Reading image file (file storage object) using OpenCV

Question

I am sending an image by curl to flask server, i am using this curl command

curl -F "file=@image.jpg" http://localhost:8000/home

and I am trying to read the file using OpenCV on the server side.

On the server side I handle the image by this code

@app.route('/home', methods=['POST'])
def home():
    data =request.files['file']
    img = cv.imread(data)
    fact_resp= model.predict(img)
    return jsonify(fact_resp)

I am getting this error-

img = cv.imread(data)
TypeError: expected string or Unicode object, FileStorage found

How do I read the file using OpenCV on the server side?

Thanks!

See https://stackoverflow.com/questions/17170752/python-opencv-load-image-from-byte-string for a (simpler) way. Note that having cv2 available implies that numpy (np) is available. — Dave W. Smith, Dec 01 '17 at 14:27

score 42 · Accepted Answer · edited Jul 23 '22 at 15:34

42

I had similar issues while using opencv with flask server, for that first i saved the image to disk and read that image using saved filepath again using cv.imread()

Here is a sample code:

data =request.files['file']
filename = secure_filename(file.filename) # save file 
filepath = os.path.join(app.config['imgdir'], filename);
file.save(filepath)
cv.imread(filepath)

But now i have got even more efficient approach from here by using cv.imdecode() to read image from numpy array as below:

#read image file string data
filestr = request.files['file'].read()
#convert string data to numpy array
file_bytes = numpy.fromstring(filestr, numpy.uint8)
# convert numpy array to image
img = cv.imdecode(file_bytes, cv.IMREAD_UNCHANGED)

edited Jul 23 '22 at 15:34

Christoph Rackwitz

11,317
4
27
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answered Dec 28 '17 at 06:17

flamelite

2,654
3
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42

16

For 2nd trick, use img = cv2.imdecode(npimg,cv2.IMREAD_COLOR) for opencv 3.1 – user1 Feb 27 '18 at 15:37
From @user1 the same for version 4.5.5 – Sinal Jun 04 '22 at 14:18

AKSHAYAA VAIDYANATHAN · Answer 2 · 2017-12-29T15:43:27.057

After a bit of experimentation, I myself figured out a way to read the file using CV2. For this I first read the image using PIL.image method

This is my code,

@app.route('/home', methods=['POST'])
def home():
    data =request.files['file']
    img = Image.open(request.files['file'])
    img = np.array(img)
    img = cv2.resize(img,(224,224))
    img = cv2.cvtColor(np.array(img), cv2.COLOR_BGR2RGB)
    fact_resp= model.predict(img)
    return jsonify(fact_resp)

I wonder if there is any straight forward way to do this without using PIL.

score 5 · Answer 3 · edited Jul 23 '22 at 15:34

5

So incase you want to do something like ,

file = request.files['file']
img = cv.imread(file)

then do it like this

import numpy as np
file = request.files['file']
file_bytes = np.fromfile(file, np.uint8)
file = cv.imdecode(file_bytes, cv.IMREAD_COLOR)

Now you don't need to do cv.imread() again, but can use this in the next line of codes.

This applies to OpenCV v3.x and onwards

edited Jul 23 '22 at 15:34

Christoph Rackwitz

11,317
4
27
36

answered Mar 20 '20 at 10:47

Sahana M

485
6
4

score 1 · Answer 4 · edited Jul 23 '22 at 15:34

1

Two-line solution, change grayscale to what you need

 file_bytes = numpy.fromfile(request.files['image'], numpy.uint8)
 # convert numpy array to image
 img = cv.imdecode(file_bytes, cv.IMREAD_GRAYSCALE)

edited Jul 23 '22 at 15:34

Christoph Rackwitz

11,317
4
27
36

answered Sep 21 '18 at 04:44

James

523
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19

Reading image file (file storage object) using OpenCV

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