Python-opencv: Read image data from stdin

Question

How can I read image data from stdin rather than from a file?

With the C++ interface, it seems to be possible: https://stackoverflow.com/a/5458035/908515. The function imdecode is available in Python as well. But it expects a numpy array as (first) argument. I have no idea how to convert the stdin data.

This is what I tried:

import cv
import sys
import numpy

stdin = sys.stdin.read()
im = cv.imdecode(numpy.asarray(stdin), 0)

Result: TypeError: <unknown> data type = 18 is not supported

@Igonato: as JPEG data, e.g. `my_program < example.jpeg`. Actually, I'm getting the data from a webcam with another program and I want to avoid creating temporary files. — undur_gongor, Mar 14 '13 at 16:59

score 12 · Accepted Answer · edited May 23 '17 at 12:02

12

Looks like python stdin buffer is too small for images. You can run your program with -u flag in order to remove buffering. More details in this answer.

Second is that numpy.asarray probably not right way to get numpy array from the data, numpy.frombuffer works for me very well.

So here is working code (only I used cv2 instead of cv hope it wont matter too much):

import sys
import cv2
import numpy

stdin = sys.stdin.read()
array = numpy.frombuffer(stdin, dtype='uint8')
img = cv2.imdecode(array, 1)
cv2.imshow("window", img)
cv2.waitKey()

Can be executed this way:

python -u test.py < cat.jpeg

edited May 23 '17 at 12:02

Community

1
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answered Mar 14 '13 at 18:27

Igonato

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Thank you. I did not encounter issues with stdin buffering. It is working without `-u`. – undur_gongor Mar 15 '13 at 20:22
1

I've been trying to figure this out for a long time. Thanks for posting this. Didn't know of the frombuffer function. – Profane Jul 03 '13 at 06:20

score 2 · Answer 2 · answered Jul 04 '21 at 12:54

I've tried the above solution but could not make it work. Replacing sys.stdin.read() by sys.stdin.buffer.read() did the job:

import numpy as np
import sys
import cv2

data = sys.stdin.buffer.read()
image = np.frombuffer(data, dtype="uint8")
image = cv2.imdecode(image, cv2.IMREAD_COLOR)
cv2.imshow('stdin Image',image)
cv2.waitKey()

Python-opencv: Read image data from stdin

2 Answers2