VideoFrame.map

Use info and buffer to fill in the values of frame. frame is usually allocated on the stack, and you will pass the address to the #GstVideoFrame structure allocated on the stack; gstvideo.video_frame.VideoFrame.map will then fill in the structures with the various video-specific information you need to access the pixels of the video buffer. You can then use accessor macros such as GST_VIDEO_FRAME_COMP_DATA(), GST_VIDEO_FRAME_PLANE_DATA(), GST_VIDEO_FRAME_COMP_STRIDE(), GST_VIDEO_FRAME_PLANE_STRIDE() etc. to get to the pixels.

GstVideoFrame vframe;
...
// set RGB pixels to black one at a time
if (gst_video_frame_map (&vframe, video_info, video_buffer, GST_MAP_WRITE)) {
  guint8 *pixels = GST_VIDEO_FRAME_PLANE_DATA (vframe, 0);
  guint stride = GST_VIDEO_FRAME_PLANE_STRIDE (vframe, 0);
  guint pixel_stride = GST_VIDEO_FRAME_COMP_PSTRIDE (vframe, 0);

  for (h = 0; h < height; ++h) {
    for (w = 0; w < width; ++w) {
      guint8 *pixel = pixels + h * stride + w * pixel_stride;

      memset (pixel, 0, pixel_stride);
    }
  }

  gst_video_frame_unmap (&vframe);
}
...

All video planes of buffer will be mapped and the pointers will be set in frame->data.

The purpose of this function is to make it easy for you to get to the video pixels in a generic way, without you having to worry too much about details such as whether the video data is allocated in one contiguous memory chunk or multiple memory chunks (e.g. one for each plane); or if custom strides and custom plane offsets are used or not (as signalled by GstVideoMeta on each buffer). This function will just fill the #GstVideoFrame structure with the right values and if you use the accessor macros everything will just work and you can access the data easily. It also maps the underlying memory chunks for you.