityTransferFunction>();
+	}
 }
 
 boost::optional<ColourConversion>
@@ -186,6 +192,10 @@ ColourConversion::as_xml(xmlpp::Element* element) const
 	if (auto gf = dynamic_pointer_cast<const dcp::GammaTransferFunction>(_out_j2k)) {
 		cxml::add_text_child(element, "OutputGamma", fmt::to_string(gf->gamma()));
 	}
+
+	if (auto gf = dynamic_pointer_cast<const dcp::GammaTransferFunction>(_out_mpeg2)) {
+		cxml::add_text_child(element, "OutputMPEG2Gamma", fmt::to_string(gf->gamma()));
+	}
 }
 
 optional<size_t>
@@ -240,6 +250,10 @@ ColourConversion::identifier() const
 		digester.add(gf->gamma());
 	}
 
+	if (auto gf = dynamic_pointer_cast<const dcp::GammaTransferFunction>(_out_mpeg2)) {
+		digester.add(gf->gamma());
+	}
+
 	return digester.get();
 }
 
diff --git a/src/lib/mpeg2_encoder.cc b/src/lib/mpeg2_encoder.cc
index 38388431d..c9ee6c491 100644
--- a/src/lib/mpeg2_encoder.cc
+++ b/src/lib/mpeg2_encoder.cc
@@ -22,6 +22,7 @@
 #include "mpeg2_encoder.h"
 #include "writer.h"
 #include <dcp/ffmpeg_image.h>
+#include <dcp/transfer_function.h>
 extern "C" {
 #include <libavutil/pixfmt.h>
 }
@@ -43,7 +44,24 @@ MPEG2Encoder::encode(shared_ptr<PlayerVideo> pv, dcpomatic::DCPTime time)
 {
 	VideoEncoder::encode(pv, time);
 
-	auto image = pv->image(force(AV_PIX_FMT_YUV420P), VideoRange::VIDEO, false);
+	shared_ptr<Image> image;
+
+	if (auto conversion = pv->colour_conversion()) {
+		if (!conversion->in()->about_equal(conversion->out_mpeg2(), 0.05)) {
+			image = pv->image([](AVPixelFormat) { return AV_PIX_FMT_RGB24; }, VideoRange::VIDEO, Image::Alignment::PADDED, false);
+			dcp::convert_rgb_gamma(image->data()[0], image->size(), image->stride()[0], *conversion);
+			image = image->convert_pixel_format(conversion->yuv_to_rgb(), AV_PIX_FMT_YUV420P, Image::Alignment::COMPACT, false);
+		}
+	}
+
+	if (!image) {
+		image = pv->image(
+			[](AVPixelFormat) { return AV_PIX_FMT_YUV420P; },
+			VideoRange::VIDEO,
+			Image::Alignment::COMPACT,
+			false
+			);
+	}
 
 	dcp::FFmpegImage ffmpeg_image(time.get() * _film->video_frame_rate() / dcpomatic::DCPTime::HZ);
 
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