A theory regarding 4:1:1 compression and clean bluescreen keys

Okay, this is just a random thought I've had.

We know that miniDV compression is 4:1:1, which means that for every four pixels, only one instance of a color is sampled. This causes fringes when attempting to key because of artifacting along the edges.

So. I was thinking, and perhaps this is off-the-wall (but really, aren't all the best theories? ), what if you shrank the image to 1/4th the vertical resolution prior to keying?

I'm aware that this would result in an image at a resolution of 180 X 480 (in the case of NTSC), but think about it:

Using this squeezed image (which according to theory should have intact, uncompressed color since the 4 interpolated color samples have been condensed to 1 clean sample), you can create your squeaky-clean black-and-white matte.

Rendering out this matte, stretching it BACK out to 720 (matching the compression of the original footage), and using it as a track matte for the original footage layer, should (again, in theory) generate some pretty clean chromakeying.

Maybe this would even work with shrinking the whole image down to 1/4th and using it that way, but you'd be compromising the vertical resolution that way, which isn't so much the issue.

There's probably some technical detail that would prevent my theory from working, and since I have no miniDV bluescreen footage I have no way to test it. But this hit me this evening and I thought I'd share it.

M. Scott

 

Sounds like someone needs to make a plugin...



Just make sure to give it to me for free

 

Sounds sound (ejit!)

Someone test it! Quick!!

 

What you are seeking is matte blur, really. All the good keyers have it. DVMatte, the production bundle matte choker, etc.

It helps, but in 4:1:1 it is still visible. PAL is 4:2:0 and is a little better.

By the way, you are mixing up horizontal and vertical my friend. You are talking about the HORIZONTAL resolution that you want to divide by 4, not the VERTICAL. Just a hint

/Z

 

No, I had it right. I was talking about horizontal resolution. I just made the comment that shrinking the ENTIRE video to 1/4th the size would be a compromise of vertical resolution. The rest of the time I was talking about horizontal resolution.

M. Scott

 

The shrink/stretch you mention is really just a work around to get the blur you'll get when the software resizes the image up and bilinear interportates the image. You then need to extract the matte and re-using it later because you know you've lost important image details.

The real point of the procedure is to blur the U and V channels horizontally. That's what is required to smooth out the image's chroma information for a smoother key.

In a tool like Shake, you would simply build a tree that converts the RGB images to YUV, blurs the U and V channels horizontally, and recombine it to RGB. This is what the DV matte plug-ins are doing, there is no need for special plug-ins if your software has RGB to YUV converters.

BTW, green screens will work better with DV than blue screens, because it has a higher luminance value which the DV hardware reacts to better

 

Dorkman, my man, where you been? Mister X already came up with this method, and yes, it works quite nicely. Good thinking though. Even if someone else came up with it first.

 

I think it could work. Someone needs to post a Pic of DV footage against bluescreen.

 

Way to piss on someones parade pixel! =P

Anyways there are also filters that you can use in conjunction with a matte box (yes that means you people with cameras that cant use matte boxes *cough the panasonic24p(iece of crap)* there is a filter you can buy that is not horribly expensive that (dont ask me how but ive seen it work and its amazing) does that basic method when you shoot. Looking at the filter I have no idea what the science behind it is, since its basicly a clear filter with small pink lines on evenly spaced across the filter. But you get a much cleaner key and sharper edges on blue screens with these things. I dont know if they have one for green screens or not, I only saw the ones designed for blue. I have the documentation around somewhere, ill see if i can dig it up.

 

Panasonic 24p piece of crap you say? You'll eat those words when you see my fanfilm my friend. That camera is awesome.

 

I downloaded a few movies from a website somewhere that were shot with the Panasonic in question, and they didn't look too special. Maybe I'm missing something?

 

What website was that cause I plan on buying the new Panasonic 24f HD DV Camera.

 

buying a 24P HD??? You got that kinda money layin around. If you plan on buying that you better call a panasonic rep and have em come demo it for you first if your really serious.

 

That'd probably be one of the first things I'd buy if I ever won Powerball, along with a decent motion control rig to go with it and all the necessary training I'd need to use it.

We can all dream