opencv的getTickCount()用法

转载地址： https://blog.youkuaiyun.com/chaipp0607/article/details/71056580

double t1 = (double)getTickCount();
.
.
.
double t2 = (double)getTickCount();
cout<<"time:"<<(t2-t1)*1000/(getTickFrequency())<<endl;
   
   
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getTickCount()与getTickFrequency()都被定义在core.hpp文件下：

//! Returns the number of ticks.

/*!
  The function returns the number of ticks since the certain event (e.g. when the machine was turned on).
  It can be used to initialize cv::RNG or to measure a function execution time by reading the tick count
  before and after the function call. The granularity of ticks depends on the hardware and OS used. Use
  cv::getTickFrequency() to convert ticks to seconds.
*/
CV_EXPORTS_W int64 getTickCount();

/*!
  Returns the number of ticks per seconds.

  The function returns the number of ticks (as returned by cv::getTickCount()) per second.
  The following code computes the execution time in milliseconds:

  \code
  double exec_time = (double)getTickCount();
  // do something ...
  exec_time = ((double)getTickCount() - exec_time)*1000./getTickFrequency();
  \endcode
*/
CV_EXPORTS_W double getTickFrequency();

/*!
  Returns the number of CPU ticks.

  On platforms where the feature is available, the function returns the number of CPU ticks
  since the certain event (normally, the system power-on moment). Using this function
  one can accurately measure the execution time of very small code fragments,
  for which cv::getTickCount() granularity is not enough.
*/
CV_EXPORTS_W int64 getCPUTickCount();

   
   
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getTickCount()：用于返回从操作系统启动到当前所经的计时周期数，看名字也很好理解，get Tick Count(s)。
getTickFrequency()：用于返回CPU的频率。get Tick Frequency。这里的单位是秒，也就是一秒内重复的次数。

所以剩下的就很清晰了：
总次数/一秒内重复的次数 = 时间(s)
1000 *总次数/一秒内重复的次数= 时间(ms)

这个逻辑很清晰，没什么问题，但是这里有一个小坑，那就是C版本的cvGetTickFrequency()函数和C++版本的getTickFrequency()的单位不一样，前者以ms计算频率，后者以s为单位计算频率，所以如果使用C版本的cvGetTickFrequency()计算时间的话，应该是：
总次数/一秒内重复的次数*1000 = 时间(ms)
总次数/一秒内重复的次数*1000000 = 时间(s)

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