Preblur for DV Keying

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  03 March 2003
Preblur for DV Keying

I've got a bit of DV chromakeying to do, & I want to do a bit of pre-blur on the chroma channels (in YUV colours space). I know someone who has 'Video Finesse' on Premiere, which allows you to do a horisontal blur to just the U&V channels, but I was hoping there was some way of doing it within combustion.

Anyone know of a workaround or plugin (preferrably a cheap one) that can do this?
  03 March 2003
Here is a cut and paste from a reply i posted a ways back...

The number of components per channel on MiniDV footage is 4:1:1 - which is bad news for Green and Blue screen keying, which is why it is so difficult to properly key DV. (4:1:1 refers to RGB channels). By blurring the Green and Blue channels a little bit and recombining the channels into a new image (before sending through a keyer) you can achieve much better results, or at least get rid of some nasty edges much easier.

Included is a diagram...basically what is going on is this...

Original footage is sent to three layer operators, so now we have 3 layers in the comp. However- we send the original red channel into all 3 RGB channels of the new Red Layer. Then we send the original Green channel into all 3 RGB channels of the new Green Layer. Then, we send the original Blue channel into all 3 RGB channels of the new blue layer. Then, apply a blur (4 pixels or so) to only the Blue and Green layers. Then, recombine the 3 New layers into one. At first glance the resulting output image won't look much different, but Ultimatte or another keyer has a much easier time with it, and you won't get near the aliasing around the edges. I tried it on several pieces of footage with varying degrees of improvement, but it's worth a shot if you find something difficult to key. It also works better if you apply the blur in YUV space instead of RGB.

Sorry for the complicated diagram. I had to black out the thumbnails due to NDAs
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Fix it in pre.
  04 April 2003
Cheers for the reply

Theres only one problem with it, the 4:1:1 sampling does not refer to RGB channels, but rather to YUV channels, so you get 1 sample for the two colour difference channels, for every 4 samples of the luminance channel (so U is Blue - Yellow, & V is Red - Yellow). Thats where the whole blurring U & V comes from, you blur the poorly sampled colour information, but the Luma (which retains your 'detail' remains unblurred).

That seems to have been 'chinese whispered' into the method you've got there - but it would probably be just as good to blur all channels (RGB) than just the green & blue, as all channels end up with the poorly sampled UV info (the Y+UV gets translated into RGB) so you'd effectivly get softening on your image detail.

I had a really good link on DV colour sampling, I'll see if I can hunt it out...

<edit> found it!

I just wish combustion could do a horisontal channel blur in YUV space...

Last edited by Chris : 04 April 2003 at 12:20 AM.
  04 April 2003
I knew it was something like that, blindly cut and pasted that from a reply from a real old thread, but you get the idea

I'm gonna try to knock out a workaround, been doing 3d exclusively for a few months so my compings a bit rusty.
Fix it in pre.
  04 April 2003
If you were using shake, you could probably do some crazy colorX stuff to get your keyin image to YUV space (or is that YCrCb?) Since you're not, though, you might check out Walker Effects, which has a channel extractor plugin that looks like it might let you get at the U and V channels long enough to blur them before you pull the key.
  04 April 2003
A guy on the combusion Webforum (Thanks Ludvig! ) posted an idea which could work, I'll test it tonight:

1. apply an unconstrained 4 pixel horisontal blur to the footage
2. apply a compound HSV operator after the blur
3. connect the V channel of the HSV operator to the unblurred footages value channel

Its kind of going about it in revers, instead of separating off the chroma & blurring it (which is a bit hard in RGB ) you are blurring everything, then reconnecting the unblurred Luma (hopefully)

edit - I tested it out last night, it works perfectly!

Last edited by Chris : 04 April 2003 at 08:57 PM.
  01 January 2006
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