Forging Slow Motion

February 28, 2005

Hi,

To my disappointment I have found that the HDW750 won't do any higher than 30p. Although it does 50i which could work to my advantage.

I really want slow motion on one of the scenes, but I've been thinking of ways I can get it out of the camera.

One of my plans (and don't ask me how I'm going to do it) is to shoot 50i, on 1 frame I will stretch field A over field B, and then for the next frame I will stretch field B over field A. Therefore creating a 50p look, but just with the shutter angle halved.

Is there any kind of software that might do this?

Has it even been done before?

Thanks for any input.

Edited February 28, 2005 by Daniel J. Ashley-Smith

Nate Downes · February 28, 2005

It would look jerky if you do it that way. Honestly, the only reliable method for slow-mo is to use film. You can rent a camera capable of 128fps relatively cheaply and use just 1 roll.

Thomas Worth · February 28, 2005

You can shoot 50i and interpolate the fields to create a 50p clip. Here is an article I wrote on how to create slow-mo sequences from interlaced footage:

http://rarevision.com/articles/slow_motion.php

Elhanan Matos · March 1, 2005

What framerate are you trying to achieve?

David Cox · March 1, 2005

We did some tests last week with field doubling. This involves taking a field, interpolating it to a frame (to avoid loosing too much resolution) and thus getting a "P (ish)" version of the "I". We did a test at 60i for some slow motion shots that would be inserted into sequences that would otherwise be shot 24P.

Although the theory works, we found the frames to be a little stroby (is that a word?). The motion blur on the frames just didn't seem right somehow, so our client has opted to shoot the slow mo stuff on film. Of course that then gives you DI issues of making your slow mo shots look like your HD stuff, if they cut back-to-back. I would encourage you to view tests with various settings to see if you can get over the strobyness (!!)

Software does exist that can create amazing slo-mos - we have something that does it. But the problem is that it does it by detecting edges and then morphing them to create additional frames. All very clever and when it works its very good. But that's the problem, it is very very image specific. This is why we will never recommend that anyone shoots with the intention of relying on it to provide a slo-mo. Its just there for the "what if we slo-mo'd it" moment in the edit room!

For ultra slo-mo's, there is the phantom camera from green door films in the UK www.greendoorfilms.co.uk. It can go up to a few thousand frames per second, but that might be a bit of an overkill if you're not shooting a bullet through an apple. (or maybe you can squeeze that scene in somewhere!!)

David Cox

Managing Director

Baraka Post Production Ltd

London

baraka.co.uk

Bob Hayes · March 1, 2005

I just shot 3,000 fps with a new video camera called a Photron worked great.

Elhanan Matos · March 3, 2005

We have a camera that does 1,000 fps in HD and up to 4,000 fps in SD that is available for rental. It's called the Cine Speed-Cam.

Michael Nash · March 3, 2005

The slo-mo shots in Once Upon A Time in Mexico were shot 60i. I don't know the process they used to get it back to 24P, but both on film and DVD the effect cuts seamlessly with the 24fps stuff. But then, I don't don't know if Rodriguez had any ILM help for that... ;)

Elhanan Matos · March 6, 2005

You can do the slow-mo effects (60i to 24P) right on the studio decks, I've done it on the SRW-5000, but I think the F500 does it too.

May 24, 2005

2 x slow motion on a basic video camera:

50i (PAL) or 60i (NTSC) gives you double the number of (half spatial quality) frames. So shoot interlaced and in post, time stretch the footage to double it's length, effectively using 1 field for 1 frame. You should use field blending to interpolate the missing field, which creates it by blending the two fields either side. It's not bad - giving the same spatial quality as deinterlacing (just that you're using the second field not discarding it). The better field blenders can to some extent ananlyze the image and follow the lines, creating a better interpolated field. As ever, slow moving sharp edges that are a few degrees off horizontal suffer worst. After Effects is a good, easily available program to control how you deinterlace an image.

Motion blur / shutter speed....

Remember, you also need to adjust your shutter angle. A default shutter speed for a (PAL) video camera is 1/50th. If you're shooting for 2x slow mo, to get the equivalent motion blurring of a film camera with 180 degree shutter angle, you'd need 1/100th shutter speed on video. For 4x slow mo effect, you'd need to use 1/200th. (All shutter speeds are for PAL cameras 'cos I'm in the UK. I guess it's 1/60th, 1/120th, 1/240th etc. for NTSC.)

3 x and beyond slow motion on video.

To go slower on video is a much harder task. Basically to get a smooth motion, you need to interpolate the missing frames. The simplest and worst technique is frame blending in post, where you effectively cross fade between the frames. Otherwise, you need a piece of software that can analyse adjacent frames and by morphing and cross-fading create the frames that should exist between them (or try to at least) It's not perfect, but can give impressive results, especially with low or moderate amounts of movement.

There are plug-ins for Avid, Discreet and After Effects that can do this. I think the Avid one is called Fluid motion and the After Effects / Discreet one is called Reelsmart Twixtor. There's probably loads of others too.

Regards,

Ben.

http://www.fink-base.com