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White chroma key

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leonardo barone
White chroma key
on Jun 29, 2016 at 8:07:35 pm

Painting a wall is a better alternative than muslin,foam or others matherials green screen, because a wall can't have any whinkles, I was thinking to paint my wall when I thought, hey, wait a moment, because I can use also blu and white color for get a key, why i should paint it ? Why I can't get key just using my white wall ? I tried to do it without softboxs, the parts of the walls where was enlighted by the sun get a good key, so why I can't get a good chroma simple enlighting my wall with two softboxs ?


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John Rofrano
Re: White chroma key
on Jun 29, 2016 at 11:41:44 pm

[leonardo barone] "...so why I can't get a good chroma simple enlighting my wall with two softboxs ?"
Because if there are people in the shot, the whites of their eyes will get keyed out a will there white socks or shirt or pants. The reason chroma key uses such a bright green is because that usually won't appear in the foreground that you want to keep. Green is also selected because most video is compressed to 4:2:2, 4:2:1, or 4:2:0 with 4: being the luma and the green channel so there is twice as much information for green in the video signal as there are for red or blue. This is why green makes a better key.

Feel free to use white if you want to but at that point I would use a Luma keyer rather than a Chroma keyer. You can also use the Mask Generator to pull a key with a white background by making an Alpha Mask from the footage.

~jr

http://www.johnrofrano.com
http://www.vasstsoftware.com



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Graham Bernard
Re: White chroma key
on Jun 30, 2016 at 3:40:40 am

I agree with John.

What I will add regarding why Sun is better than your light boxes is also white balance. The Sun will be full spectrum and your boxes probably not. You can check this using the Vegas scopes. Mixing Sun - daylight- and artificial light - your boxes - can be fraught. I have a great LED light but this has actual holes or gaps in the Spectrum and no amount of White Balance can correct it. What I'd need to do, to "improve" matters, is place a correctional Lee filter over the front. If your boxes have some means of balance correction, try that.

Simply put, mixing Sun and artificial is problematic. Doing CK work, with an even outcome, requires quite a controlled environment. But that doesn't necessarily mean complicated. Mixing Sun and artificial light is complicated and, as you've discovered, most likely counter productive. I've used GREEN material and a steam iron to remove the wrinkles. The steam iron is a far better analogue solution than relying on mixed light sources!

* Grazie

Video Content Creator and Potter
PC 7 64-bit 16gb * Intel® Core™i7-2600k Quad Core 3.40GHz * 2GB NVIDIA GEFORCE GTX 560 Ti
Cameras: Canon XF300 + PowerShot SX50HS Bridge


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leonardo barone
Re: White chroma key
on Jun 30, 2016 at 6:39:40 am

Yes guys, I know that green is better then white or also blue for the reasons you mentioned, but what I meant was " if there aren't any difference between using a normal green screen background (muslin or other materials ) and painting the wall in green, why there should be any difference between a normal white screen background and a normal white wall ?
That's the question, lol...


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Graham Bernard
Re: White chroma key
on Jun 30, 2016 at 7:08:45 am

I understood you were lighting the background with two different sources of light - Sun and Boxes.

* Grazie

Video Content Creator and Potter
PC 7 64-bit 16gb * Intel® Core™i7-2600k Quad Core 3.40GHz * 2GB NVIDIA GEFORCE GTX 560 Ti
Cameras: Canon XF300 + PowerShot SX50HS Bridge


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