gstreamer: keep same transfer with that in negotiated caps

The conversions back and forth between GStreamer colorimetry and
libcamera color space are not invariant for the bt601 colorimetry.
The reason is that Rec709 transfer function defined in GStreamer
as GST_VIDEO_TRANSFER_BT709 (5), is to be replaced by its alias
GST_VIDEO_TRANSFER_BT601 (16) only for the case of bt601 (aka 2:4:16:4)
colorimetry - see [1].

Currently the composition of the GStreamer/libcamera conversions:
colorimetry_from_colorspace (colorspace_from_colorimetry (bt601))
returns 2:4:5:4 instead of the expected 2:4:16:4 (bt601). This
causes negotiation error when the downstream element explicitly
expects bt601 colorimetry.

Minimal example to reproduce the issue is with a pipeline handler
that do not set the optional color space in the stream configuration,
for instance vimc or imx8-isi:
export LIBCAMERA_PIPELINES_MATCH_LIST="vimc,imx8-isi"
gst-launch-1.0 -v libcamerasrc ! video/x-raw,colorimetry=bt601 ! fakesink

Above pipeline fails to start. This change memorizes downstream required
transfer function when mapped libcamera transfer is Rec709 in
gst_libcamera_configure_stream_from_caps(), and restores the transfer
function in gst_libcamera_stream_formats_to_caps().

[1] https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/724

Bug: https://bugs.libcamera.org/show_bug.cgi?id=150
Signed-off-by: Hou Qi <qi.hou@nxp.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

3 files changed, 19 insertions, 12 deletions

diff --git a/src/gstreamer/gstlibcamera-utils.cpp b/src/gstreamer/gstlibcamera-utils.cpp
index 732987ef..a466b305 100644
--- a/src/gstreamer/gstlibcamera-utils.cpp
+++ b/src/gstreamer/gstlibcamera-utils.cpp
@@ -85,7 +85,7 @@ static struct {
 };
 
 static GstVideoColorimetry
-colorimetry_from_colorspace(const ColorSpace &colorSpace)
+colorimetry_from_colorspace(const ColorSpace &colorSpace, GstVideoTransferFunction transfer)
 {
 	GstVideoColorimetry colorimetry;
 
@@ -113,6 +113,8 @@ colorimetry_from_colorspace(const ColorSpace &colorSpace)
 		break;
 	case ColorSpace::TransferFunction::Rec709:
 		colorimetry.transfer = GST_VIDEO_TRANSFER_BT709;
+		if (transfer != GST_VIDEO_TRANSFER_UNKNOWN)
+			colorimetry.transfer = transfer;
 		break;
 	}
 
@@ -144,7 +146,8 @@ colorimetry_from_colorspace(const ColorSpace &colorSpace)
 }
 
 static std::optional<ColorSpace>
-colorspace_from_colorimetry(const GstVideoColorimetry &colorimetry)
+colorspace_from_colorimetry(const GstVideoColorimetry &colorimetry,
+			    GstVideoTransferFunction *transfer)
 {
 	std::optional<ColorSpace> colorspace = ColorSpace::Raw;
 
@@ -188,6 +191,7 @@ colorspace_from_colorimetry(const GstVideoColorimetry &colorimetry)
 	case GST_VIDEO_TRANSFER_BT2020_12:
 	case GST_VIDEO_TRANSFER_BT709:
 		colorspace->transferFunction = ColorSpace::TransferFunction::Rec709;
+		*transfer = colorimetry.transfer;
 		break;
 	default:
 		GST_WARNING("Colorimetry transfer function %d not mapped in gstlibcamera",
@@ -379,7 +383,8 @@ gst_libcamera_stream_formats_to_caps(const StreamFormats &formats)
 }
 
 GstCaps *
-gst_libcamera_stream_configuration_to_caps(const StreamConfiguration &stream_cfg)
+gst_libcamera_stream_configuration_to_caps(const StreamConfiguration &stream_cfg,
+					   GstVideoTransferFunction transfer)
 {
 	GstCaps *caps = gst_caps_new_empty();
 	GstStructure *s = bare_structure_from_format(stream_cfg.pixelFormat);
@@ -390,7 +395,7 @@ gst_libcamera_stream_configuration_to_caps(const StreamConfiguration &stream_cfg
 			  nullptr);
 
 	if (stream_cfg.colorSpace) {
-		GstVideoColorimetry colorimetry = colorimetry_from_colorspace(stream_cfg.colorSpace.value());
+		GstVideoColorimetry colorimetry = colorimetry_from_colorspace(stream_cfg.colorSpace.value(), transfer);
 		g_autofree gchar *colorimetry_str = gst_video_colorimetry_to_string(&colorimetry);
 
 		if (colorimetry_str)
@@ -405,9 +410,8 @@ gst_libcamera_stream_configuration_to_caps(const StreamConfiguration &stream_cfg
 	return caps;
 }
 
-void
-gst_libcamera_configure_stream_from_caps(StreamConfiguration &stream_cfg,
-					 GstCaps *caps)
+void gst_libcamera_configure_stream_from_caps(StreamConfiguration &stream_cfg,
+					      GstCaps *caps, GstVideoTransferFunction *transfer)
 {
 	GstVideoFormat gst_format = pixel_format_to_gst_format(stream_cfg.pixelFormat);
 	guint i;
@@ -495,7 +499,7 @@ gst_libcamera_configure_stream_from_caps(StreamConfiguration &stream_cfg,
 		if (!gst_video_colorimetry_from_string(&colorimetry, colorimetry_str))
 			g_critical("Invalid colorimetry %s", colorimetry_str);
 
-		stream_cfg.colorSpace = colorspace_from_colorimetry(colorimetry);
+		stream_cfg.colorSpace = colorspace_from_colorimetry(colorimetry, transfer);
 	}
 }
 
diff --git a/src/gstreamer/gstlibcamera-utils.h b/src/gstreamer/gstlibcamera-utils.h
index cab1c814..4978987c 100644
--- a/src/gstreamer/gstlibcamera-utils.h
+++ b/src/gstreamer/gstlibcamera-utils.h
@@ -16,9 +16,10 @@
 #include <gst/video/video.h>
 
 GstCaps *gst_libcamera_stream_formats_to_caps(const libcamera::StreamFormats &formats);
-GstCaps *gst_libcamera_stream_configuration_to_caps(const libcamera::StreamConfiguration &stream_cfg);
+GstCaps *gst_libcamera_stream_configuration_to_caps(const libcamera::StreamConfiguration &stream_cfg,
+						    GstVideoTransferFunction transfer);
 void gst_libcamera_configure_stream_from_caps(libcamera::StreamConfiguration &stream_cfg,
-					      GstCaps *caps);
+					      GstCaps *caps, GstVideoTransferFunction *transfer);
 void gst_libcamera_get_framerate_from_caps(GstCaps *caps, GstStructure *element_caps);
 void gst_libcamera_clamp_and_set_frameduration(libcamera::ControlList &controls,
 					       const libcamera::ControlInfoMap &camera_controls,
diff --git a/src/gstreamer/gstlibcamerasrc.cpp b/src/gstreamer/gstlibcamerasrc.cpp
index 8efa25f4..5e9e843d 100644
--- a/src/gstreamer/gstlibcamerasrc.cpp
+++ b/src/gstreamer/gstlibcamerasrc.cpp
@@ -433,6 +433,8 @@ static bool
 gst_libcamera_src_negotiate(GstLibcameraSrc *self)
 {
 	GstLibcameraSrcState *state = self->state;
+	std::vector<GstVideoTransferFunction> transfer(state->srcpads_.size(),
+						       GST_VIDEO_TRANSFER_UNKNOWN);
 
 	g_autoptr(GstStructure) element_caps = gst_structure_new_empty("caps");
 
@@ -448,7 +450,7 @@ gst_libcamera_src_negotiate(GstLibcameraSrc *self)
 
 		/* Fixate caps and configure the stream. */
 		caps = gst_caps_make_writable(caps);
-		gst_libcamera_configure_stream_from_caps(stream_cfg, caps);
+		gst_libcamera_configure_stream_from_caps(stream_cfg, caps, &transfer[i]);
 		gst_libcamera_get_framerate_from_caps(caps, element_caps);
 	}
 
@@ -476,7 +478,7 @@ gst_libcamera_src_negotiate(GstLibcameraSrc *self)
 		GstPad *srcpad = state->srcpads_[i];
 		const StreamConfiguration &stream_cfg = state->config_->at(i);
 
-		g_autoptr(GstCaps) caps = gst_libcamera_stream_configuration_to_caps(stream_cfg);
+		g_autoptr(GstCaps) caps = gst_libcamera_stream_configuration_to_caps(stream_cfg, transfer[i]);
 		gst_libcamera_framerate_to_caps(caps, element_caps);
 
 		if (!gst_pad_push_event(srcpad, gst_event_new_caps(caps)))