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rambomhtri
rambomhtri
Known Participant
July 28, 2020
해결됨

How forgiving is the green screen for other different green shade?

  • July 28, 2020
  • 2 답변들
  • 17489 조회

Hi, I'm quite new to editing, I plan to do a green screen video with an object that is going to be all the time (would kill me to mask it manually, whatever this means) which color is far different from the "chroma key" green, but still green. It's a turquoise-mint-seafoam green, quite light/washed and noticeable different from the green screen's shade. The second problem is that the object is going to be directly surrounded by the green screen. I mean, it's not a green shade in the middle of another color object, it's an object which edge is going to be in contact with the green screen color.

Here are the colors:

 

Is it going to be a huge problem?

Or as long as the green is not very similar, you can work it out easily?

 

I chose green screen because I read it's the brightest for cameras, the most professional and it's easy to use due to tech specs of the color. The other option is buying a blue screen, but I just bought the green screen to do this project, and I've read that blue requires more advanced lightning as it's harder to work with.

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최고의 답변: Phillip Harvey

Blue screen would be ideal if your subject is going to be green. Honestly, any color would be good so long as it's not similar to the subject. Blue and Green are common because they aren't anything like skin tones/hair, etc., but in theory you can use any color as a key.

You can also use light and dark to help you key, usually called a luma matte. If your object was dark and you used a white background (or vice versa) you could create a matte to separate your object.

Can you use a green object with a green screen? Yes... But you better be very good at lighting your green screen. If your two green colors were as perfect as the colors you posted here then you'd likely be fine, the problem is that every tiny variation in green on your green screen is going to require you to expand the threshold to include more shades of green, and since your object is a shade of green not far off, you don't have a very big margin for error.

2 답변

rambomhtri
rambomhtri작성자
Known Participant
July 28, 2020

I've done some testing and I can totally chroma key the seafoam object, but there's a huge problem: the object, although correctly keyed and defined, loses its color, it goes from seafoam/mint to a very light grey. In other words, it loses its color. What parameter should I adjust so the effect only makes transparent the selected green shade and doesn't change or removes any other shade of green?

Now that I think about it, many, if not all, colors in a video have some kind of green into them, only perfect reds or blues don't have any kind of green. So, there must be a way to work it out, because if not, green screening only would work with red or blues.

M
Michael Grenadier
Legend
July 28, 2020

you can actually create a hi-con that will "cut the hole" and then, theoretically, color correct your foreground image.  I used to know how to do this in fcp7 but haven't had to figure it out in Premiere but, often you think you've got a clean key but when you actually composite it, you see flaws...  That's why chroma-keying filters have all sorts of controls to clean up the edges, etc...and adjust the color of the foreground...

rambomhtri
rambomhtri작성자
Known Participant
July 29, 2020

It makes me wonder if your keyed images aren't so cleanly keyed but are a bit transparent, like.. it took out your green values but left some luma values in your subject.

 

At any rate, if you do have a clean key you can potentially turn that layer into an alpha or luma matte. I'm not sure if Premiere will recognize an alpha matte based on Ultra Key, but you may be able to create a luma matte. If you play around with Levels and a Tint effect you should be able to crush the scene into two values: black and white. In this instance, if your key is clean, you'd actually want everything to be white, because what would be black is what's keyed out already. Then you can use the Trackmatte Key on a version of the original clip (with no ultra key or VFX), direct it to source from the layer with your Key/Tint/Levels (this layer is traditionally above the original-coming from working in AE-I'm not sure if it makes a difference in Premiere), and set it to luma.

Now you're not really using Ultra Key to punch out the background, but using a Track Matte. The Ultra Key was just helpful for creating the Track Matte (if this works out).

 


Hi, this is what I mean:

First picture is the original, below, the composite result (with black background) after keying the red color. As you can see, the red color is unique in that picture, the problem is, if I'm not wrong, all the different colors of that picture are a mix of Red, Green and Blue, if we are using the RGB profile. In a RGB profile, that kind of yellow in the wall is the sum of red + green. So, if I key the red color of the stick, that red color and its shades are removed from the entire picture, and I end up with a perfectly transparent stick (more or less) but a green color in the wall, because Yellow - Red = Green. I've learned that the only setting that avoids this whole red removing is the Range in color spill. If I lower the range, the yellow wall is not affected by the red removal of the key, and after some tweaks in all the other settings, I can perfectly manage to accomplish a perfect red keying, without the red of all the other colors being removed.

 

My question is... I haven't read any tutorial that says that I should first change the range, and then tweak all the other values. In my testing, this is exactly what I've done to get a wonderful almost perfect keying: first, you set the range, then you start tweaking everything else, using alpha channel and composite to see the results. Am I doing something wrong?

P
Community Expert
Phillip HarveyCommunity Expert답변
Community Expert
July 28, 2020

Blue screen would be ideal if your subject is going to be green. Honestly, any color would be good so long as it's not similar to the subject. Blue and Green are common because they aren't anything like skin tones/hair, etc., but in theory you can use any color as a key.

You can also use light and dark to help you key, usually called a luma matte. If your object was dark and you used a white background (or vice versa) you could create a matte to separate your object.

Can you use a green object with a green screen? Yes... But you better be very good at lighting your green screen. If your two green colors were as perfect as the colors you posted here then you'd likely be fine, the problem is that every tiny variation in green on your green screen is going to require you to expand the threshold to include more shades of green, and since your object is a shade of green not far off, you don't have a very big margin for error.

M
Michael Grenadier
Legend
July 28, 2020

shoot some tests... and even then you may be in for a bellyfull of pain.   My understanding is that the color space of the format you're shooting in can be critical...  Which means the camera you're shooting with is critical.  Do not expect great results if you're shooting h264...  Many cameras allow you to send the video to an external recorder with a much better color space...  but to a great degree, it's about your budget.  and having a dp who know's how to light for color key..., and making sure you have the time to have to light properly...  I was asked by a student of mine many years ago (when I was teaching avid editing)   about shooting a green screen program.  We'e talking maybe 25 years ago...  (I'm old, what can I say). I told her that the best thing was to have an ultimatte and switcher in the studio to be able to actually test the key before you shoot.  They chose NOT to spend the money and they landed up calling me in to handle the postproduction and. spending much more in post cause they cheaped out in production...  The software has gotten much more sophisticated since then, but still it's not simple...

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