Conversion of cv::Mat to GstBuffer

[Michael Gruner]

Hi Saurabh

Yo may convert from a cv::Mat to a GstBuffer without a memory copy by using gst_buffer_new_wrapped:

https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstBuffer.html#gst-buffer-new-wrapped

Something like: 
buffer = gst_buffer_new_wrapped (mat.data, mat.total()*mat.elemSize());

In order to make a cv::Mat from a GstBuffer you first need to take the data out of the buffer. 

GstMapInfo info;
gst_buffer_map (buffer, &info, GST_MAP_READ); // or write
mat = cv::Mat(height, width, CV_8C3, map.data); // change your format accordingly
...
gst_buffer_unmap (buffer, &info);

Hope it helps.

Michael
www.ridgerun.com

> On Mar 13, 2019, at 7:15 AM, Saurabh Bora <saurabh9bora at gmail.com> wrote:
> 
> Hi Experts!!, 
> 
> I am trying to read opencv images locally and push them into gstreamer pipeline. I am using appsrc element with "need-data" signal attached to call-back function.
> Please look at the code below. 
> 
> //------------------------------------------------------------------------------------------------------
> #include <gst/gst.h>
> #include <opencv2/opencv.hpp>
> #include <opencv2/highgui.hpp>
> 
> static GMainLoop *loop;
> 
> static void
> cb_need_data(GstElement *appsrc,
> 	guint       unused_size,
> 	gpointer    user_data)
> {
> 	static GstClockTime timestamp = 0;
> 	GstBuffer *buffer;
> 	guint size, depth, height, width, step, channels;
> 	GstFlowReturn ret;
> 	guchar *data1;
> 	GstMapInfo map;
> 
> 	cv::Mat img = cv::imread("C:/Users/212730892.LOGON/Desktop/lena.jpg");
> 	height = img.size().height;
> 	width = img.size().width;
> 	//step = img.widthStep;
> 	channels = img.channels();
> 	//depth = img->depth;
> 	data1 = (guchar *)img.data;
> 	size = height * width* channels;
> 
> 	// Copy cv::Mat to GstBuffer
> 	buffer = gst_buffer_new_allocate(NULL, size, NULL);
> 	gst_buffer_map(buffer, &map, GST_MAP_WRITE);
> 	memcpy((guchar *)map.data, data1, gst_buffer_get_size(buffer));
> 
> 	//Convert GstBuffer back to cv::Mat and write it, to check if data was correctly copied. //This is where I get to know that data is not correct.
> 	cv::Mat img2(cv::Size(134, 134), CV_8UC3, (char*)(buffer));
> 	cv::imwrite("C:/Users/212730892.LOGON/Desktop/dummy1.jpg", img2);
> 
> 	GST_BUFFER_PTS(buffer) = timestamp;
> 	GST_BUFFER_DURATION(buffer) = gst_util_uint64_scale_int(1, GST_SECOND, 2);
> 
> 	timestamp += GST_BUFFER_DURATION(buffer);
> 
> 	g_signal_emit_by_name(appsrc, "push-buffer", buffer, &ret);
> 	//gst_buffer_unref(buffer);
> 
> 	if (ret != GST_FLOW_OK) {
> 		/* something wrong, stop pushing */
> 		g_main_loop_quit(loop);
> 	}
> }
> 
> gint
> main(gint   argc,
> 	gchar *argv[])
> {
> 	GstElement *pipeline, *appsrc, *conv, *videosink;
> 
> 	/* init GStreamer */
> 	gst_init(&argc, &argv);
> 	loop = g_main_loop_new(NULL, FALSE);
> 
> 	/* setup pipeline */
> 	pipeline = gst_pipeline_new("pipeline");
> 	appsrc = gst_element_factory_make("appsrc", "source");
> 	conv = gst_element_factory_make("videoconvert", "conv");
> 	videosink = gst_element_factory_make("autovideosink", "videosink");
> 
> 	/* setup */
> 	g_object_set(G_OBJECT(appsrc), "caps",
> 		gst_caps_new_simple("video/x-raw",
> 			"format", G_TYPE_STRING, "RGB",
> 			"width", G_TYPE_INT, 134,
> 			"height", G_TYPE_INT, 134,
> 			"framerate", GST_TYPE_FRACTION, 1, 1,
> 			NULL), NULL);
> 	gst_bin_add_many(GST_BIN(pipeline), appsrc, conv, videosink, NULL);
> 	gst_element_link_many(appsrc, conv, videosink, NULL);
> 	//g_object_set (videosink, "device", "/dev/video0", NULL);
> 	/* setup appsrc */
> 	g_object_set(G_OBJECT(appsrc),
> 		"stream-type", 0,
> 		"format", GST_FORMAT_TIME, NULL);
> 	g_signal_connect(appsrc, "need-data", G_CALLBACK(cb_need_data), NULL);
> 
> 	/* play */
> 	gst_element_set_state(pipeline, GST_STATE_PLAYING);
> 	g_main_loop_run(loop);
> 
> 	if (gst_element_set_state(pipeline, GST_STATE_NULL) == GST_STATE_CHANGE_FAILURE)
> 	{
> 		g_printerr("Unable to set the pipeline to the playing state.\n");
> 	}
> 
> 	// Free resources
> 	gst_object_unref(GST_OBJECT(pipeline));
> 	g_main_loop_unref(loop);
> 
> 	return 0;
> }
> //-------------------------------------------------------------------------------------------------------------------------------------
> 
> In cb_need_data, the part where copying I am copying to GstBuffer seems to be wrong and it does not work as expected. 
> Can anyone, please help me understand, how can I copy the image data perfectly and pass it downstream to other elements without losing any data?
> 
> -- 
> Thanks and Regards,
> Saurabh Bora
> 
> PH NO : 7038166900
> EMAIL : saurabh9bora at gmail.com
>              saurabh9bora at outlook.com
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