server.py

from aiohttp import web
import asyncio

data = open("interlaced.png", "rb").read()

# offsets found via trial and error
# (although I now know how to calculate these programmatically, I might put something on github later)
a = data[:10500]
b = data[10500:33000]
c = data[33000:78500]
d = data[78500:]

CHUNK_SIZE = 512

async def handle(request):
        response = web.StreamResponse(
                status=200,
                reason="OK",
                headers={
                        "Cache-Control": "no-store"
                },
        )
        response.content_length = len(data)
        response.content_type = "image/png"

        await response.prepare(request)

        await response.write(a)
        await asyncio.sleep(3)
        await response.write(b)
        await asyncio.sleep(3)
        await response.write(c)
        await asyncio.sleep(3)
        await response.write(d)
        await response.write_eof()
        return response


app = web.Application()
app.add_routes([
        web.get('/', handle),
        web.get('/image.png', handle)
])


web.run_app(app, host="127.0.0.1", port=8081)