Accessing video RGB data

[Stefan Sauer]

On 10/01/2011 10:16 AM, Sami Pietila wrote:
> Hi,
>
> I am trying to access RGB data of image frames in a mov -file. Below
> is code that I have constructed using gstreamer examples and
> documentation. cb_have_data() doesn't seem to give me RGB data. I am
> setting the data to 0 (=black?), but I can see shortly a green screen
> on the video and then comes invalid pointer error.
when you see green on all 0, you are in a yuv format and not rgb. In the
code below the ffmpegcolorspace wont convert anything as xvimagsink will
prefer yuv. If you change the later to ximagesink you'll get RGB.

> How can I read RGB data from a video, frame by frame? I need also a
> way to control the flow of frames, in a way that once I have done with
> processing one frame I could ask gstreamer to give me next frame. Code
> examples would be great.

If you wants to modify the image data you better write an element.

Stefan

> Thanks
>
>
> #include <iostream>
> #include <gst/gst.h>
>
> GstElement *pipeline, *video;
>
> static gboolean
> cb_have_data (GstPad *pad, GstBuffer *buffer, gpointer u_data) {
>
>   guint16 *data = (guint16 *) GST_BUFFER_DATA (buffer);
>   GstCaps *caps = gst_buffer_get_caps(buffer);
>
>   char m_width[] = "width";
>   char m_height[] = "height";
>
>   GstStructure *structure = gst_caps_get_structure(caps, 0);
>
>   const GValue *width_gval = gst_structure_get_value(structure, m_width);
>   const GValue *height_gval = gst_structure_get_value(structure, m_height);
>
>   int width = g_value_get_int (width_gval);
>   int height = g_value_get_int(height_gval);
>
>   for (int y = 0; y < height; y++) {
> 	  for (int x = 0; x < width; x++) {
> 		  data[width*y + x] = 0;
> 	  }
>   }
>
>   return TRUE;
> }
>
> static void
> cb_newpad (GstElement *decodebin,
> 	   GstPad     *pad,
> 	   gboolean    last,
> 	   gpointer    data)
> {
>   GstCaps *caps;
>   GstStructure *str;
>   GstPad *videopad;
>
>   videopad = gst_element_get_static_pad (video, "sink");
>   if (GST_PAD_IS_LINKED (videopad)) {
>     g_object_unref (videopad);
>     return;
>   }
>
>   caps = gst_pad_get_caps (pad);
>   str = gst_caps_get_structure (caps, 0);
>   if (!g_strrstr (gst_structure_get_name (str), "video")) {
>     gst_caps_unref (caps);
>     gst_object_unref (videopad);
>     return;
>   }
>   gst_caps_unref (caps);
>
>   gst_pad_link (pad, videopad);
>
>   g_object_unref (videopad);
> }
>
> gint main (gint argc, gchar *argv[])
> {
>   GMainLoop *loop;
>   GstElement *src, *dec, *filter, *cs, *sink;
>   GstPad *videopad;
>   GstBus *bus;
>
>   gst_init (&argc, &argv);
>   loop = g_main_loop_new (NULL, FALSE);
>
>   pipeline = gst_pipeline_new ("pipeline");
>
>   bus = gst_pipeline_get_bus (GST_PIPELINE (pipeline));
>
>   src = gst_element_factory_make ("filesrc", "source");
>   g_object_set (G_OBJECT (src), "location",
> "/home/sammy/src/SamScanPlus/src/testi.mov", NULL);
>   dec = gst_element_factory_make ("decodebin", "decoder");
>   g_signal_connect (dec, "new-decoded-pad", G_CALLBACK (cb_newpad), NULL);
>   gst_bin_add_many (GST_BIN (pipeline), src, dec, NULL);
>   gst_element_link (src, dec);
>
>   video = gst_bin_new ("videobin");
>   cs = gst_element_factory_make ("ffmpegcolorspace", "cs");
>   videopad = gst_element_get_static_pad (cs, "sink");
>   sink = gst_element_factory_make ("xvimagesink", "sink");
>   gst_bin_add_many (GST_BIN (video), cs, sink, NULL);
>   gst_element_link_many(cs, sink, NULL);
>   gst_element_add_pad (video, gst_ghost_pad_new ("sink", videopad));
>   gst_object_unref (videopad);
>   gst_bin_add (GST_BIN (pipeline), video);
>
>   GstPad *pad = gst_element_get_pad (cs, "src");
>   gst_pad_add_buffer_probe (pad, G_CALLBACK (cb_have_data), NULL);
>   gst_object_unref (pad);
>
>   gst_element_set_state (pipeline, GST_STATE_PLAYING);
>   g_main_loop_run (loop);
>
>   gst_element_set_state (pipeline, GST_STATE_NULL);
>   gst_object_unref (GST_OBJECT (pipeline));
>
>   return 0;
> }
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