163

1) CMTimeMake(1,10) means duration of 1 second and timescale of 10, or 10 frames per second. This means 1s duration of video with 10 frames?

2) 

CMTime lastTime=CMTimeMake(1,10);
CMTime frameTime=CMTimeMake(1, 10);
CMTime currentTime=CMTimeAdd(lastTime, frameTime)

= (2, 10) ?

2 seconds of video and with 10 frames per second of the currentTime?

PaulG
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lilzz
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    if your video is 48 FPS (frame per second) then you would easily do `CMTimeMake(1, 48)` to run a block of code every 1/48 of a second ie 1 block per frame – mfaani May 04 '17 at 14:27

5 Answers5

185

1) CMTimeMake(1,10) actually means a value of 1 and a timescale of 10. They are a numerator and denominator, so it is 1/10 of a second, not 1 second.

2) The result will be like CMTimeMake(2, 10), which is 2/10ths of a second.

Peter DeWeese
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150

Peter is right. The following code makes the concept more clear:

1)

Float64 seconds = 5; 
int32_t preferredTimeScale = 600;
CMTime inTime = CMTimeMakeWithSeconds(seconds, preferredTimeScale);
CMTimeShow(inTime);

The above code gives: {3000/600 = 5.000}

Which means a total duration of 5 seconds, with 3000 frames with a timescale of 600 frames per second.

2)

int64_t value = 10000;
int32_t preferredTimeScale = 600;
CMTime inTime = CMTimeMake(value, preferredTimeScale);
CMTimeShow(inTime);

This one gives {10000/600 = 16.667}

Which means a total duration of 16.667 seconds, with 10000 frames with a timescale of 600 frames per second.

Notice the difference between CMTimeMake(int64_t value, int32_t timescale) and CMTimeMakeWithSeconds(Float64 seconds, int32_t preferredTimeScale)

Hope this explanation helps. For further clarifications, please don't hesitate to post further questions on this post.

Raunak
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  • I wonder why `setMaxRecordedDuration` needs the `preferredTimeScale` when `frame_rate` is set differently. Everyone is just blindly using 600, and no good explanation as to exactly what it is doing. – dashesy Sep 11 '15 at 01:26
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    @dashesy 600 is a multiple of the common video frame rates. Warren Moore explains it at http://warrenmoore.net/understanding-cmtime pretty well. – danimal Sep 06 '16 at 23:21
41

With CMTimeMake(A, B) you store a rational number, an exact fraction A / B seconds

  • CMTimeMake(1, 4) -> the time interval 0.25 seconds

With CMTimeMakeWithSeconds(A, B) you store A seconds to a resolution of B steps

  • CMTimeMakeWithSeconds(0.25, ...) -> the time interval 0.25 seconds

You commonly see CMTimeMakeWithSeconds(time, NSEC_PER_SEC). The NSEC_PER_SEC effectively means "max resolution".

bcattle
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12

Interval for 1 Second

If you only want to know how to make an interval for 1 second (like me), this is your answer:

int seconds = 1;

CMTime interval = CMTimeMakeWithSeconds(seconds, NSEC_PER_SEC);
MGY
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3

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

See See this SO Answer which is clear

Community
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kandelvijaya
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