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kCMTimeInvalid is invalid CMTime, but based on Apple document, there are more invalid CMTime, what are they? What does CMTime "invalid" means? It's overflow, uninitiated or anything else?

https://developer.apple.com/documentation/coremedia/kcmtimeinvalid

All fields are 0, so you can calloc or fill with 0's to make lots of them. Do not test against this using (time == kCMTimeInvalid), there are many CMTimes other than this that are also invalid. Use CMTIME_IS_INVALID(time) instead.

I found some cases when CMTime is invalid:

  • When flags(CMTimeFlags) kCMTimeFlags_Valid not set.

    kCMTimeFlags_Valid Must be set, or the CMTime is considered invalid.

  • Some operations of CMTimeAdd

+infinity + +infinity == +infinity

  • -infinity + -infinity == -infinity
  • +infinity + -infinity == invalid
  • -infinity + +infinity == invalid
    • allenlinli
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    2 Answers2

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    There are five possible states:

    1. +Infinity: This is similar to Float.Infinity. This is a valid value, just greater than any finite number. How might you use it? For example, imagine an API that gives you information about a time range within a video, identified by two CMTimes. You might invoke it with (-Infinity, +Infinity) to ask for information about the entire video.
    2. -Infinity: This is again similar to -Float.Infinity.
    3. Indefinite: This is similar to Float.NaN, as I understand. Use this when you don't know what value to use, like the duration of a live stream, as Apple suggests. It wouldn't be right to use infinity, for example, since a live stream doesn't go on for forever. It has a finite duration; we just don't know it yet.
    4. Invalid: This is a CMTime structure that doesn't obey the rules of CMTime. I assume that means things like a zero or negative denominator. Since CMTime is a C struct, it's not encapsulated, so someone can create one with invalid values like this. C structs can't have initialisers that throw an exception or return nil.
    5. Numeric: This is the normal case of a finite value. Use CMTIME_IS_NUMERIC to check for this. It returns false for all the weird cases above.
    Kartick Vaddadi
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    From the documentation you posted, it says:

    Use CMTIME_IS_INVALID(time) instead.

    From CMTime.h, it looks like CMTIME_IS_INVALID is defined as:

    #define CMTIME_IS_INVALID(time) (! CMTIME_IS_VALID(time))

    And CMTIME_IS_VALID is defined as:

    #define CMTIME_IS_VALID(time) ((Boolean)(((time).flags & kCMTimeFlags_Valid) != 0))

    So it looks like the only thing that determines if a CMTime is valid or not is whether the kCMTimeFlags_Valid flag is set.

    TylerP
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