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Having issues with multiple nested velocity envelopes.
I’ve just spent many hours figuring why these velocity envelopes don’t seem to be working correctly, but I’m at a dead end.
I have very simple project involving some 60mph motorcycle footage. I wanted to start it out at 60mph, and then use the velocity envelope to smoothly and steadily bring it up to around 1000mph or more by the end. It must be smooth and consistent so I can properly calculate speed. Seems simple, just use multiple nested velocity envelopes, (first one can get it up to 300%, next up to 900%, next up to 2700%, etc).
Heres the issue though. Even though I am selecting “linear” on every single velocity envelope, the end result is FAR from linear. It takes a very long time to start picking up speed in the beginning, and shooting up faster and faster near the end. Best way to describe it is look at the 5 primary fade types avaialble for your audio tracks, see the middle one, where it starts slow and waits till the end to drastically shoot up.
For example, I have one clip that has been through 5 tiers of nesting, all velocity envelopes were linear, so that in the beginning it should be playing at 100% aka 60mph, and at the end should be playing at 8100% aka 4860MPH, and should be a completely straight linear increase all the way up to that. However its not. Even if I go 1/4 into the clip, where the speed should be well over 1000mph, its really only looking like about 200mph. Around the middle point where it should be around 2400mph, its really only looking like around 500mph.
Why is it doing this, and is it possible to solve it and get the linear velocity envelope I want?
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3 Replies
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The problem with nesting linear velocity envelopes is that the end result will never be linear. If you nest 2 linear envelopes, you’ll get a square-law result, 3 envelopes will give you cubic, and so on. They’re not strictly logarithmic or exponential, but are still a curve increase that starts gradually and then gets quicker and quicker.
You can experiment with using some linear and some log envelopes to equalise the resultant envelope into an approximately linear one. You may have to use some keyframes to get the correct result. The individual envelopes need to ramp up relatively quickly to start with, then flatten out.
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So if the issue is with the multiple velocity envelopes being nested, could I get around it by rendering each time? As in do one linear velocity envelope, render that, and then use the rendering for the next velocity envelope, and so on?
I know that normally this will degrade quality with each render, but considering this doesn’t need to be production quality, and considering nesting doesn’t work, perhaps its my best bet? For me the linear speed accuracy is most important.
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No, rendering each time you apply a linear envelope will give exactly the same non-linear speed ramp as nesting linear envelopes, except of course the image quality will suffer due to the repeated re-render.
You need to use an envelope shape that, when multiplied by itself several times, results in a straight line… ie for a total of 2700% increase (300% x 300% x 300%) you would need 3 velocity envelopes that each form a cube root, which looks like the plot here: https://en.wikipedia.org/wiki/Cube_root
You can try approximating this curve with one of the non-linear keyframe types in Vegas, or manually draw keyframes that approximate this curve and see if the resulting overall speed ramp looks about right.
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