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Background Replacement
with Green Screen - CS5

By Craig Stocks

Green screen photography, or chroma key as it's called in the broadcast industry, has been around for a long time. The technique was originally developed by feature film producers in the 1930s, but you're probably most familiar with it from nightly news and weather reports. Green is the most commonly used background color, but blue is also used in some situations. The most important criteria is that the background color doesn't appear anywhere else in the image.

Now, with digital capture and readily available digital darkroom tools, green screen replacement is within the range of every photographer. Mastering the tools opens up new possibilities for special effects, and also provides a very cost effective way to expand the variety of backgrounds you can offer to clients. As an added benefit, you can freely change the color of the background to fine tune the background to best complement the subject.

With still photography, the biggest challenge is to cleanly cut out the subject. (A digital photo is typically much higher resolution than video or HDTV, which has about the same resolution as a 4 X 6 inch print.) The process normally involves three steps; first you need to remove the green background by creating a mask around the subject, then you normally need to fine tune the mask, and lastly you will drop in a new background. Most of the steps are similar whether the starting point is a green screen, blue screen, or any other background.

All of the steps can be done effectively in Adobe Photoshop, which is the focus of this tutorial. There are also a number of dedicated programs that specialize in green screen removal. such as Green Screen Wizard and Primatte. You can also buy kits that bundle a green backdrop with software, such as the Westcott Digital Green Screen Kit.

You'll probably want to avoid showing your subjects full length. It can be done, but there are numerous new challenges when the image includes the ground and a horizon line.

To get started, you need a suitable backdrop. There are a number of good options, including paper rolls, pop-up backdrops, and bulk material. You can also create your own by painting a wall or other suitable surface with an appropriate paint formulation. The best backgrounds won't show any glare or reflections, so be sure to use something with a dull finish, such as felt or flat wall paint.

A good end result starts with a good image. Make sure the background is brightly and evenly light. Ideally, you want a uniform, well saturated color that will be easy to select. It's important to avoid shadows on the background from the subject. In this example, there is a background light positioned behind the subjects specifically to light the backdrop.

You may run into a situation where the background isn't large enough to completely fill the frame. No problem, just make sure that the green screen is behind the most important parts, especially around hair or other difficult to extract parts. It's very easy to deal with corners and other areas of the frame with normal selection tools.

Be sure to keep some space between your subject and the background. If the background is too close, or too bright, you can get quite a bit of color contamination on the subject. In the sample photo, the backdrop was about six feet behind the subjects.

For Green Paint:
oz 48 96
Ax Perm yellow 2 12 0
D Thalo Green 7 24 0
Kx White 2 24 0
Kx Magenta 0 2 0

For Blue Paint:
E Thalo Blue 7 32 0
Ex White 3 0 0
V Magenta 3 0 0

Premium Plus Interior Flat wall Low voc
Infinity and beyond (Dc3c-20-4)
Deep Base (21300)

This was copied off the label on a can of paint from Home Depot.

The new Photoshop CS5 includes a number of really great, new features, but here we'll focus on how the new Refine Edge command makes green screen extraction even better. In fact, it's so good, you may find that you don't need any additional cleanup work on the mask.

Load the image into Photoshop to get started. Unlike with previous versions, you don't need to duplicate the background layer. CS5 will do that for us, and automatically create the mask.

The last step in this dialog box is to adjust the Fuzziness slider back and forth until you get a good selection where all of the background is masked, and all of the subject is visible.

The first step is to create a selection. You can use either Select Color Range, or any selection tool will work as a starting point. Many times you can get outstanding results using the Quick Selection tool as well. With the new Refine Edge tool, the quality of the initial selection is less important than it was with earlier versions.

Create an initial selection using the Select Color Range command, which will open the dialog box shown here. To see the selected area, I've chosen the Quick Mask preview mode. I've also checked the Invert box to invert the selection. That way, the colors I choose will be masked out, and the remainder will be actually be the selected region.

Click on the left-most eyedropper tool and click on the image in the green area. To fill in the entire green area, click on the middle eyedropper and continue clicking in other areas of the background to add those colors to the selection.

The last step in this dialog box is to adjust the Fuzziness slider back and forth until you get a good selection where all of the background is masked, and all of the subject is visible.

Now the CS5 magic! Select the new Select - Refine Edge tool from the menu. As shown here, I've chosen to view the resulting image on a white background, so it's easy to see how the settings will be applied. There is also a new Smart Radius setting that tells Photoshop how far out to look for adjoining details. I've also used the Paint Edge tool (just to the left of the Edge Detection box) to paint around the hair where I want CS5 to look a little further out. I've also checked the box to remove color contamination, which does even more to perfect the selection.

Lastly, I've chosen output to New Layer with Layer Mask. With this setting, CS5 will automatically duplicate the Background layer and add the mask. Talk about easy! Just fill the Background layer with white and you're ready to move on.

Even as amazing as the new Refine Edge command is, there may still be some areas of the mask that need to be cleaned up. It really depends on the size and resolution of the finished image. If it's going to be used as a small avatar on Facebook, then you're probably done here. If you want to print an 8 X 10 inch portrait, you'll probably need to fine tune the mask.

No matter which process you use to create the mask, you probably still need to do some fine tuning. In this tutorial, I'll describe the two most common tasks, fine tuning the mask to perfect the cutout and fixing color contamination that may occur. The most difficult area is around hair, so we'll concentrate on those areas.

This area exhibits both kinds of problems. There are some areas where the green background is still partially visible through the mask. Also, notice that some of the fine hair has taken on a green color cast. The detail is there, it's just the wrong color.

First, let's work on the mask. Just like we used Overlay mode when we filled the "a copy" channel with white and black, we can paint in overlay mode to fine tune the edges. First, make sure the mask is selected in the Layers pallet, and choose a soft brush. Set your foreground color to black. Lastly, change the brush's Mode to Overlay

As you begin to brush over the offending areas on the mask, the mask edges will contract and increase in contrast. In this example, I made one stroke at 100% opacity, but that was too much of an adjustment. I immediately selected Edit - Fade to adjust the brush's opacity to 15%.

If you to make the opposite adjustment, simply change the brush color to white. Then, painting on the mask will reveal the detail.

Next, we'll deal with the color contamination. The goal here is to restore the correct color to the fine hairs by painting the color on a new layer.

Add a new layer and set its blending mode to "Hue." Then, right-click to the right of the layer name and select "Create Clipping Mask." That will ensure that whatever is on this layer only affects the visible pixels on the layer below.

Now, select a soft edged brush and set the foreground color by Alt-clicking on a nearby region of hair. Then simply paint that color onto the layer. The color you paint only affects the color (because of Hue blending mode ) of the visible hair (because the layer is clipped to the one below).

The big advantage of the traditional filter tools is that they can be applied to just a portion of the mask. The Masks panel works on the entire mask - all or nothing. But, the Filter tools can be applied to just a selected area of the mask. With the mask active, select the region to be affected (using the Rectangular Marquee or Polygonal Lasso tool) and apply the filters. A typical change is to grow or shrink the mask by 1 or 2 pixels, then apply a Gaussian blur of that amount or less.

It's that easy. Just work your way around the subject inspecting and fine tuning as you go. You may also find it useful to occasionally invert the white background to black to make sure the result looks good on different backgrounds. Depending on the subject, some backgrounds are much more forgiving than others.

Now it's time for the creative fun - dropping in a new background. I'll show a couple of options, but you're only limited by your imagination, and perhaps good taste.

There are three broad categories of replacement background. You can create a background entirely within Photoshop by applying filters and layer styles to a new layer. You can also use images of traditional backgrounds. You may have a background you can photograph, but you'll probably want to get one from Wetzel & Company that specializes in providing high quality files that can be used as a digital background.

Of course, you can also place your subjects in an entirely new setting. Here's where taste and judgment comes into play. If you want a natural looking result, you'll need to choose a setting that's appropriate for the subject's pose, style and lighting. The example shown here would look very out of place with a beach sunset as a background, whereas a model in a swimsuit would look out of place with a formal studio background.

Next, let's try a background with a more current feel. Here, I've chosen the Wetzel DPB-12 from the Digital Pastel collection. I chose this background because it has the look of a traditional hand-painted canvas backdrop.

I've added the new background image as a Smart Object, which provides continued flexibility. As in the previous example, I added a Gradient Overlay layer effect. I also added a Gaussian Blur as a Smart Filter so that the background will appear out of focus.

To fine tune the brightness, contrast and color of the background, I added two additional adjustment layers, both clipped to the layer with the new background.

Again, notice that I haven't deleted any of the earlier alternatives. I can easily go back to either of the earlier backgrounds.

One really nice feature of digital backgrounds is their flexibility. If you like the pattern, but the color doesn't fit the subject, you can easily change it. In this example, I've simply clipped a Hue Saturation adjustment layer to the background Smart Object and shifted adjusted the Hue, Saturation and Brightness sliders to get a completely different look.

Be sure to check out Wetzel's Download Library where you can find thousands of art and photographic backgrounds - ready to use in your latest project.

The preceding tutorial is an excerpt from an extensive "Green Screen" tutorial by Craig Stocks. You can read his entire tutorial at his website, where Craig discusses Lab Mode, and Select Color as alternative selection techniques.



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