FFMPEG fade in/out expressions

I've been playing with FFMPEG lately mulling over a plugin for Appcelerator Titanium and figured we'll need a text rendering with nice fading in/out effect. There's text drawing filter in FFMPEG, but not a function to easily fade it, so I came up with a couple of expressions of my own. I'm working with alpha channel here, but it can easily be applied to other properties.

Fading in

alpha='if(lt(t,t1),0,(t-t1)/t2)'

  • t1 - start time (in seconds),
  • t2 - length (in seconds).

Fading out

alpha='if(lt(t,t1),1,(t2-(t-t1))/t2)'

  • t1 - start time (in seconds),
  • t2 - length (in seconds).

Fading in and out

alpha='if(lt(t,t1),0,if(lt(t,t1+t2),(t-t1)/t2,if(lt(t,t1+t2+t3),1,if(lt(t,t1+t2+t3+t4),(t4-(t-t1-t2-t3))/t4,0))))'

  • t1 - fade in start time (in seconds),
  • t2 - fade in length (in seconds),
  • t3 - time to keep fully opaque (in seconds),
  • t4 - fade out length (in seconds).

If you happen to know a better way or shorter expressions, please let me know.

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