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Tracking 3D camera, but placing objects in the mirror?

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Laco GaalTracking 3D camera, but placing objects in the mirror?
by on Mar 29, 2013 at 12:22:02 pm

I have a shot where there is a mirror visible. I can track the camera movement using PFTrack, or the built-in camera tracker, but the problem is that I will have to place an object in the mirror.

Do I have to "simply" mirror the camera movement on the X axis?


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Tom DaigonRe: Tracking 3D camera, but placing objects in the mirror?
by on Mar 29, 2013 at 4:10:15 pm

For tracking an object like a mirror where you want to insert an image in the mirror, I would use Mocha, which is an Object Tracking program that comes free with AE.

Tom Daigon
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Darby EdelenRe: Tracking 3D camera, but placing objects in the mirror?
by on Mar 29, 2013 at 4:39:11 pm

[Laco Gaal] "Do I have to "simply" mirror the camera movement on the X axis?"

Not exactly. You need to mirror across the, erm, mirror :) You also don't need to mirror the camera, you can mirror the layers you want to reflect.

To do this you'll need good tracks around the mirror to determine it's location and orientation. Then I'd recommend creating a solid layer with those transforms to use as a matte for objects appearing in the mirror. This is also a good way to check your track of the mirror, the solid should stay locked to it.

Next you can duplicate the layers you need to be reflected, parent them to your mirror solid and enter a scale of [100,100,-100] on it (flipping its z-axis). This will flip all of the duplicated reflection layers across the plane of the mirror. Last step is to matte the reflection layers with the mirror solid.

This is all assuming that your reflection layers are 3D layers. Also, you could optionally pre-compose all of the layers you need to reflect and collapse transformations on that pre-comp, then you only need to duplicate the pre-comp and parent it to the mirror solid to reflect/flip all the layers. If you also enable the collapse transformations/continuously rasterize switch on the mirror solid you made earlier you can use the Set Matte effect to matte the pre-comp with collapse transformations. No messy track mattes that way, pretty straight forward.

Here's a sample file with a simulated camera track:

5739_mirrortrack.aep.zip

Darby Edelen


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Laco GaalRe: Tracking 3D camera, but placing objects in the mirror?
by on Mar 30, 2013 at 12:03:50 am

Darby,
thanks for the answers, and the project file. Actually this will be a shooting two weeks later, and I will have to remove the DOP from the mirror.
I did a little test at home, if you're interested, I've uploaded the source file, the Pftrack export (please add the ".ma" extension), and the "clean plate" (need to freeze frame) here: http://we.tl/LhDn6nGNr7

I was able to solve the movement by parenting the plate to a layer with position set to 100,100,-100 (thanks for that!)
And I was also able to mask the mirror with your instructions. So it seems that It is solved.
The only thing I'm missing is the parallax movement of the clean plate...But it seems we'll have to live with that:)


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Darby EdelenRe: Tracking 3D camera, but placing objects in the mirror?
by on Mar 30, 2013 at 6:45:14 pm

[Laco Gaal] "The only thing I'm missing is the parallax movement of the clean plate...But it seems we'll have to live with that:)"

I'd love to take a look at your test file and advise on how to overcome the parallax issue, but the file transfer site you used is blocked on my corporate network and on my home ISP.

You can upload files directly to the COW by using the button on the far right above the new message text box (at the right of all of the text formatting buttons, it looks like a floppy disk).

Darby Edelen


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Laco GaalRe: Tracking 3D camera, but placing objects in the mirror?
by on Mar 30, 2013 at 7:07:59 pm

thanks, I've uploaded it here, because it's bigger than 100MB: http://carousel.hu/stuff/mirrorAE.zip


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Tudor "Ted" JelescuRe: Tracking 3D camera, but placing objects in the mirror?
by on Mar 29, 2013 at 7:30:32 pm

All the answers are good in theory, it would help though to see what you have in your mirror and to know what kind of object you need to add. It makes a world of difference if your object is static or if it is moving. Also, the type of camera move matters.
In theory you could track just the image reflected in the mirror and add a static object (flipped on the Y axis) in there using the camera move as tracked in the mirror.

Tudor "Ted" Jelescu
Senior VFX Artist


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