Why does this python program convert blue color to yellow when working on the image?

Question

The thing is, I have this code:

import numpy as np
import cv2
from matplotlib import pyplot
img = cv2.imread('C:\Users\Niranjan\Desktop\FINAL YEAR PROJECT\Python Codes\obj.jpg')
rows,cols,ch = img.shape
pts1 = np.float32([[170,220],[466,221],[110,540],[528,541]])
pts2 = np.float32([[0,0],[530,0],[0,542],[530,542]])
M = cv2.getPerspectiveTransform(pts1,pts2)
dst = cv2.warpPerspective(img,M,(530,542))
pyplot.subplot(121),pyplot.imshow(img),pyplot.title('Input')
pyplot.subplot(122),pyplot.imshow(dst),pyplot.title('Output')
pyplot.show()

So blue is getting converted to yellow... is this a hint that RGB is converted to CMYK?

what should i do to keep my colors same as original?

*note: This is not an ad

I think it is because you are reading the file as RGB but pyplot is treating it as BGR — Amitay Nachmani, Mar 05 '17 at 10:57

score 3 · Answer 1 · edited May 23 '17 at 12:17

3

For historical reasons, OpenCV uses a BGR color representation. You can convert your images before displaying them using:

image = cv2.cvtColor(image, cv2.COLOR_BGR2RGB)

edited May 23 '17 at 12:17

Community

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1

answered Mar 05 '17 at 11:14

Étienne Millon

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i tried this but it gives me "Assertion Failed open cv error: (-215) scn == 3 || scn == 4 in function cvtColor – Niranjan Dixit Mar 05 '17 at 12:13

score 3 · Accepted Answer · answered Mar 05 '17 at 16:52

3

Use img = img[:,:,::-1] to shuffle the color channels form BGR to RGB.

pyplot.subplot(121),pyplot.imshow(img[:,:,::-1] ),pyplot.title('Input')
pyplot.subplot(122),pyplot.imshow(dst[:,:,::-1] ),pyplot.title('Output')
pyplot.show()

Here is the matplotlib input and output images.

answered Mar 05 '17 at 16:52

thewaywewere

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Thank you very much sir! – Niranjan Dixit Mar 05 '17 at 17:32
@NiranjanDixit could you give me a vote up if my answer helps? – thewaywewere Apr 26 '17 at 19:17

Why does this python program convert blue color to yellow when working on the image?

2 Answers2