Convert YUVj420p pixel format to RGB888 using gstreamer - opencv

Question

im using gstreamer 1.2 to feed frames from my IP camera to opencv program
the stream is (640*368 YUVj420p) and i want to convert it to RBG888 to be able to use it in my opencv program
so is there a way to use gstreamer to do that conversion ?
or do i have to do it by myself?
if so please give me the equation that do this conversion

Answer 1

After some trials with gstreamer i decided to do the conversion myself and it worked
First we have to understand the YUVj420p pixel format
As shown in the above image, the Y', U and V components in Y'UV420 are encoded separately in sequential blocks. A Y' value is stored for every pixel, followed by a U value for each 2×2 square block of pixels, and finally a V value for each 2×2 block. Corresponding Y', U and V values are shown using the same color in the diagram above. Read line-by-line as a byte stream from a device, the Y' block would be found at position 0, the U block at position x×y (6×4 = 24 in this example) and the V block at position x×y + (x×y)/4 (here, 6×4 + (6×4)/4 = 30).(copied)
here is the code to do it (python)
This code will show how to inject frame to opencv using gstreamer and make the converstion
import gi
gi.require_version('Gst', '1.0')
from gi.repository import GObject, Gst
import numpy as np
import cv2
GObject.threads_init()
Gst.init(None)
def YUV_stream2RGB_frame(data):
w=640
h=368
size=w*h
stream=np.fromstring(data,np.uint8) #convert data form string to numpy array
#Y bytes will start form 0 and end in size-1
y=stream[0:size].reshape(h,w) # create the y channel same size as the image
#U bytes will start from size and end at size+size/4 as its size = framesize/4
u=stream[size:(size+(size/4))].reshape((h/2),(w/2))# create the u channel its size=framesize/4
#up-sample the u channel to be the same size as the y channel and frame using pyrUp func in opencv2
u_upsize=cv2.pyrUp(u)
#do the same for v channel
v=stream[(size+(size/4)):].reshape((h/2),(w/2))
v_upsize=cv2.pyrUp(v)
#create the 3-channel frame using cv2.merge func watch for the order
yuv=cv2.merge((y,u_upsize,v_upsize))
#Convert TO RGB format
rgb=cv2.cvtColor(yuv,cv2.cv.CV_YCrCb2RGB)
#show frame
cv2.imshow("show",rgb)
cv2.waitKey(5)
def on_new_buffer(appsink):
sample = appsink.emit('pull-sample')
#get the buffer
buf=sample.get_buffer()
#extract data stream as string
data=buf.extract_dup(0,buf.get_size())
YUV_stream2RGB_frame(data)
return False
def Init():
CLI="rtspsrc name=src location=rtsp://192.168.1.20:554/live/ch01_0 latency=10 !decodebin ! appsink name=sink"
#simplest way to create a pipline
pipline=Gst.parse_launch(CLI)
#getting the sink by its name set in CLI
appsink=pipline.get_by_name("sink")
#setting some important properties of appsnik
appsink.set_property("max-buffers",20) # prevent the app to consume huge part of memory
appsink.set_property('emit-signals',True) #tell sink to emit signals
appsink.set_property('sync',False) #no sync to make decoding as fast as possible
appsink.connect('new-sample', on_new_buffer) #connect signal to callable func
def run():
pipline.set_state(Gst.State.PLAYING)
GObject.MainLoop.run()
Init()
run()

Answer 2

How exactly are you getting the frames from your camera? And how you inject it into your opencv application?
Supposing you get your frames outside of gstreamer you should use a pipeline like:
appsrc caps="video/x-raw, format=I420, width=640, height=368" ! videoconvert ! capsfilter caps="video/x-raw, format=RGB" ! appsink
And then use appsrc to inject the data and use appsink to receive it back. If you are getting your data from camera from http or v4l2 you can replace appsrc with souphttpsrc or v4l2src.