[FFmpeg-user] Fade in filter and alpha channel fade in text

Joshua Grauman jnfo-c at grauman.com
Mon Aug 13 11:48:51 EEST 2018

>>>>  Hi all,
>>>>  I am combining various effects, and I noticed that when I fade in a
>>>>  video
>>>>  using the [in]fade=t=in:st=2:n=24[out] syntax, and I fade in text using
>>>>  alpha='if(lt(t,2),0,if(lt(t,3),t-2,1))', that they both fade in roughly
>>>>  at
>>>>  the same time, but both are not linear. I'm guessing the fade=in uses a
>>>>  sine wave or something to fade in the video, while clearly the text fade
>>>>  is based on a linear fading of the alpha channel. Am I correct that the
>>>>  video fade in is a sinusoidal fade? Does anyone know offhand how I could
>>>>  get the text fade to match the video one?
>>>  Fade is always linear in this filter.
>>  Good to know, obviously I have something else wrong then. Thanks!
> Well, if you are using both fading methods simultaneously, then the text has 
> a quadratic fading curve.
> because in the middle of the fading interval you have a factor 0.5 from the 
> fade filter, and also a factor 0.5 from the alpha function. 0.5 * 0.5 = 0.25.
> Michael

Ah, yes, I think this is it. Thanks.


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