slow motion : looking for "the maths"...

FuTz wrote on 1/31/2006, 5:26 PM

There was a post some time ago (months ago / year ago) and somebody would ask about slow motion with Vegas.
What I'd like to find out is this "formula" one came with. The essence of what was being said is that if you stretch or pull down velocity to 50%, the clip is not actually going 50% of its normal speed.
Or something like that.
I tried to search for the answer using (slow down, slowing down, slow motion, accelerate, etc...) but there's so many answers (around 13000) that I've become discouraged and decided to ask in case somebody remembers.
Wish me good luck, I know...
A big thanks in advance if you can help !

Comments

johnmeyer wrote on 1/31/2006, 5:53 PM

I've made several long posts on this subject. Some of them were not entirely accurate. What "formula" are you looking for? If you set the clip to 0.50, Vegas will create one additional frame between each existing frame. That additional frame is created by blending the two frames. As far as I know, the speed will indeed be exactly 50% of the original playback speed.

Somehow, however, I don't think that is answering the question you are trying to ask.

FuTz wrote on 1/31/2006, 9:34 PM

Mmm... checked again but still can't find.

It was about something that would sound logical for everyone but if you thought about it and analyzed, there were maths that proved otherwise. Maybe it was about deceleration then bringing back to normal speed.
It's just that I'd like to take a clip that's got this effect on a rendered clip (gradually going from normal speed to around 10% of speed) and bring back to normal speed from begin to end. Get rid of the effect on an existing clip.

Chienworks wrote on 2/1/2006, 4:53 AM

If you're changing the speed of the clip rather than having it at a fixed 50% then the final duration won't be 2x. Let's take an extremely simple case and say that you have a 10 second clip that you want to slow from 100% at the beginning to 50% at the end. The average speed would seem to be 75% which would make the clip will play for 13.333 seconds. In fact this is an oversimplification because the duration would be weighted more towards the slower speed so the clip would actually play slightly longer.

This also assumes that the speed change is linear. Vegas usually uses curves for the velocity envelope and this introduces other variations that can be difficult to calculate, indeed impossible if we don't know the algorithm that Vegas uses to generate the curves. Now add in that you may be slowing down for part of the duration, playing a constant speed for part of it, then speeding up again ... and it's no wonder that Vegas doesn't calculate the end point automatically!

The other issue that has been reported is that if you drag the line to 50% with your mouse, the "50%" value you see is the rounded value of the actual speed. You may have actually set the velocity to 49.7% or 50.2%. This can affect the final speed. The solution to this is to right-mouse-button click on the node and type in 50% to get the exact speed.

If you set the velocity to 50%, then you get 50% and the clip will play at exactly half speed.