Convert YUVj420p pixel format to RGB888 using gstreamer

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()