# Python and OpenCV

Python-based software can publish streams to the server with the OpenCV library and its GStreamer plugin, acting as a [RTSP client](06-rtsp-clients.md). OpenCV must be compiled with support for GStreamer, by following this procedure:

```sh
sudo apt install -y libgstreamer1.0-dev libgstreamer-plugins-base1.0-dev gstreamer1.0-plugins-ugly gstreamer1.0-rtsp python3-dev python3-numpy
git clone --depth=1 -b 4.5.4 https://github.com/opencv/opencv
cd opencv
mkdir build && cd build
cmake -D CMAKE_INSTALL_PREFIX=/usr -D WITH_GSTREAMER=ON ..
make -j$(nproc)
sudo make install
```

You can check that OpenCV has been installed correctly by running:

```sh
python3 -c 'import cv2; print(cv2.getBuildInformation())'
```

Check that the output contains `GStreamer: YES`.

Videos can then be published with `cv2.VideoWriter`:

```python
from datetime import datetime
from time import sleep, time

import cv2
import numpy as np

fps = 15
width = 800
height = 600
colors = [
    (0, 0, 255),
    (255, 0, 0),
    (0, 255, 0),
]

out = cv2.VideoWriter('appsrc ! videoconvert' + \
    ' ! video/x-raw,format=I420' + \
    ' ! x264enc speed-preset=ultrafast bitrate=600 key-int-max=' + str(fps * 2) + \
    ' ! video/x-h264,profile=baseline' + \
    ' ! rtspclientsink location=rtsp://localhost:8554/mystream',
    cv2.CAP_GSTREAMER, 0, fps, (width, height), True)
if not out.isOpened():
    raise Exception("can't open video writer")

curcolor = 0
start = time()

while True:
    frame = np.zeros((height, width, 3), np.uint8)

    # create a rectangle
    color = colors[curcolor]
    curcolor += 1
    curcolor %= len(colors)
    for y in range(0, int(frame.shape[0] / 2)):
        for x in range(0, int(frame.shape[1] / 2)):
            frame[y][x] = color

    out.write(frame)
    print("%s frame written to the server" % datetime.now())

    now = time()
    diff = (1 / fps) - now - start
    if diff > 0:
        sleep(diff)
    start = now
```

The resulting stream will be available on path `/mystream`.