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Tough compositing query

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simon mountneyTough compositing query
by on May 23, 2011 at 8:17:12 am

Hi all, I've got a job in the works where I have to film and then key out some bottles of alcohol on a rotating turntable for use on the internet and an iphone app. I've done jobs much like this before and the keying has become very clean through practice.

One issue that has normally been cheated around is the issue of transparent areas of glass. In the past I have used a second shot of the interior and masked it onto the keyed out footage and this works very well.This technique only works when I was fully aware of what the background colour is supposed to be.

Now I have been asked to shoot some footage and provide transparent aspects within the glass so that different backgrounds come through. This process would be fairly simple using a still image in Photoshop, where you either mask or erase through to the background. A little bit of skill and this looks very good, however the process doesn't seem to cross over to After Effects. Keying is quite destructive, and removing the green information doesn't help the quality of information within see-through areas of glass.

Can anybody offer some advice as to what methods I could attempt to accomplish what I want in film? I have time to tinker around as the job has as of now only been quoted on, but I want to get it right, and as of this moment I am uncertain how successful I will be using what I currently know. Even if the job doesn't go ahead it would be a useful skill to know.

Kindest regards

Simon Mountney

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Tudor "Ted" JelescuRe: Tough compositing query
by on May 23, 2011 at 1:59:41 pm

Not having an idea of the background you will have behind transparent glass can be tricky since glass tends to pick up color from the environment and it distorts whatever is behind. Having movement makes it even harder and if you have anything but a cylindrical bottle ... well, I would recommend 3D. However, here's some ideas:
Make sure you can get a constant rotation.
Shoot the bottle on chroma. - main pass
Shoot the bottle on white. - highlights
Paint the bottle in white- except labels- and shoot on black. - create distortion map for the background
You can use just the regular distortion, however, if you have the budget get FreeForm Pro and use 3d displacement with reflection mapping on the distortion- this way you can have a more realistic distortion of the background.

Tudor "Ted" Jelescu
Senior VFX Artist

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simon mountneyRe: Tough compositing query
by on May 23, 2011 at 5:33:17 pm

Unfortunately for me, the budget for this doesn't include me buying and learning a 3d program from scratch, as nice as that would be. Are there any options left within AE? It seems like I might have to cheat with the interiors with artificial versions of whatever backgrounds get decided on. It's not the end of the world, but it is a pain in the arse as it'll have to be unique for each of a long list of bottles. I'm afraid that distortion maps are well out of the realm of my limited understanding.


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Darby EdelenRe: Tough compositing query
by on May 23, 2011 at 6:34:16 pm

You can use the properties of glass to your advantage here and shoot against a black background.

Once you have your footage in AE you can adjust the luminance of the image so that the background drops just into black and use this as a luma track matte for your footage. The last step is to remove the black that appears blended in with the semi-transparent areas. To do this, pre-compose your bottle with it's track matte and then set the pre-comp's blend mode in your main composition to Luminescent Premultiply (this will remove the 'premultiplied' black from the image).

As with any technique in AE this could be done in a few different ways. Other options would be to apply a Channel > Set Matte effect to your original footage and point it to the effected pure black background footage and use the Luminance for the matte. You could then apply the Channel > Remove Color Matting effect, or change the blend mode as above.

The benefits of this technique are that reflections in the glass should be preserved entirely and you should have no color spill to remove (as with a green screen). One downside is that if there are any opaque objects on the glass object (e.g. a label, or a bottle cap) you will need to roto those elements separately. Personally I think the benefit of having good looking transparent glass outweighs this downside.

I would recommend shooting a test. If you have any more questions the COW is here :)

Darby Edelen

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