main.py

from uuid import uuid4
from os import remove
from PIL import Image, ImageDraw
from pydantic import BaseModel
from fastapi import FastAPI
from fastapi.staticfiles import StaticFiles
from starlette.responses import FileResponse
from starlette.background import BackgroundTask


class ImageData(BaseModel):
    strokes: list
    box: list


app = FastAPI()


@app.post("/transform")
async def transform(image_data: ImageData):
    filepath = "./images/" + str(uuid4()) + ".png"
    img = transform_img(image_data.strokes, image_data.box)
    img.save(filepath)

    return FileResponse(filepath, background=BackgroundTask(remove, path=filepath))


app.mount("/", StaticFiles(directory="static", html=True), name="static")


def transform_img(strokes, box):
    # Calc cropped image size
    width = box[2] - box[0]
    height = box[3] - box[1]

    image = Image.new("RGB", (width, height), color=(255, 255, 255))
    image_draw = ImageDraw.Draw(image)

    for stroke in strokes:
        positions = []
        for i in range(0, len(stroke[0])):
            positions.append((stroke[0][i], stroke[1][i]))
        image_draw.line(positions, fill=(0, 0, 0), width=3)

    return image.resize((28, 28))