Monday, February 13, 2012

Background Replacement By Shooting On A Chroma Green Screen

Chroma Green Screen Background
Chroma Green Screen Background
Impact Chroma Sheet Background - 10 x 12' (Chroma Green)

When readers and my students ask me how to do background replacements on shots and generally I’ll tell them to just shoot on white or black and use the techniques found in Photoshop Compositing Secrets or The Photoshop Channels Book to mask out the background. This works for me, and if I do a good enough job lighting the background then I can also just drop a U-Point control on the entire background and dial up the brightness in Viveza shown in this example here:

Mouse over to see before, mouse out to see after
Viveza background substitution from white background
shot using a
Elinchrom Quadra Ringflash

However, the industry practice for video (i.e., your local weather forecast where the weatherman walks around with the map behind them) is done with what is called a chroma green screen.

To do this technique yourself you need a lighting setup that can be as complex as mine, or as simple as this kit that you can pick up for less than $100 at B&H:

Impact Digital Flash Umbrella Mount Kit
Impact Digital Flash Umbrella Mount Kit (<$100 at B&H)

For this article, I used the kit above but I’ll admit that I prefer this kind of umbrella adapter rather than the cheap ones included in this kit. In fact, I don’t even use a cold shoe – I just screw my 580 EX II flash directly into them using the little stand that comes with my flash (it has a screw mount on the bottom). I’m pretty sure this works for Nikon’s too, but honestly I haven’t ever tried it with something like a SB-900 or SB-910.

You also need a green screen of course, so for this article I chose the Impact Chroma Sheet Background - 10 x 12' (Chroma Green) which retailed for < $59 at B&H. I tossed this on a Impact Background Support System (12' Wide) and I had Kimberly sit on a posing stool. I positioned the white shoot through umbrellas (with the black covers removed) on the left and right of Kimberly and let ETTL do the rest. My handheld shot with my Canon 1D Mark IV was taken using a 70-200mm lens set to 102mm at f/2.8 using ISO 320 for 1/60 sec. The result was the shot at the top of this article and also shown later when you hover over the shot where I’ve replaced the background.

Replacing the Background in Photoshop

The key to replacing the background in Photoshop is the Color Range dialog that you’ve probably never used before. However, it’s easily found under the Select menu of CS5 (and presumably in the same place in earlier versions).

Color Range from the Select Menu of Photoshop

Once you have this dialog the easiest way to get the results you want is to HOLD THE SHIFT KEY and drag downwards over the areas where you know your background appears. REPEAT this process for the big areas until you get something that looks like the screen shot above.  Click OK and you get a selection like this:

Green Screen Selected using Color Range

To fix the eyes you can just use your favorite method of removing an unwanted selection. I like the quick mask tool, but a minus selection using lasso would work (as would many other ways).

Once your selection is good, then go to the select menu and inverse your selection. This selects the subject instead of the background. You can now do a CTRL+J (or CMD+J) to create a new layer with just your subject as shown here:

image

From here you can either fill your background layer with another color or just replace the background layer with a new background. For this example, I chose to replace it with black:

image

This of course reveals what I hate about using Chroma Green Screens – the horrid green fringe! If you are lucky you can go select the layer with your subject (Layer 1 in this example) and then go to Layer –> Matting –> Defringe. Enter a value (usually try 1 and work your way up from there) and voila the green halo goes away. However, that never seems to work perfectly from me (although it does help).

The easy way to remove the green screen

I use the free version of a tool called Easy Green Screen found at http://www.photoshopgreenscreen.com/ that just works at getting rid of most of that nasty green screen. Here’s a quick video that shows how it works:


Play in HD

With a simple few clicks I ended up with a much better separation of my subject from the green screen and doing a 1px Defringe fixed 99% of the problem. The net result of the before and after (using a new background is shown here):

Mouse over to see before, mouse out to see after
Mouse in and out to see the before and after

There’s a little area by her right arm, hair and shirt that isn’t 100% perfect but I honestly called it close enough for this article. In a real shot I might spend the time fixing it, but honestly that’s what I don’t like about green screens – you have to fight the green halo issue.

Of course you can use tools like Remask and Perfect Mask to quickly deal with contamination issues like this, but they can also be used to remove more complex background when you didn’t use a green screen in the first place.

Conclusion

This article shows that you don’t have to spend much money to do simple background replacements and that there are a wealth of tools to help you get perfect results. Lightroom users are probably better off using a white background and my Viveza trick for the cheapest way to just get a pure white background. However, Photoshop (and equivalent) users can use a product like Easy Green Screen and get great results in a hurry. The biggest tip to remember though is to be sure you get enough light on your background as the more evenly lit and bright it is, the easier it will be to remove it.

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6 comments:

Pompo Bresciani said...

to make it even easier you should also have the model stand further from the green background to prevent the green color from spilling on the hair and making it hard to select. Also a rim light (behind the model pointing at camera) will further separate the subject from thebackground and make the selection easier and cleaner

SUPERMHR said...

To add to the point made above, the actual key in rpelacing background in photoshop CS5 is the redesigned "Refine Edge" dialogue box, which throws every other channel and color based techinques out the window.
just make a quick selection (with the quick selection tool) and then, click on the "Refine Edge" button in the options bar.
using the "Refine Radius" tool (just making a stroke on the edge of the slection) will give out outstanding redults in no time.
to reduce the "Green Fringe" just hover over to the "Decontaminate Colors" check box and you're set.
a lot better results in a LOT less time.
Cheers.
MH.Rahmani

Ron Martinsen said...

SUPERMHR,

I use Refine Edge all of the time and that's actually why I reference "Photoshop Compositing Secrets" in this article because I didn't want to get into a big discussion on how to use that feature. That book covers it better than I would in a short blog article.

For this particular image Refine Edge didn't save the day. It's great sometimes, but not in this case (even when using its paint brush to include/exclude areas). Decomaninate colors being checked didn't help on the left side of the image but it did on the right.

Ron

Damon Bell said...

Ron,

Well Stated about the CS5 refine edge tool. I agree, the results vary widely per image (I've done extensive testing on with it).

It sometimes helps the transparency mask but other times makes it worse (fills in gaps that should be transparent).

The decontaminate colors is sometimes OK for the edge but usually not good for large spill areas.

Anyway, I just ran across this so I thought I'd post a reply. I am the developer of EGS.

Regarding your comment about defringing by 1 pixel, I'm planning to build a this option into version 4.

Also, thanks for posting this article. :)

Damon

SUPERMHR said...

the whole point was getting results faster.
and about the green fringe...
i found the method in this article exremely effective:
http://www.squidoo.com/photoshop-green-spill-correction

Ron Martinsen said...

SUPERMHR,

Sweet thanks for the tip! I haven't seen that one before so I'll have to give it a shot. I'll be pretty jazzed if that trick works with this shot.

Ron