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Object Removal Question

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Casey PegramObject Removal Question
by on Apr 13, 2010 at 10:14:00 am

Hey everyone, I'm a bit stumped by a VFX problem I need to solve. I've got a shot with a background element that needs to be removed. I've done this type of thing before by creating a "clean" frame in Photoshop (removing said item), tracking the shot in Mocha AE, then patching in the clean frame with the tracking data and some masks.

The technique works well, but there's an extra element to this shot, namely the focus changes drastically (it's a smooth pan with a rack focus from foreground to background). The element that needs to be removed begins as a very soft blur in the background then when the focus changes it becomes a cleanly defined box-shaped area. Obviously tracking in a single clean frame isn't going to get the job done here. I'm still trying to sketch out my solution for this, but it seems to me I'll either need to create multiple clean frames and somehow transition between them (which seems like it'll look obvious since the focus pull is a slow, gradual transition) or perhaps attempt to mirror the camera's focus change by adding a keyframed lens blur filter to the frame patch. My worry there is just getting it to match visually with the characteristics of the lens.

The only other technique I know is to duplicate the video layer, poke a hole in the top with a mask and then shift the bottom layer around to grab similar looking video from surrounding areas. This has the great advantage of containing within it already the necessary focus change, but the surrounding area of similar material (it's a wood floor) isn't large enough to cover up the hole. Maybe I can duplicate and shift things around on the bottom layer to get it up to size, though.

Apologies if this all sounds kind of vague and crazy but I figured I would ask if anyone here has any tips on how to deal with this kind of thing as it seems like it must be a pretty common thing to have focus pulls in shots where background replacement is needed. Thanks in advance for any help.

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Tudor "Ted" JelescuRe: Object Removal Question
by on Apr 13, 2010 at 10:27:46 am

Not seeing the shot it's hard to evaluate- this one is more "art" than recipe- but here's my 2 cents:
This may not work in your case, but sometimes it's actually easier to replace a bigger part of the image, like the whole floor. Start with the floor in focus, create clean plate, apply track and apply some sort of blur (with gradient mask for perspective intensity) to match the focus shift. This way, even if the focus shift it's not dead on, it will not be noticeable because you have that on a bigger surface and not just a patch.

Tudor "Ted" Jelescu
Senior VFX Artist
Bucharest, Romania

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Alperen T. AyhanRe: Object Removal Question
by on Apr 13, 2010 at 2:59:35 pm

this may work for you: duplicate layer( or comp) and draw a mask with pen tool around clean area. make only that layer 3D, create a new camera then play around the depth of field by keyframing. you will have the same focusing settings...

Alperen T. AYHAN
Always cinema...

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Casey PegramRe: Object Removal Question
by on Apr 13, 2010 at 3:17:16 pm

Thanks for the suggestions, guys. For right now I've solved it in editorial with some creative cropping (meaning zooming in the NLE - it's 1080p acquisition for 720p delivery, so I've got the wiggle room). I played around with it for several hours but couldn't get my track working to my satisfaction. It's a very, very shallow focus shot at one point and there's really nothing to track back there, even Mocha gets tripped up and starts sliding. It seems I'd probably have to track it by hand, which I don't have time for right now (although that could change).

My plan was to take my track data, paste it onto a solid matte, then use masks on that layer to cut out a couple floorboards in the scene (using alpha track matte). I got this working but of course my floorboards ended up in the air pretty quickly because of the sliding track.

Since you guys have given helpful answers though I'll post a couple grabs of the shot in question in case I'm overlooking the obvious:

Look at the table on both shots to get an idea of the connection between the two grabs.

Thanks again.

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