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Using a Video as a Key

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Phil Baur
Using a Video as a Key
on Feb 7, 2015 at 7:29:24 pm

Remember back in the early days of digital video when people would film an empty frame, then, without changing the camera, they would have someone in the frame. Then, in post, they would put the second clip at 50% opacity to create a "ghost" effect on the person?

Good. Here's the question: in Motion 5, is there a way to use the empty frame as a Key to the second clip, effectively giving me just the movements of the person against an alpha channel? Basically I want to Chroma key, using all the information of the first clip as the Chroma. I only want the differences between the clip to show up.

Thanks!


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Robin S. Kurz
Re: Using a Video as a Key
on Feb 9, 2015 at 7:54:30 am
Last Edited By Robin S. Kurz on Feb 9, 2015 at 7:55:43 am

That is what is called a "Difference Matte", yes. The sloppiest form of matting by far, unless you're using a) uncompressed footage and b) the camera is most definitely locked 100%. In which case you simply place one clip on top of the other with the top one set to the "Difference" blend mode. That will (or should, given a and b are met) a black and white matte which you in turn can use as a matte for either one after the fact.

- RK

____________________________________________________
Deutsch? Hier gibt es ein umfassendes FCP X Training für dich!


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Simon Ubsdell
Re: Using a Video as a Key - Creating a Difference Keyer in Motion
on Feb 10, 2015 at 2:49:42 pm

[Robin S. Kurz] "That is what is called a "Difference Matte", yes. The sloppiest form of matting by far, unless you're using a) uncompressed footage and b) the camera is most definitely locked 100%. In which case you simply place one clip on top of the other with the top one set to the "Difference" blend mode. That will (or should, given a and b are met) a black and white matte which you in turn can use as a matte for either one after the fact."

While this is correct, it is only part of the story.

The Difference Blend Mode will create the basis of a matte but it will be full colour and not the grayscale image that is needed for a matte. It will look like this:



The extra steps needed are as follows.

Make a group containing your two plates, in which you have applied a Difference Blend Mode to the top layer.

Clone this group and hide the original group. Call your new group Difference Matte. Duplicate the clone two times (don't clone it!).

Add a Channel Mixer filter to each clone. Set the switch at the bottom of Monochrome. Set the top layer's Channel Mixer filter to Red-Red 1.0 and everything else to zero (apart from the Alpha-Alpha value which needs to be left at 1.0). Set the next layer's Channel Mixer filter to Red-Green 1.0. Set the bottom layer's Channel Mixer filter to Red-Blue 1.0.

Finally set the Blend mode for the top two layers to Add, leaving the bottom layer set to Normal.

The result is your Difference Matte.

You now simply need to create a new group containing the source image that you to isolate and any background you want to use. Add an Image Mask to your foreground layer and add the Difference Matte (group) to the image well, making sure to set the Source Channel to Luminance.

The raw Difference Matte will almost certainly be inadequate without some extra work so you will need to add a Levels filter to the Image Mask to punch up the whites (White In) and/or crush down the blacks (Black In).

Here's the Raw Matte and then the Raw Matte with a Levels filter applied to make it solid:





Robin is quite right in saying that a Difference Key is a very rough and ready effect and should be avoided wherever possible but sometimes it's the only way of isolating a foreground object (other than roto) so it's a technique well worth knowing.

Simon Ubsdell
tokyo-uk.com


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Robin S. Kurz
Re: Using a Video as a Key - Creating a Difference Keyer in Motion
on Feb 10, 2015 at 3:04:45 pm

Oh yeah... Motion is one of the COLOR difference matte-rs. Forgot about that, especially since FCP does it now, too. And again, depending on both image and most of all compression, you should be lucky to get that good of a matte with a clip. ;) But it's certainly less work than roto and worth a try!

- RK

____________________________________________________
Deutsch? Hier gibt es ein umfassendes FCP X Training für dich!


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Simon Ubsdell
Re: Using a Video as a Key - Creating a Difference Keyer in Motion
on Feb 11, 2015 at 6:44:15 pm
Last Edited By Simon Ubsdell on Feb 11, 2015 at 6:47:45 pm

[Robin S. Kurz] "Oh yeah... Motion is one of the COLOR difference matte-rs. "

Surely all Difference Blend modes return a color image from a color input and not grayscale, or am I missing something?

As I'm sure you know, purists recommend a different method (rather than using the Difference Blend mode) for optimum results in building a difference keyer:

1) Subtract Layer A from Layer B.

2) Subtract Layer B from Layer A.

3) Add the result of Step 1 and Step 2 together.

The function is as follows: (abs (Ra-Rb) + abs (Ga-Gb) + abs (Ba-Bb))

where R= red channel, G= green channel, B= blue channel; and a is Layer A, and b is Layer B.

Obviously these are functionally the same, in principle - this is just the long way of writing a difference function.

The good news is that in Motion there is no "difference" between these two methods and indeed you can verify that by subtracting the result of method 1 from the result of method 2.

Simon Ubsdell
tokyo-uk.com


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