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Old March 28th, 2008, 07:15 PM   #1
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Chroma subsampling

I plan to shoot HDV 1280x720 which is 4:2:0. What would it be if I subsampled the footage to 640x360? Can I get 4:4:4? What will it be if I use Vegas to do the subsampling? I plan to do chromakey and want to get the best result I can. Also is there a way that I can tell what the chroma is on clips?

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Jerry
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Old March 28th, 2008, 07:28 PM   #2
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Originally Posted by Jerry Jesion View Post
I plan to shoot HDV 1280x720 which is 4:2:0. What would it be if I subsampled the footage to 640x360? Can I get 4:4:4? What will it be if I use Vegas to do the subsampling? I plan to do chromakey and want to get the best result I can. Also is there a way that I can tell what the chroma is on clips?

Regards,
Jerry
You can't get more out than you put in. Which is why HDV is generally not recommended for chromakey. If memory serves, when you do anything with vegas effects, it resamples to 4:4:4, but you still aren't going to be adding anything to the video you shot 4:2:0.
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Old March 28th, 2008, 07:45 PM   #3
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Yes, one cannot get something for nothing, but I would be giving up spatial resolution - a factor of 2 for the width and height. My understanding is that 4:2:0 has the chroma subsampled by 2x2. I.E. each 2x2 block of pixels has a single chroma value. If this is subsampled by a factor of 2 for the x and y then one would get a chroma value for each pixel. Is there a flaw in my reasoning?

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Jerry
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Old March 28th, 2008, 08:30 PM   #4
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Yes I think I see where you're going. You're asking if you have a dime, and cut it in half, will you get two nickels.. :)

Someone more knowledgeable than me is going to have to answer your question for you. :)
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Old March 28th, 2008, 11:48 PM   #5
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Yes, one cannot get something for nothing, but I would be giving up spatial resolution - a factor of 2 for the width and height. My understanding is that 4:2:0 has the chroma subsampled by 2x2. I.E. each 2x2 block of pixels has a single chroma value. If this is subsampled by a factor of 2 for the x and y then one would get a chroma value for each pixel. Is there a flaw in my reasoning?
There's different variations on 4:2:0. The progressive flavours are like how you describe it. The interlaced flavours are hurt by interlacing, which kind of reduces the vertical effectiveness by half... so it's like chroma was reduced 2X horizontally and 4X vertically (instead of 2X vertically).

There are other technical details (e.g. in other cameras you'd have problems if the camera's sensor wasn't up to stuff, noise)... but you don't really need to worry about that.

2- Anyways, in practice:

a- You can try to make the most out of Vegas' chroma key. Using the chroma blur and the secondary CC filters to clean up your keys is one trick.

b- Or, you can use keying software like Ultra. (And a bunch of others on the market.)
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