为什么关闭优化程序后时间变短

Question

我启动了cv.setUseOptimized，为什么时间变短了？你知道吗

import cv2 as cv
print(cv.useOptimized())
img1 = cv.imread('./image_lib/1.jpg')
e1 = cv.getTickCount()
img1 = cv.medianBlur(img1, 49)
e2 = cv.getTickCount()
t = (e2 - e1) / cv.getTickFrequency()
print(t)
cv.setUseOptimized(False)
print(cv.useOptimized())
img2 = cv.imread('./image_lib/1.jpg')
e1 = cv.getTickCount()
img2 = cv.medianBlur(img2, 49)
e2 = cv.getTickCount()
t = (e2 - e1) / cv.getTickFrequency()
print(t)

结果如下所示

True
0.3181056
False
0.0906346

Answer 1

我试过用timeit。你知道吗

def img1():
    print(cv.useOptimized())
    img1 = cv.imread('./image_lib/1.jpg')
    img1 = cv.medianBlur(img1, 49)
    return img1


def img2():
    print(cv.useOptimized())
    img2 = cv.imread('./image_lib/2.jpg')
    img2 = cv.medianBlur(img2, 49)
    return img2


n = 1
t1 = timeit.timeit(stmt=img1, number=n)
cv.setUseOptimized(False)
t2 = timeit.timeit(stmt=img2, number=n)
print(t1)
print(t2)

结果也是如此

True
False
0.31975700000000007
0.11581930000000007

为什么？你知道吗

---

获取构建信息

General configuration for OpenCV 4.1.0 =====================================
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  Platform:
    Timestamp:                   2019-04-11T18:54:08Z
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      requested:                 SSE4_1 SSE4_2 AVX FP16 AVX2 AVX512_SKX
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    C Compiler:                  C:/Program Files (x86)/Microsoft Visual Studio 14.0/VC/bin/x86_amd64/cl.exe
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                                -

---

def img1():
    print(cv.useOptimized())
    img1 = cv.imread('./image_lib/1.jpg')
    img1 = cv.medianBlur(img1, 49)
    return img1


def img2():
    print(cv.useOptimized())
    img2 = cv.imread('./image_lib/1.jpg')
    img2 = cv.medianBlur(img2, 49)
    return img2


n = 1
cv.setUseOptimized(False)
t1 = timeit.timeit(stmt=img1, number=n)
cv.setUseOptimized(True)
t2 = timeit.timeit(stmt=img2, number=n)
print(t1)
print(t2)

结果呢

False
True
0.1122399999999999
0.3290590999999998

Answer 2

您的测试很可能不可靠，因为您在代码中读取同一个文件两次

img1 = cv.imread('./image_lib/1.jpg')
.
.
.
img2 = cv.imread('./image_lib/1.jpg')

操作系统可能将图像保存在一些缓冲区中，这意味着第二次读取比第一次快得多。 您还应该使用timeit模块来定义时间函数。你知道吗