Monday, June 04, 2007

Slow Motion in Premiere

With the new Premiere CS3, there's lots of talk about using the time remapping feature for variable slow motion (progressively slowing the clip down or speeding it up.) This is something that previously required After Effects, so it's pretty exciting to have that in Premiere. But that's not what I want to write about - I'm more interested in writing about how to achieve good slow motion results in Premiere Pro, especially when dealing with interlaced footage.

As a wedding videographer (and a sports videographer, where this article is even MORE relevant), I use slow motion ALL THE TIME. A little slow motion and the right song can do wonders for otherwise ordinary footage. But sometimes that slow motion causes your footage to become jerky, especially when using very low speeds with interlaced footage. What's the best way to get smooth slow motion (aside from filming at a higher frame rate)? I was struggling with this issue just tonight and came up with what seems (so far anyway - I need to test with more clips) to be a good solution.

First, let me address what is SUPPOSED to work, but doesn't seem to, or doesn't work all that great. First, Adobe will tell you to use the frame blend option. In my tests, this seemed to make little or no difference. Some will tell you to use integer multiples for your speed so that Premiere doubles (or triples, etc.) the frame rate evenly (50% = 2x, 33.33% = 3x, 25% = 4x, etc.). Again, with the particular clip I was having trouble with tonight, that seemed to help a LITTLE, but barely. Then there's always the time remapping feature in After Effects, but I wanted a solution that just uses Premiere. I've also come across suggestions to use the ReelSmart Twixtor plug-in - I haven't tried it - I don't really want to drop several hundred dollars on a plugin just to get good slow motion - shouldn't Premiere be able to handle this natively?

Enter the Posterize Time effect. Generally I've used this effect for slowing down frame rates when I actually want the footage to be jerky. Ever seen those commercials when they want to make something look really bad (like the competitor's product) or news stories about some nefarious scheme and they play footage in black and white with really jerky footage? There are rare occasions (think billion dollar man slomo sequences) where you actually want jerky footage. The plugin is also sometimes used to attempt to make 60i footage look like 24p to achieve a film look.

ANYWAY, I discovered that I can actually use this effect to INCREASE the frame rate of my clip and give me better slow motion. Of course,I'm not literally increasing the frame rate of the source footage - that would require reshooting at a higher frame rate. I'm just "pretending" or telling Premiere that it's a higher frame rate. To be honest, I don't know EXACTLY what Premiere does when you use this filter with a higher frame rate than the source footage, but from my empirical evidence, whatever it does, it works pretty well.

First, I asked myself what Premiere actually has to do to slow down footage. Well, it has to increase the number of frames playing in a given time frame. If I play back my footage at 50%, it needs twice as many frames to make the same clip take twice as long at the same frame rate. The extra frames are accomplished through interpolation. So taken to an extreme, at 10%, each frame would now be doubled 10 times. You can see how this would lead to jerky footage - instead of the picture changing 30 times per second (or each field changing 30 times per second, or 60 times overall, to be more accurate dealing with interlaced footage), the frame will only change 3 times per second. So I came up with this formula, which may be completely baseless, so take it for what it's worth - it just seemed to work. Original FPS / speed = FPS for Posterize Time filter. So my original clip is 30fps, I want to slow it down to 40%. 30 / .4 = 75. I apply the posterize time effect, set the frame rate to 75, and by golly, it looks pretty good. Of course, it still looks pretty good when the fps is set to 30, so the formula really isn't that important, just the concept of using the posterize time effect.

Realistically, I've found that Premiere does slow motion just fine at 50% and above. It's only when you drop below that that you start to have problems...and using progressive footage is a whole different story, as is the definition many consumer camera manufacturers use to define "progressive." The reason I mentioned sports videography at the beginning of this article is because jerky footage (as well as interlacing artifacts) is much worse when there is more action happening in the clip, or when the camera itself is moving quickly. There is more change in the image from frame to frame (or field to field). I've found that as you increase the fps on the posterize time effect, you can end up smoothing the video, but blurring it more, especially for high action shots. The frame blending and interpolation process attempts to sort of take the middle ground when creating extra frames, so when there's greater differences between each frame and each field, those created frames can be quite blurry. When slowing high action shots down to extreme slow speeds, you have a trade off between clarity and smoothness with the posterize time effect.

Related to this article is achieving good frame holds - ones that don't shake violently. That's another topic related to interlacing - maybe I'll address that another time...

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At 1:09 PM, Blogger Rusty said...

Thanks for your comments on this. This has been haunting me for some time now. I can never seem to get the smooth slowmo results I'm looking for in Premier. I do have one question for you... It seems, based on previous posts, that you shoot with a Canon GL2. Do you shoot in frame mode or not, and have you noticed any difference in slowmo results from one to the other? Thanks for your help. Rusty

At 5:23 PM, Blogger Josh said...

I do not shoot in frame mode. For one, I've never been happy with the results - it appears too choppy. Especially when the camera is panning or there is fast action. Also, while I can't remember the exact details, I remember reading an article about how frame mode takes a lot of detail out of your image because of the way the GL2 accomplishes full frames. Something about the fact that the chip is still an interlaced chip and so it does some kind of weird alternating and duplicating of the red, blue, and green channels to get a full frame, which effectively lowers your resolution. I'm probably using all the wrong terms as I'm not taking the time to specifically look it up but in short, you don't get as much detail in your image when shooting in frame mode.

At 2:23 AM, Anonymous Smumdax said...

Thanks for your article... I was trying to find a solution to that exact problem, and everywhere I turned, forums told me that Premiere cannot do that, Premiere isn't good, no you CANNOT, impossible, use After Effects, use this, try that... then I've found your page...

Goes to show that you cannot rely on one source of info. When someone says: No you cannot... I usually don't believe them... and I find it very sad to imagine that some other people had the same problem, but stopped their search after reading some know-it-all say "Nope, impossible".


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