Reference Systems

VRPN - UNC

URL

http://www.cs.unc.edu/Research/vrpn/vrpn_getting_started.html

Time Stamp Structure

struct timeval
  {
    __time_t tv_sec;		/* Seconds.  */
    __suseconds_t tv_usec;	/* Microseconds.  */
  };

OpenCV

• include/opencv/cv.h: double timestamp
• src/cv/_cvipp.h: float timestamp
• 3rdparty/include/ffmpeg_/avformat.h: int64_t timestamp;

KWSys

• static double GetTime();

ITK RealTimeClock

• itkRealTimeClock.h
• itkRealTimeClock.cxx

System Time

• http://en.wikipedia.org/wiki/System_time

Scales

If counting from a given starting point (forward):

• int32 : can count in microseconds, up to 4.5 years
  • 2**32 / (30 * 3600 * 24.0 * 365 ) = 4.5 years

• double : can count in microseconds, up to 285 years (before losing any precision)
  • 2^53 / ( 1000000 * 3600 * 24.0 * 365 ) = 285
  • after that it will be counting in tens of microseconds...

• int64 : can count in microseconds, up to 584942 years
  • 2**64 / ( 1000000 * 3600 * 24.0 * 365 ) = 584942
  • (or in nanoseconds up to 584 years)

If counting relative times:

• int32 -> will use the sign, and can count (-2.4, +2.4) years
• double -> and can count (-142, +142) years
• int64 -> (-292471, +292471) years

Units

• Should the time units be specified ?
  • Use microseconds ?

Proposal

• Use double as internal representation
• Use micro-seconds as unit
• Have convenience methods to return time in other units
  • seconds, hours, days
• Have convenience methods for computing differences between two times
• Encapsulated type:
  • External time representation = double in microseconds
  • Internal time representation is private and is not exposed
• Two Classes
  • TimeStamp
  • TimeInterval
  • TimeInterval = TimeStamp - TimeStamp
  • TimeStamp = TimeStamp + TimeInterval
  • Time operations will be implemented in these two classes.
  • They will have to be as efficient as double or int64 arithmetics.
• "Get" methods only available for debugging
  • TimeStamp.GetTimeInSecondsSince1970();
• Serialization
  • ostream << (controlled by ITK)
  • istream >> should reconstruct the object.
  • when sent to file formats then we will be subject to the file format representation (just as we are with images).
• Manage conversion between Dates and microseconds by using the Unix conversion algorithms.
• Internal implementation (private)
  • TimeStamp will use a timeval struct
  • TimeInterval will use a double
• Error management: Needs more discussion
  • Do we want an ErrorValue to be stored in the TimeStamp and / or the TimeInterval ?
  • Should error be managed only through exceptions ?
  • Should there be a default value ?
    • If so, should there be a way of checking for it ?
    • How to avoid the subsequent code to be saturated with "if"s.