通過測量CPU時間嘗試相同。
int main()
{
clock_t startTime;
clock_t endTime;
int height =1024;
int width =1024;
// 700 ms
cv::Mat in(height,width,CV_8UC1, cv::Scalar(255));
std::cout << "value: " << (int)in.at<unsigned char>(0,0) << std::endl;
cv::Mat out(height,width,CV_8UC1);
startTime = clock();
out = in/4;
endTime = clock();
std::cout << "1: " << (float)(endTime-startTime)/(float)CLOCKS_PER_SEC << std::endl;
std::cout << "value: " << (int)out.at<unsigned char>(0,0) << std::endl;
startTime = clock();
in /= 4;
endTime = clock();
std::cout << "2: " << (float)(endTime-startTime)/(float)CLOCKS_PER_SEC << std::endl;
std::cout << "value: " << (int)in.at<unsigned char>(0,0) << std::endl;
//40 ms
cv::Mat in2(height,width,CV_8UC1, cv::Scalar(255));
startTime = clock();
for (int y=0; y < in2.rows; ++y)
{
//unsigned char* ptr = in2.data + y*in2.step1();
unsigned char* ptr = in2.ptr(y);
for (int x=0; x < in2.cols; ++x)
{
ptr[x] /= 4;
}
}
std::cout << "value: " << (int)in2.at<unsigned char>(0,0) << std::endl;
endTime = clock();
std::cout << "3: " << (float)(endTime-startTime)/(float)CLOCKS_PER_SEC << std::endl;
cv::namedWindow("...");
cv::waitKey(0);
}
與結果:
value: 255
1: 0.016
value: 64
2: 0.016
value: 64
3: 0.003
value: 63
你看到的結果不同,可能是因爲mat.divide()
不執行浮點除法和舍入到下一個。雖然在更快的版本中使用整數除法,但速度更快但結果不同。
另外,在openCV計算中有一個saturate_cast,但我猜計算負載差別較大的是雙精度分割。
你可以試試* = 1.0f/4.0; ?你沒有初始化元素btw – Micka
浮點乘法的結果與我的測試中對整像素乘法和整像素乘法的整數除法與大約20%的不確定性/差異相同。 –
你可以運行perf嗎? https://perf.wiki.kernel.org/index.php/Main_Page – auselen