Python Opencv img.item() performance too slow

Question

i wrote this little code to compare the pixels greyscale values of two 100×100 jpeg images.
However, the performance is very disappointing (1.5 Seconds for 10.000 comparisons). Is there a way to achieve a better performance?

Here is the code:

import cv2
import numpy as np
import math
import datetime


img1 = cv2.imread('Testbild 2014-08-23 17:27:25.141362.jpeg', cv2.IMREAD_GRAYSCALE)
img2 = cv2.imread('Testbild 2014-08-23 17:27:25.061802.jpeg', cv2.IMREAD_GRAYSCALE)

height, width =img1.shape
cnt = 0
threshold = 10

print("start:" + str(datetime.datetime.now()))
for y in range(0 , height):
    for x in range(0 , width):
        val1 = img1.item(y,x)
        val2 = img2.item(y,x)
        diff_abs = math.fabs(int(val1)-int(val2))
        if diff_abs > threshold:
            cnt += 1      
        if x == height and y == width:
            break
        if x == height:
            x=0
        if y == width:
            y=0     

print("end:" + str(datetime.datetime.now()))
print("Result: " + str(cnt))

Many thanks for your answers!

Asked By: flo

||

Source

Answer 1

The double loop:

for y in range(0 , height):
    for x in range(0 , width):
        val1 = img1.item(y,x)
        val2 = img2.item(y,x)
        diff_abs = math.fabs(int(val1)-int(val2))
        if diff_abs > threshold:
            cnt += 1      
        if x == height and y == width:
            break
        if x == height:
            x=0
        if y == width:
            y=0

can be replaced by:

diff_abs = np.abs(img1-img2)
cnt = (diff_abs > threshold).sum()

This takes advantage of NumPy array’s ability to do fast element-wise arithmetic.

The condition

 x == height and y == width

is never true. If height < width, then y will never equal width (since y is in range(0, height)). If height > width, then x will never equal height. and if height == width, then neither x nor y will ever equal height.

The condition

    if x == height:
        x=0

does nothing useful, since even if x == height, the assignment to x is lost on the next iteration of the loop.

The same goes for

    if y == width:
        y=0

Answered By: unutbu

Python Opencv img.item() performance too slow

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