Read frames from GStreamer pipeline in opencv (cv::Mat)

pchaurasia pchaurasia at
Mon May 29 19:03:50 UTC 2017

Hi Folks,

I am looking to read frames from my Gstreamer pipeline into opencv data
structure. I am using 'new_sample' signal of appsink to read frames. The map
derived from the sample, read off of appsink - does not seem to give right
size of the frame. 

  if (!gst_buffer_map ((buffer), &map_info, GST_MAP_READ)) {
    gst_buffer_unmap ((buffer), &map_info);
    return GST_FLOW_ERROR;

I believe map_info.size --> should give size of the frame. The frame size
that I see does not appear correct.

Following is my code -

#include "opencv2/objdetect/objdetect.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include "opencv/cv.h"
#include <gst/gst.h>
#include <gst/app/gstappsink.h>
#include <gst/gstelement.h>
#include <stdio.h>
#include <stdlib.h>

#include <unistd.h>
#include <pthread.h>

using namespace cv;
GstSample* buffer;        
cv::Mat frame;

int sampleno = 0;
GstFlowReturn CaptureGstBuffer(GstAppSink *sink, gpointer user_data)

  //prog_data* pd = (prog_data*)user_data;

  GstSample* sample = gst_app_sink_pull_sample(sink);

  if(sample == NULL) {
    return GST_FLOW_ERROR;

  GstBuffer* buffer = gst_sample_get_buffer(sample);
  GstMapInfo map_info;

  if (!gst_buffer_map ((buffer), &map_info, GST_MAP_READ)) {
    gst_buffer_unmap ((buffer), &map_info);
    return GST_FLOW_ERROR;

  //render using
//  frame = Mat::zeros(1080, 1920, CV_8UC3);
 // frame = cv::Mat(1080, 1920, CV_8UC3, (char *),

    //Mat grey;
    //cvtColor(frame, grey, CV_BGR2GRAY);

//if (!frame.empty())
//  imshow("test-gstreamer-video",grey);
//  waitKey(1);

  fprintf(stderr,"Got sample no  %d  %d\n",sampleno++,(int)map_info.size);

  gst_buffer_unmap ((buffer), &map_info);
  //gst_memory_unmap(memory, &map_info);

  return GST_FLOW_OK;

int main(int argc, char *argv[]) {
  GstElement *pipeline, *source, *caps, *sink;
  GstBus *bus;
  GstCaps *filtercaps;
  GstElement *tee, *vq1;
  GstMessage *msg;
  GstBin     *recorder;
  GstStateChangeReturn ret;
  GstPad      *srcpad,*sinkpad; 

  /* Initialize GStreamer */
  gst_init (&argc, &argv);

  /* Create the elements */
  source        = gst_element_factory_make ("nvcamerasrc", "source");
  sink          = gst_element_factory_make ("appsink", "sink");
  tee           = gst_element_factory_make ("tee", "videotee");
  vq1           = gst_element_factory_make ("queue", "q1");

  recorder = GST_BIN(gst_bin_new("recording-bin"));

  /* Create the empty pipeline */
  pipeline = gst_pipeline_new ("test-pipeline");

  if (!pipeline || !source || !sink || !tee ||  !vq1 ) {
    g_printerr ("Not all elements could be created.\n");
    return -1;

  caps = gst_element_factory_make ("capsfilter", "filter");
  g_assert (caps != NULL); /* should always exist */

  filtercaps = gst_caps_from_string("video/x-raw(memory:NVMM),
width=(int)1920, height=(int)1080, format=(string)I420,
framerate=(fraction)30/1 ");
  g_object_set (G_OBJECT (caps), "caps", filtercaps, NULL);
  gst_caps_unref (filtercaps);

  /* Modify the source's properties */
  //g_object_set (source, "pattern", 0, NULL);
  g_object_set (sink, "drop" , TRUE, NULL);
  g_object_set (sink, "new_sample" , TRUE, NULL);
  g_object_set (sink, "max-buffers" , 1, NULL);

   GstAppSinkCallbacks* appsink_callbacks =
   appsink_callbacks->eos = NULL;
   appsink_callbacks->new_preroll = NULL;
   appsink_callbacks->new_sample = CaptureGstBuffer;
   gst_app_sink_set_callbacks(GST_APP_SINK(sink), appsink_callbacks,  
(gpointer)NULL, free);

  /* Build the pipeline */
  gst_bin_add_many (GST_BIN (pipeline), source, caps, tee, vq1, sink, NULL);
  if (gst_element_link_many (source,caps,tee, vq1, sink, NULL) != TRUE) {
    g_printerr ("Elements could not be linked1.\n");
    gst_object_unref (pipeline);
    return -1;

  srcpad                = gst_element_get_request_pad(tee,"src_%u");
  sinkpad               = gst_element_get_static_pad(vq1,"sink");

  /* Start playing */
  ret = gst_element_set_state (pipeline, GST_STATE_PLAYING);
    g_printerr ("Unable to set the pipeline to the playing state.\n");
    gst_object_unref (pipeline);
    return -1;

  /* Wait until error or EOS */
  bus = gst_element_get_bus (pipeline);
  msg = gst_bus_timed_pop_filtered (bus, GST_CLOCK_TIME_NONE,

  /* Parse message */
  if (msg != NULL) {
    GError *err;
    gchar *debug_info;

    switch (GST_MESSAGE_TYPE (msg)) {
        gst_message_parse_error (msg, &err, &debug_info);
        g_printerr ("Error received from element %s: %s\n", GST_OBJECT_NAME
(msg->src), err->message);
        g_printerr ("Debugging information: %s\n", debug_info ? debug_info :
        g_clear_error (&err);
        g_free (debug_info);
      case GST_MESSAGE_EOS:
        g_print ("End-Of-Stream reached.\n");
        /* We should not reach here because we only asked for ERRORs and EOS
        g_printerr ("Unexpected message received.\n");
    gst_message_unref (msg);

  /* Free resources */
  gst_object_unref (bus);
  gst_element_set_state (pipeline, GST_STATE_NULL);
  gst_object_unref (pipeline);
  return 0;

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