Colour conversion from BGR to RGB in cv2 is slower than on python

edit

@Santhosh1, your experiment is unfair because ima2 = ima[..., ::-1] permutes only indices but not the data. That means to check identical operations you have to call numpy.ascontiguousarray In example,

import numpy as np

a = np.zeros([4, 5, 3], dtype=np.uint8)
a[:,:,0] = ord('b')
a[:,:,1] = ord('g')
a[:,:,2] = ord('r')

print a
print a.flags
print a.data

gives

[[[ 98 103 114]
  [ 98 103 114]
  [ 98 103 114]
  [ 98 103 114]
  [ 98 103 114]]

 [[ 98 103 114]
  [ 98 103 114]
  [ 98 103 114]
  [ 98 103 114]
  [ 98 103 114]]

 [[ 98 103 114]
  [ 98 103 114]
  [ 98 103 114]
  [ 98 103 114]
  [ 98 103 114]]

 [[ 98 103 114]
  [ 98 103 114]
  [ 98 103 114]
  [ 98 103 114]
  [ 98 103 114]]]

  C_CONTIGUOUS : True
  F_CONTIGUOUS : False
  OWNDATA : True
  WRITEABLE : True
  ALIGNED : True
  WRITEBACKIFCOPY : False
  UPDATEIFCOPY : False

bgrbgrbgrbgrbgrbgrbgrbgrbgrbgrbgrbgrbgrbgrbgrbgrbgrbgrbgrbgr

But

a = a[:,:,::-1]

print a
print a.flags
print a.data

cannot get raw data because an array became discontinuous

[[[114 103  98]
  [114 103  98]
  [114 103  98]
  [114 103  98]
  [114 103  98]]

 [[114 103  98]
  [114 103  98]
  [114 103  98]
  [114 103  98]
  [114 103  98]]

 [[114 103  98]
  [114 103  98]
  [114 103  98]
  [114 103  98]
  [114 103  98]]

 [[114 103  98]
  [114 103  98]
  [114 103  98]
  [114 103  98]
  [114 103  98]]]

  C_CONTIGUOUS : False
  F_CONTIGUOUS : False
  OWNDATA : False
  WRITEABLE : True
  ALIGNED : True
  WRITEBACKIFCOPY : False
  UPDATEIFCOPY : False

Traceback (most recent call last):
  File "test.py", line 16, in <module>
    print a.data
AttributeError: cannot get single-segment buffer for discontiguous array

Inspired by hearing about the "contiguous RAM" problems mentioned by dkurt, I went deeper and wrote a separate post with a long answer for everyone else who wants the fastest RGB/BGR conversion. It contains very important information. Spoiler alert: Do NOT use Numpy tricks!

https://answers.opencv.org/question/2...