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DPChallenge Forums >> Hardware and Software >> How do I use a Chroma Key Green-Blue Background?
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04/13/2006 09:48:54 AM · #1
I just got this for an Easter Gift....

Reversible / Collapsible Chroma Key Green-Blue Background by Civichrome featuring light absorbent velour for digital background replacement

Now what do I do with it. I think I understand that the color is to enable ease of replacing the background using Photoshop? (I have Photoshop Elements) Is that correct? Do I just use layers to replace the background color with a gradient fill or something like that?

Sorry for showin my stupidity!!

04/13/2006 10:01:55 AM · #2
You can literally replace the background with ANYTHING. Another photo, a group of photos, a gradient fill, a pure color, so forth.

The idea is that the green/blue is a color that rarely occurs in anyting else, and it is also very uniform and consistent, so a Magic Wand selection is usually a pretty easy way to "cut" it out and put the remaining "stuff" on top of whatever you want to be the new background. Elements doesn't have, I don't think, some of the fancy features that basically automate Chroma Key removal, but it's still a good tool.

Please note that most of the things I have mentioned are not legal for DPC entries, even in advanced editing. Just a disclaimer.
04/13/2006 10:06:30 AM · #3
Thank you nards......that's what I kinda thought...oh boy....can't wait to go home and try some pictures!!

04/13/2006 10:12:25 AM · #4
Chroma key is most often used to create video/film special effects, but it could be used in stills to provide a background (or foreground if it was placed there).

The colour selection tools in PS are able to select the very specific colours in a Chroma key backdrop quite easily and allow a 'matte' to be generated. Any image or fill can then be added in place of the coloured backdrop.

There are three important points to remember:

1. Light the backdrop as evenly as possible to provide the best 'key', ideally with a dedicated light source, rather then the spill that comes from the subject light

2. Keep the backdrop as far away from the subject as possible to avoid any coloured light spill. If blue or green light spills onto the subject then the key will be more difficult to generate (bits of the image that you want to keep will 'disappear').

3. Don't use a backdrop that has similar colours to those in your subject, for the same reason as point 2. In other words, blue jeans and blue chrome key would make life difficult :-)


Hope this is of some use
04/13/2006 10:33:00 AM · #5
Front....thank you for the added information. I am just trying to learn to take photos with Backdrops. 'He' thought this would be a good idea as I had indicated that I wanted to purchase a backdrop. I'm sure that with the non-professional photos that I take that I will be able to utilize this!

In any event, I am excited to have a new toy! Now when I want to get proper lighting equipment I can blame him because I 'need it for my backdrop that you bought'

Sound like a plan??


04/13/2006 11:27:44 AM · #6
There are a number of ways to replace the BG in PS - the magic wand is the easiest. you can set it's sensitivity, so if it grabs too much or too little it's easy to fix.

once it's selected you can do one of two things - I usually Select-Inverse (i want the subject, NOT the BG) and then Ctrl-J - pops the person onto a new layer - you can then put a layer below this layer and you're done.

Now there are ways to tweak this - once you have the person for example, go to (working from a bad memory here)'selection-modify-shrink' and et it to 1 pixel. this helps cut out the BG a bit better.

There are ways to make the selection a mask, and then you can gaussian blur the mask to ease the edges a bit too.

when substituting a BG, make sure:
The lighting is from the same direction
it the same color/brightness other wise it can look pasted together
you might blur the BG image a tiny bit - too sharp looks wrong.

I find abstract type BGs work best. I am not having success putting a shot I took of the great outdoors behind a person. Not sure why, but it looks really really fake to me.

I don't have a chroma key BG, but here are some i have done:
320510.jpg
the image on the right was on a BG like the one on the left - you can cut out easily with a chroma BG and it makes creating collages easy.

309213.jpg
This has a new BG - a greatly enlarged and blurred shot of my christmas tree. Rascal would NOT sit still, so the owner sat and held the dog, and I replaced the BG. They liked it so much they were here today (my first repeat customer!) and I get to do it again on their other dog, Odie.

278264.jpg
this is still my favorite - the eagle was shot at the National Aviary, the flag outside a VFW, same lens (maybe that's a secret?).

and finally...
278761.jpg
Again, an Aviary shot. This BG was generated by PS - under filter-render-clouds (on a blank new layer)
04/13/2006 11:42:08 AM · #7
Prof- Great examples and very detailed explanation....Thank you. (and congrats on your repeat customer!)

Thanks to each of you for taking the time to give me help. Now if I can just convince the boss that I am 'catching a cold.... ahem' maybe I can go home and test what you are teaching me.

I'll let you all know how I do!
Thanks again!
04/13/2006 12:28:20 PM · #8
All the previous are right on! I use chroma-key in this way:

1. load the chromakey image and the new background image into photoshop.
2. using the magic wand tool, select the chroma background.
3. select the background image.
4. using the 'move selection tool', drag the new background into the chroma area and position it as desired. Alternatively; select the background then using edit, cut and paste into the selected chroma area and repositon using the 'move selection' tool.
5. deselect and flatten the new image.

Something to watch for: make sure the brightness and contrast of the new background is correct to acomplish the effect you envisioned!
04/13/2006 01:04:52 PM · #9
Originally posted by ElGordo:

All the previous are right on! I use chroma-key in this way:

1. load the chromakey image and the new background image into photoshop.
2. using the magic wand tool, select the chroma background.
3. select the background image.
4. using the 'move selection tool', drag the new background into the chroma area and position it as desired. Alternatively; select the background then using edit, cut and paste into the selected chroma area and repositon using the 'move selection' tool.
5. deselect and flatten the new image.

Something to watch for: make sure the brightness and contrast of the new background is correct to acomplish the effect you envisioned!


I'm not sure I'm following this precisely, but I do it a little differently:

1. Load up original
2. Duplicate BG layer
3. Open Image that will become new BG, drag and drop new BG image onto original
4. Drag this layer in layers palette so it's between the two layers I have there
5. Make top layer (duplicate layer) active, magic wand to select chromakey, delete

Now you have the new BG showing behind the knocked-out subject. Check the edges to see how clean they look. Usually you need to fiddle with the amount of feathering in the chromakey selection. Use history to back up and adjust the feathering as necessary, then delete again. Do this at heavy magnification so you can watch your edges.

When I am doing multiple-image montages, I create a new document the size I want it to be out of the image that will be the montage BG. Then I open up, in turn, each of the smaller images, slect the chroma-key and delete, invert the selection, copy it, and paste it into the master document, where I can drag the selectiona round and free-transform it (rotation & resizing usually). I do this with each of the components in turn, and end up with something like this I did for a local library:

320534.jpg

Robt.
04/13/2006 01:23:32 PM · #10
I do a bunch of chromakey work...
my solution is a bit more expensive but ultimatte advantage makes the key very very clean.

I agree with what has been said about a even background and controlling the spill (reflections) from the key onto the subject.

here is one of mine 279822.jpg

Message edited by author 2006-04-13 13:25:02.
04/13/2006 01:37:38 PM · #11
Can some guide more about Chroma Key background
04/13/2006 01:38:50 PM · #12
Originally posted by General:

Can some guide more about Chroma Key background


Care to expand that cryptic statement/question?

R.
04/13/2006 01:42:20 PM · #13
Originally posted by Bear_Music:

Originally posted by General:

Can some guide more about Chroma Key background


Care to expand that cryptic statement/question?

R.


Its is hardly cryptic, i dont how more simple can i get.

All i wanted to know what is Chroma key Background
04/13/2006 01:49:21 PM · #14
Originally posted by General:

Originally posted by Bear_Music:

Originally posted by General:

Can some guide more about Chroma Key background


Care to expand that cryptic statement/question?

R.


Its is hardly cryptic, i dont how more simple can i get.

All i wanted to know what is Chroma key Background


well, okay... The grammar in the first is rather confusing...

Chromakey is BG material of a pure, bright color, typically green or blue, that is intended to facilitate removal of BG. The theory is to use a pure, saturated BG color that does not naturally appear in most subjects, so you can use color selection tools to select and delete all traces of the BG. In other words, it simplifies the selection process, essentially automates it.

R.

One vendor: http://www.chroma-key.com/

Message edited by author 2006-04-13 13:51:38.
04/13/2006 01:54:13 PM · #15
This is what I just gotbut in a 5x7

The grammar appeared cryptic....didn't need to be simpler, just clearer.

oops, Bear beat me to the Chroma Key answer.

Message edited by author 2006-04-13 13:57:55.
04/13/2006 01:56:12 PM · #16
Originally posted by General:

Originally posted by Bear_Music:

Originally posted by General:

Can some guide more about Chroma Key background


Care to expand that cryptic statement/question?

R.


Its is hardly cryptic, i dont how more simple can i get.

All i wanted to know what is Chroma key Background


Ummm if that's all you need to know, Google is a LOT faster and involves less typing for your friends.
04/13/2006 01:57:26 PM · #17
Thanks
04/13/2006 02:02:46 PM · #18
Does this background need to be of a special material, or just say two color mentioned are good enough and material doesnt matter
04/13/2006 03:28:31 PM · #19
Originally posted by General:

Does this background need to be of a special material, or just say two color mentioned are good enough and material doesnt matter


It isn't so much the material as it is the purity of the colors. Most colors "render" with sloppy channels, so to speak, where the chromakey is very precisely cut off. It's available in a number of materials, including at least fabric and paper; I've seen those two. I think you can get sheets of it also.

I tried painting a backdrop once so I could drop it out easier, "imitation chromakey", and it wasn't anywhere near as good, the color didn't mask as crisply. I'm honestly not 100% sure why this is the case, but...

Robt.
04/13/2006 03:51:31 PM · #20
Originally posted by General:

Does this background need to be of a special material, or just say two color mentioned are good enough and material doesnt matter

The material should be non-reflective (like cotton or velvet), not shiney (like silk or polyester) to minimize strong reflections -- that is, you don't want the light reflecting off the backdrop onto the subject.

You only need one color. The blue/green is blue on one side, green on the other. Other colors will work, but reds, oranges, yellows, browns are all tones found in skin while blue and green are not (usually). The reason for blue and green is so that if the subject is wearing blue, you use the green side. If they are wearing green, you use the blue side.

Message edited by author 2006-04-13 15:52:58.
04/13/2006 04:23:29 PM · #21
Of course you can make your own chromakey backgrounds. Chromakey paint is available from a few suppliers, google to find them. If the background will have seams, then use chromakey gaffers tape to cover the seams. But almost any FLAT finish paint will work and can be almost any color (blue or green is most effective) that does not coincide with the subject colors. Uniformity of the color is far more important than actual wavelength of the color so use whatever color you like. I use a sheet of masonite that has a very smooth surface and carefully spray the paint so that a uniform layer is deposited with no runs. If there are ripples are other nonuniformities they will not be illuminated properly for a uniform chromakey background. Lighting is crucial for a good effect. The background must be illuminated separately from the foreground and be as uniformly lighted as is possible. Make sure the chromakey background is far enough behind the subject (five feet or more works well) so that there is little or no reflection back onto the foreground subject. Make sure that subject lighting does not cast a shadow on the background!
04/13/2006 04:40:44 PM · #22
Poor man's chromakey! Works well in a studio setting:

1. Photograph the background, don't change anything on the camera such as zoom or exposure parameters.
2. Place the subject in front of the background and photograph the subject, make sure that no shadows are cast on the background from the subject.
3. Load both images into Photoshop and subtract the background photo from the image with the subject (actually use a 'difference' selection). Now you have an image with the subject only which can be copied and pasted into any background you desire.

This takes a bit of care and practice but will work almost anywhere that you can control the lighting.

Message edited by author 2006-04-13 16:42:34.
04/13/2006 06:29:20 PM · #23
Originally posted by ElGordo:

Make sure that subject lighting does not cast a shadow on the background!


Interstingly, the most convincing Chromakey shots (In video and film at least) are those that do allow shadows to be cast and also to appear over the inserted background image.

The trick is to maintain a good 'clip' off the shadow portions of the Chromakey backdrop.

I've never actually tried this with stills, but I have a feeling that one of the blending modes in PS might be able to make this work.

Hmmmm, I feel an experimental phase coming on....
04/13/2006 10:45:45 PM · #24
ok next question....this background comes all twisted/flexible into a small circle. This has made the material very wrinkled. what method should I use to get the marks out of it.?

04/13/2006 11:31:27 PM · #25
Another question, even if i have Chrome Key Background and i am planning a Portrait than real sucess of how easily how can i change background , doesn't it depend on my lighting also?.

I mean If i have shoot portrait with Chroma Key background and it is without shadows than changing background is easier as compared to Portrait shot with Chroma Key Bakground with shadows.
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