CMTime: How Time Is Represented in AVFoundation

CMTime

How is CMTime structured?

CMTime is a C structure that represents time using

  1. Numerator (int64_t)
  2. Denominator (int32_t)

Example

// time1 and time2 both represent 100 seconds, but using different timescales.
CMTime time1 = CMTimeMake(200, 2); // 200 half-seconds
CMTime time2 = CMTimeMake(400, 4); // 400 quarter-seconds

Special CMTime constants

  • kCMTimeZero
  • kCMTimeInvalid
  • kCMTimePositiveInfinity
  • kCMTimeNegativeInfinity

Special CMTime constant testers

  • CMTIME_IS_INVALID
  • CMTIME_IS_POSITIVE_INFINITY

  • CMTIME_IS_INDEFINITE

    CMTime myTime = CMTimeMake(200, 2); if (CMTIME_IS_INVALID(myTime)) { NSLog(@“Error”); }

CMTimeRange

How is CMTimeRange structured?

  1. Start time (CMTime)
  2. Duration (CMTime)

Example

CMTime time1 = CMTimeMake(200, 2);
CMTime time2 = CMTimeMake(400, 4);
CMTimeRange range = CMTimeRangeMake(time1, time2)

Testing for inclusion

CMTimeRangeContainsTimeRange(range, CMTimeRangeGetEnd(range));
// This is false

CMTimeRange special constants

  • kCMTimeRangeZero
  • kCMTimeRangeInvalid

CMTimeRange special constant testers

  • CMTIMERANGE_IS_VALID
  • CMTIMERANGE_IS_INVALID
  • CMTIMERANGE_IS_EMPTY
  • CMTIMERANGE_IS_EMPTY