试图了解CMTime [英] Trying to understand CMTime

问题描述

我见过一些示例 of CMTime （三个单独的链接），但我仍然没有得到它。我正在使用AVCaptureSession和AVCaptureVideoDataOutput，我想设置输出的最大和最小帧速率。我的问题是我只是不理解CMTime结构。

显然CMTimeMake（value，timeScale）应该每隔1 / timeScale秒给我一个值帧，总值/ timeScale秒，或者我错了？

为什么不记录这个是什么？

如果它真的像那样工作，我怎么能让它有无限数量的帧？

如果真的很简单，我就是对不起，但还没有点击任何内容。

解决方案

A CMTime struct表示存储为有理数的时间长度（请参阅 CMTime参考）。 CMTime 有一个值和一个时间刻度字段，并代表时间价值/时间刻度秒。

CMTimeMake 是返回 CMTime 结构的函数，例如：

  CMTime t1 = CMTimeMake（1,10）; // 1/10秒= 0.1秒
 CMTime t2 = CMTimeMake（2,1）; // 2秒
 CMTime t3 = CMTimeMake（3,4）; // 3/4秒= 0.75秒
 CMTime t4 = CMTimeMake（6,8）; // 6/8秒= 0.75秒
  

最后两个时间值 t3 和 t4 代表相同的时间价值，因此

  CMTimeCompare（t3，t4）== 0 
  

如果设置

<$ p $> AVCaptureSession 的> videoMinFrameDuration 没有区别p> connection.videoMinFrameDuration = CMTimeMake（1,20）; //或
connection.videoMinFrameDuration = CMTimeMake（2,40）;


在这两种情况下，帧之间的最小时间间隔设置为1/20 = 0.05秒。

I have seen some examples of CMTime (Three separate links), but I still don't get it. I'm using an AVCaptureSession with AVCaptureVideoDataOutput and I want to set the max and min frame rate of the the output. My problem is I just don't understand the CMTime struct.

Apparently CMTimeMake(value, timeScale) should give me value frames every 1/timeScale seconds for a total of value/timeScale seconds, or am I getting that wrong?

Why isn't this documented anywhere in order to explain what this does?

If it does truly work like that, how would I get it to have an indefinite number of frames?

If its really simple, I'm sorry, but nothing has clicked just yet.

解决方案

A CMTime struct represents a length of time that is stored as rational number (see CMTime Reference). CMTime has a value and a timescale field, and represents the time value/timescale seconds .

CMTimeMake is a function that returns a CMTime structure, for example:

CMTime t1 = CMTimeMake(1, 10); // 1/10 second = 0.1 second
CMTime t2 = CMTimeMake(2, 1);  // 2 seconds
CMTime t3 = CMTimeMake(3, 4);  // 3/4 second = 0.75 second
CMTime t4 = CMTimeMake(6, 8);  // 6/8 second = 0.75 second

The last two time values t3 and t4 represent the same time value, therefore

CMTimeCompare(t3, t4) == 0

If you set the videoMinFrameDuration of a AVCaptureSession is does not make a difference if you set

connection.videoMinFrameDuration = CMTimeMake(1, 20); // or
connection.videoMinFrameDuration = CMTimeMake(2, 40);

In both cases the minimum time interval between frames is set to 1/20 = 0.05 seconds.

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