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what went wrong? extreme interlacing problems

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Jason Berger
what went wrong? extreme interlacing problems
on Jul 1, 2013 at 12:00:19 am

Okay, so I shot a martial arts competition at 60i and 1/500th shutter speed using the Vixia G20. From my readings on the internet this seemed like the best possible way to shoot because it could be converted down to 30p for slow-motion sequences and the high shutter speed would eliminate blur.

The interlacing of images came out really, really extreme. The worst I have ever seen. Every single movement, no matter how small, reduces the people on screen to indistinct figures with zig-zag contours.

Was the shutter speed too high? Was it a mistake to shoot 60i? Is there any way to salvage this?


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Todd Terry
Re: what went wrong? extreme interlacing problems
on Jul 1, 2013 at 2:50:44 am

Hi Jason...

Firstly, I don't know the exact capabilities of your camera, so some of this is a bit of a guess...

I'd say that some of the choices of settings that you made are probably not what most people would consider optimal for the situation.

We have two things to deal with here, interlacing and shutter speed...

First, interlacing... yes some people shoot 60i specifically for slo-mo footage. You can shoot 60i footage and turn it into the equivalent of a 40% slow-mo in a 24p project (60 x 40% = 24). But to do this you must extract the fields and turn each into its own frame so that effectively you have a 60fps clip (and yes, you'll lose half the vertical resolution, but that's the price you pay). Different editing platforms handle this different ways, but you have to do this step. If you can't extract the fields into frames, at minimum you have to deinterlace the footage or you'll get that zig-zaggy interlaced look that you're now familiar with.

Ok, now shutter speed. This is not primarily what is causing your crappy images (although I doubt it's helping) but your 1/500th speed is probably way too high, so it's also something you should keep an eye on. Unless you are shooting specifically for a special effect (such as purposely shooting to get the stuttery "narrow shutter" look), you'll probably want to most always shoot with a "normal" shutter for whatever your actual or simulated playback speed would be. A normal shutter (the equivalent of a 180° shutter in a film camera) is one over twice the frame rate. In the 24p world, a normal shutter speed is 1/48th. In the 60i world, it's 1/60th (since 60i is actually 30fps). So, lets say you were going to do a field-to-frame extraction so you can do a 40% slow-mo of your 60i footage. Your effective frame rate in that case is 60fps, so a "normal" shutter would be 1/120th of a second. Higher than that and you'll get a stuttery look.

That 40% slow-mo is the slowest actual slow-mo you can do in your case. You can simulate slower than that using frame interpolation software such as Twixtor. If you use one of those plug-ins, then yes shoot with a frame rate appropriate to how much you anticipate simulating. So yes, the 1/500th shutter speed would be the right choice if you planned to slow the footage down enough to simulate a 250fps frame rate (which would be pretty slow slow-mo indeed...about one-tenth speed).

Can you salvage the footage? Maybe. You definitely need to extract the fields to frames. That will eliminate that jagged look you are getting. The shutter speed will probably look a fair bit too high, but it should be a lot better than you are getting already.

One of the big problems with this is that there isn't one "best" group of settings that's good for everything. Folks are often wanting to shoot footage that they might want to view at normal speed, but they might want to slo-mo it as well, and naturally want settings that make it look the very best possible for both. Sadly, there's no such thing. The best settings for great looking normal footage won't at all be the best settings for slow-mo... and vice versa. While I know you can't always do this, the only real optimal settings come from knowing in advance at what framerate you want for playback. It's not something you can decide after the fact... at least not and get perfect results.

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Keith Slavin
Re: what went wrong? extreme interlacing problems
on Jul 1, 2013 at 4:02:04 am

Hi Jason,

Firstly, interlace artifacts become more pronounced with motion and a fast shutter speed. A high quality deinterlacer should take care of it. Secondly, normally filmed clips at 30 frames/sec requires about 1/60 sec shutter speed, but slowed-down clips require about 1/2 this at 1/120 sec. This is still more blur than your 1/500 sec, so you may need to add motion blur.

However, you did do the right thing shooting at 60i, instead of 30p! This is because the temporal sampling rate is 60, not 30, which helps with motion estimation in general. Also fast shutter times can be adapted to a larger range of frame rates, as it is very difficult to reduce motion blur.

We have tools for dealing with both issues - high quality deinterlacing and motion-blur adjustment. If you like, you could send us a link to download a short clip, and we will convert it for you. I assume you wanted 60->30 (2:1) slowmo?

Best,

keith@isovideo.com


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