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Blurring Street lights in BG of Night Shot Without Affecting Foreground?

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Mike Healey
Blurring Street lights in BG of Night Shot Without Affecting Foreground?
on Oct 7, 2009 at 12:24:56 am

I have a video that I shot of someone in the back of a car that is driving through a city at night-time. I would like to blur out and color correct the street lights in the background to try to re-create a shallow depth of field, while not affecting the person in the car in the foreground. I obviously know that it would have been better to try to do this in production as opposed to post, but this wasn't an option due to a variety of factors (budget, very tight schedule for pre-production and production, etc).

I was wondering if there was a way to create a mask based on the luminance of the lights in the background, or find some other way to apply effects only to the background and not the foreground? Any help would be greatly appreciated. I'm also open to trying this out in an application other than AE if that might help...


Thanks!
-Mike

Mike Healey
mike@kneeon.tv
http://www.kneeon.tv


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Jon Geddes
Re: Blurring Street lights in BG of Night Shot Without Affecting Foreground?
on Oct 7, 2009 at 1:38:03 pm

Well, if the background has a much brighter luminance, you could duplicate the footage, desaturate and apply a levels adjustment to create the matte, precompose, and apply a lens blur to your original footage, using the precomp as the Depth Map Layer.

Jon Geddes
http://www.precomposed.com


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Marcello Mazzilli
Re: Blurring Street lights in BG of Night Shot Without Affecting Foreground?
on Oct 7, 2009 at 1:50:15 pm

Well the first thing is to find a way of selecting just what you want to blur. You can do this with luminance (as suggested), with chrominance (lights could be the only orange thing), but also you can combine fixed masks (like simple boxes) for darker corners etc... Viceversa you can create negative masks forcing some parts not to be cut out (i presume the lower part of the screen). If the objctive of the the mask is to work on blurs then probably it is possible to soften the mask to reduce edge imperfections. I don't have the footage (or a foto) so it's not easy just to guess. Once you have an alpha mask then you just have to use it as a matte for the whole frame. After FX will do fine for me.

siRoma di Marcello Mazzilli
Corporate video productions in Italy
http://www.siroma.com


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Dave LaRonde
Re: Blurring Street lights in BG of Night Shot Without Affecting Foreground?
on Oct 7, 2009 at 3:48:20 pm

[Mike Healey] "...I obviously know that it would have been better to try to do this in production as opposed to post, but this wasn't an option due to a variety of factors (budget, very tight schedule for pre-production and production, etc). "

I don't think you DO appreciate how much better it would have been... effects work normally costs three times as much as just shooting it.

To address your problem, you might try the Compound Blur effect in AE. It applies blur based on greyscale.

This won't be a "one click and I'm done!" sort of deal. You'll probably have to play around with precomps, various effects like Levels and Invert, and animating wipes to isolate only those lights you want to see.

I'm not sure what you're going to do about the rest of the background that's the same distance away as those lights.

Happy trial and error, tweaking and rotoscoping.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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