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Strange Interlacing on Propeller in Progressive File

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Mike Fizer
Strange Interlacing on Propeller in Progressive File
on Nov 3, 2014 at 6:44:22 pm

I've got an engine start up on an aircraft where the propeller is becoming "interlaced" on the Canon C100 MTS 29.97 file in both AE CC 2014 and PPRo CC 2014. You can tell from the attached the 4 blade prop is now 8 blades, what you might not see are the interlaced artifacts.

What is stranger is that the file plays fine in VLC with the BOB interlace.

Once the propeller gets up and spinning the issue goes away.

Any ideas what might be happening?

Many thanks,


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George Goodman
Re: Strange Interlacing on Propeller in Progressive File
on Nov 5, 2014 at 2:40:13 pm

Probably a rolling shutter situation.

Your camera is capturing it once at the top of the sensor, then when it spins around fast enough, it catches it again at the bottom during the scan. But when the propeller is fast enough, it would be all the way around by the bottom section of the scan so that would make the effect go away. So it's just the perfect combination of speed of the propeller and speed on the sensor read from the camera causing the effect.

Just my guess anyway.

"|_ (°_0) _|"



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Todd Terry
Re: Strange Interlacing on Propeller in Progressive File
on Nov 5, 2014 at 4:54:54 pm

Any diagnosis of that effect is just a guess, there are some unknown factors and settings at play...

It definitely is not rolling shutter though. That's just not what rolling shutter would look like. If that were the case you would not see those interlacing lines, and the props would appear to "bend," sort of corkscrewing as they turn. Those appear perfectly straight, just interlaced.

A couple of potential culprits... we know you are shooting 29.976, you told us that. You also shot interlaced, that is obvious from the footage. If you put a 29.976 interlaced file on a 29.976 interlaced timeline, it will play back perfectly normal looking on an interlaced CRT monitor, or on a progressive videovideo monitor (such as an LED, LCD or plasma television). But on a progressive computer monitor, you will see those interlacing lines.

BUT... if you put your 29.976 interlaced source footage on a 29.976 progressive timeline, you are likely to see those interlacing lines on any monitor... CRT, interlaced, progressive, whatever.

You're not seeing in the VLC player because the player's settings are only showing you a deinterlaced version on playback... i.e., it's only displaying one field per frame on playback...not both fields.

You can "fix" that in editing as well by deinterlacing so that only one field (either the upper or lower) is being used (various NLE platforms accomplish this is different ways). You won't see the interlacing then, although you will lose resolution, because only half the fields are being used. It's a tradeoff.

The way to fix this in the future, unless you want the live-instant-videoy look of 60i, then always shoot progressive... whether it be 24p or 30p. If you do want the live video look (and sometimes you might), then go ahead and shoot 60i... for projects that you know will be seen on a real TV monitor. For projects that will exclusively be viewed on computer monitors, always shooting progressive will give the best results.

Hope this helps.

By the way, unless those props were just winding down (or starting up) and were moving very slowly, you were apparently shooting at a much much higher frame rate than would be considered "normal"... was that intended?


Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.

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