Displaying opencv image using python flask

Question

I'm doing some processing on an image using opencv and using the python flask api. I'd like to display the image in the browser.

import cv2
from flask import Flask, request, make_response
import base64
import numpy as np
import urllib

app = Flask(__name__)


@app.route('/endpoint', methods=['GET'])
def process():
    image_url = request.args.get('imageurl')
    requested_url = urllib.urlopen(image_url)
    image_array = np.asarray(bytearray(requested_url.read()), dtype=np.uint8)
    img = cv2.imdecode(image_array, -1)


    # Do some processing, get output_img

    retval, buffer = cv2.imencode('.png', output_img)
    png_as_text = base64.b64encode(buffer)
    response = make_response(png_as_text)
    response.headers['Content-Type'] = 'image/png'
    return response

if __name__ == '__main__':
    app.run(debug=True)

However, I'm getting empty, invalid image as output. What am I doing wrong?

Answer 1

As mentioned in the comments, you need to return the bytes of an image, not a base64 string.

Try the following:

retval, buffer = cv2.imencode('.png', output_img)
response = make_response(buffer.tobytes())