Python OpenCV load image from byte string

I’m trying to load image from string like as PHP function imagecreatefromstring

How can I do that?

I have MySQL blob field image. I’m using MySQLdb and don’t want create temporary file for working with images in PyOpenCV.

NOTE: need cv (not cv2) wrapper function

Answers:

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Method 1

This is what I normally use to convert images stored in database to OpenCV images in Python.

import numpy as np
import cv2
from cv2 import cv

# Load image as string from file/database
fd = open('foo.jpg')
img_str = fd.read()
fd.close()

# CV2
nparr = np.fromstring(img_str, np.uint8)
img_np = cv2.imdecode(nparr, cv2.CV_LOAD_IMAGE_COLOR) # cv2.IMREAD_COLOR in OpenCV 3.1

# CV
img_ipl = cv.CreateImageHeader((img_np.shape[1], img_np.shape[0]), cv.IPL_DEPTH_8U, 3)
cv.SetData(img_ipl, img_np.tostring(), img_np.dtype.itemsize * 3 * img_np.shape[1])

# check types
print type(img_str)
print type(img_np)
print type(img_ipl)

I have added the conversion from numpy.ndarray to cv2.cv.iplimage, so the script above will print:

<type 'str'>
<type 'numpy.ndarray'>
<type 'cv2.cv.iplimage'>

EDIT:
As of latest numpy 1.18.5 +, the np.fromstring raise a warning, hence np.frombuffer shall be used in that place.

Method 2

I think this answer provided on this stackoverflow question is a better answer for this question.

Quoting details (borrowed from @lamhoangtung from above linked answer)

import base64
import json
import cv2
import numpy as np

response = json.loads(open('./0.json', 'r').read())
string = response['img']
jpg_original = base64.b64decode(string)
jpg_as_np = np.frombuffer(jpg_original, dtype=np.uint8)
img = cv2.imdecode(jpg_as_np, flags=1)
cv2.imwrite('./0.jpg', img)

Method 3

I’ve try to use this code to create an opencv from a string containing a raw buffer (plain pixel data) and it doesn’t work in that peculiar case.

So here’s how to do that for this kind of data:

image = np.fromstring(im_str, np.uint8).reshape( h, w, nb_planes )

(but yes you need to know your image properties)

if your B and G channel is permuted, here’s how to fix it:

image = cv2.cvtColor(image, cv2.cv.CV_BGR2RGB)

Method 4

I was following the solution from @jabaldonedo but it seems it’s a bit old and need some adjustments.

I am using OpenCV 3.4.8.29 by the way.

im_path = 'path/to/foo.jpg'
with open(im_path, 'rb') as fp:
    im_b = fp.read()
image_np = np.frombuffer(im_b, np.uint8)
img_np = cv2.imdecode(image_np, cv2.IMREAD_COLOR)  

im_cv = cv2.imread(im_path)

print('Same image: {}'.format(np.all(im_cv == img_np)))

Same image: True

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0