def do_transform_ip(self, inbuffer):

#2. Implement drawing with Cairo

     Have a look at PyCairo documentation to get an intuition how to use it. To implement common drawing tasks with Cairo use next guide.

# Initialize stride value in respect to buffer format and width to accelerate image processing 
# https://pycairo.readthedocs.io/en/latest/reference/surfaces.html#cairo.ImageSurface.format_stride_for_width
stride = cairo.ImageSurface.format_stride_for_width(cairo.FORMAT_RGB24, width)

# Create image surface from image buffer (buffer should be mapped with READ | WRITE flags)
# https://pycairo.readthedocs.io/en/latest/reference/surfaces.html#cairo.ImageSurface.create_for_data
surface = cairo.ImageSurface.create_for_data(buffer, cairo.FORMAT_RGB24,
                                             width, height, stride)

# Create cairo context to manipulate with image data
# Documentation: https://pycairo.readthedocs.io/en/latest/reference/context.html
context = cairo.Context(surface)

# Calculate bottom-right point to place image
overlay = self.overlay()
x = width - overlay.get_width()
y = height - overlay.get_height()

# Documentation: https://www.cairographics.org/FAQ/
# Question: How do I paint on surface?
context.set_source_surface(self.overlay(), x, y)
context.paint()

Put previous code to do_transform_ip() in GstOverlayCairo

Note: To learn how to make Gst.Buffer writable read this.

Note: To draw wih cairo we use cairo.FORMAT_RGB24.  So update Src/Sink caps should have RGBx format.

Gst.Caps.from_string("video/x-raw,format=RGBx")

#3. Implement drawing with OpenCV

     Next function draws transparent image over some other image. Approach was taken from here.

def draw_image(source, image, x, y):

    h, w = image.shape[:2]
 
    max_x, max_y = x + w, y + h
    alpha = image[:, :, 3] / 255.0
    for c in range(0, 3):
        color = image[:, :, c] * (alpha)
        beta = source[y:max_y, x:max_x, c] * (1.0 - alpha)
        source[y:max_y, x:max_x, c] = color + beta
    return source

     Put draw_image() to do_transform_ip() in GstOverlayOpenCv

#4. Test with “walking kitten” images and video

     The main goal is to draw “walking kitten” over video and measure its performance to check what plugin GstOverlayOpenCv or GstOverlayCairo works faster. For this purpose have a look at fpsdisplaysink plugin that allows to draw Average FPS on video.

     Gstreamer pipeline looks like this:

filesrc location=~/attachments/car.mpg ! decodebin ! videoconvert ! gstoverlayopencv name=overlay ! videoconvert ! fpsdisplaysink video-sink=gtksink sync=false

     Now launch with OpenCV:

python3 run.py -f ~/attachments/car.mpg  --images ~/attachments/cat --opencv --fps

     Now launch with Cairo:

python3 run.py -f ~/attachments/car.mpg  --images ~/attachments/cat --cairo --fps

Note: To load images from file with Cairo use next call: