Question

A few days back, I started using new OpenCV-Python interface, cv2.

My question is regarding the comparison of cv and cv2 interface.

Regarding the ease of use, new cv2 interface has improved far greater, and it is really easy and fun to work with cv2.

But what about speed?

I made two small code snipplets, one in cv and another in cv2, to check the performances. Both does the same function, access pixels of an image, test it, make some modifications, etc.

Below is the code:

---

cv2 interface:

import time
import numpy as np
import cv2

gray = cv2.imread('sir.jpg',0)
width = gray.shape[0]
height = gray.shape[1]
h = np.empty([width,height,3])
t = time.time()
for i in xrange(width):
    for j in xrange(height):
        if gray[i,j]==127:
            h[i,j]=[255,255,255]
        elif gray[i,j]>127:
            h[i,j]=[0,0,255-gray[i,j]]
        else:
            h[i,j]=[gray[i,j],0,0]
t2 = time.time()-t
print "time taken = ",t2

=====================================================

And result is:

time taken = 14.4029130936

======================================================

cv interface:

import cv,time

gray = cv.LoadImage('sir.jpg',0)
h = cv.CreateImage(cv.GetSize(gray),8,3)

t=time.time()

for i in xrange(gray.width):
    for j in xrange(gray.height):
        k = cv.Get2D(gray,j,i)[0]
        if k==127:
            cv.Set2D(h,j,i,(255,255,255))
        elif k>127:
            cv.Set2D(h,j,i,(0,0,255-k))
        else:
            cv.Set2D(h,j,i,(k,0,0))

t2 = time.time()-t
print "time taken = ",t2
cv.ShowImage('img',h)
cv.WaitKey(0)

======================================================

The result is:

time taken = 1.16368889809

=======================================================

See, here old cv is about 12 times faster than cv2. And resulting images are same. (input image is of size 720×540)

Why does this happen?

Is cv2 slower compared to cv?

Or am I making any mistake here? Is there a faster method in cv2 for the above code?

Answer 1

The image returned by cv2.imread() is an array object of NumPy. So you can use NumPy’s functions to speedup calculation.

The following program shows how to speedup your origin for loop version by using item(), itemset() method of ndarray object.

import time
import numpy as np
import cv2

gray = cv2.imread('lena_full.jpg',0)
height, width = gray.shape
h = np.empty((height,width,3), np.uint8)

t = time.time()
for i in xrange(height):
    for j in xrange(width):
        k = gray.item(i, j)
        if k == 127:
            h.itemset(i, j, 0, 255)
            h.itemset(i, j, 1, 255)
            h.itemset(i, j, 2, 255)
        elif k > 127:
            h.itemset(i, j, 0, 0)
            h.itemset(i, j, 1, 0)
            h.itemset(i, j, 2, 255-k)
        else:
            h.itemset(i, j, 0, k)
            h.itemset(i, j, 1, 0)
            h.itemset(i, j, 2, 0)
print time.time()-t

And the following program show how to create the palette first, and use NumPy’s array index to get the result:

t = time.time()
palette = []
for i in xrange(256):
    if i == 127:
        palette.append((255, 255, 255))
    elif i > 127:
        palette.append((0,0,255-i))
    else:
        palette.append((i, 0, 0))
palette = np.array(palette, np.uint8)

h2 = palette[gray]

print time.time() - t

print np.all(h==h2)

The output is:

0.453000068665
0.0309998989105
True

The cv version output is :

0.468999862671

Note: the length of axis 0 is the height of the image, the length of axis 1 is the width of the image