How do I add smooth slow down and speed up?

TheNickle Posts: 12

Hi there, this is the first thread I've created (I'm unsure if I have even posted in the right section) and it's about slowing down and speeding up footage smoothly. What I want to know is how to do it smoothly and affecting the video + audio at the same time (so when the footage slows down for example, the audio slows down with it). Thanks! 


  • CleverTagline
    CleverTagline Posts: 3,337 Ambassador

    While you can control the overall speed of an entire clip, either in the editor or in a comp, smooth speed changes that affect both audio and video are not currently possible as a built-in feature of HitFilm.  In comps, there is the Speed affect you can add to clips, and you can keyframe the speed over time, but it only affects the video.  Audio continues to play at normal speed for the duration of the clip.

  • TheNickle
    TheNickle Posts: 12

    Hm, didn't like the effect. It has a ghostly look to it because it adds frames. 

  • Triem23
    Triem23 Posts: 20,679 Ambassador

    Well, changing the speed of a clip to "slow mo" is going to add frames. The Rate Stretch tool just does frame duplication, the Speed effect does frame blending. Attempts to interpolate smooth frames needs Optical Flow which Hitfilm doesn't do. 

    Ultimately, for smooth slo-mo you pretty much need to shoot high frame rate. Even Twixtor, the best fake slo mo program on the planet (its about as expensive as Hitfilm Pro and only does one thing--time remapping) has visible artifacts more often than not. 

  • AndyQ
    AndyQ Posts: 66 Enthusiast

    I've just been playing with this, using both rate stretch tool and the "Speed" effect. I can understand the shortcomings with frame blending and the maths involved, so I'm not complaining. However, I've just done the same thing in Resolve and the interface is so much nicer - it adjusts the clip length on the timeline to match the actual playback length (something the Time effect doesn't do), allows easy setting of the speeds for each segment and a nice graph editor visible in the editing timeline for adjusting the ease in/out for each time change.  Also, not being able to use "speed" for the audio as well is a significant shortcoming (and there are many excellent algorithms for audio stretching)

  • TheBenNorris
    TheBenNorris Posts: 2,042 Staff

    This is something we are looking at for the future, as we are aware that the speed effects inside the software could be improved.

  • AndyQ
    AndyQ Posts: 66 Enthusiast

    Cool... but prioritise graph/curve editing of keyframes in the the edit timeline (for all properties/FX), which would be a stepping stone to time-remapping. I've never used such features in Premiere/AE so I don't know how well they do it, but Resolve was a joy and the results were good (and you don't have to break a clip up). I'm sure you guys are on top of it....I know you can't do everything at once!