[Python] How to dynamically overlay images with ffmpeg

As a part of a bigger project, I'm trying to stream a live feed using ffmpeg through RTSP while also dynamically changing subtitles depending on the situation.

As of now, I'm able to live stream with no issue. I also came across a solution for the subtitles by using a text file.

However, I'd like to avoid having this text file in my project. I thought about creating a picture with the subtitles and overlaying it with the screen stream. However, with my current solution, the data is streamed only when I kill the running code (data streamed for a few seconds).

Here is the current code for this problem :

import subprocess
import threading
import string
import random
import time
import io
from PIL import Image, ImageDraw, ImageFont

RTSP_URL = "..."
ffmpeg = None

def generate_subtitle():
width = 640
height = 100
font_size = 32
while True:
if ffmpeg:
try:
text = ''.join(random.choice(string.ascii_uppercase + string.digits) for _ in range(10))

image = Image.new("RGBA", (width, height), (0, 0, 0, 128))
draw = ImageDraw.Draw(image)

try:
font = ImageFont.truetype("arial.ttf", font_size)
except IOError:
font = ImageFont.load_default()

bbox = draw.textbbox((0, 0), text, font=font)
text_width = bbox[2] - bbox[0]
text_height = bbox[3] - bbox[1]

x = (width - text_width) // 2
y = (height - text_height) // 2

draw.text((x, y), text, font=font, fill=(255, 255, 255, 255))
buffer = io.BytesIO()
image.save(buffer, format="PNG")
ffmpeg.stdin.write(buffer.getvalue())
ffmpeg.stdin.flush()
time.sleep(5)
except Exception as e:
print("Erreur d'envoi d'image :", e)
break
else:
time.sleep(1)

def run_ffmpeg():
global ffmpeg
ffmpeg = subprocess.Popen([
'ffmpeg',

# Input 0: capture desktop
"-f", "gdigrab",
"-offset_x", "0",
"-offset_y", "0",
"-video_size", "1920x1080",
"-i", "desktop",

# Input 1: PNG overlay from stdin
"-f", "image2pipe",
"-vcodec", "png",
"-i", "-",

# Filter to overlay Input 1 on Input 0
"-filter_complex", "[0:v][1:v]overlay=(main_w-overlay_w)/2main_h-overlay_h)-10",

# Output settings
"-vcodec", "libx264",
"-preset", "ultrafast",
"-tune", "zerolatency",
"-g", "30",
"-sc_threshold", "0",
"-f", "rtsp",
RTSP_URL
], stdin=subprocess.PIPE)

threading.Thread(target=run_ffmpeg, daemon=True).start()

threading.Thread(target=generate_subtitle, daemon=True).start()

while True:
time.sleep(1)



My question is how can I stream the data correctly ? If there is another solution to change dynamically the subtitles without using a text file or a temporary file I'd be glad to hear it.

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