Setting up the Key for a Difficult Green Screen Shot
In this tutorial, we’ll walk you through the “tweakage” involved in getting a very nice key from a difficult green screen shot. Don’t be intimidated by the length of this tutorial – in most cases, the key should never take this much tweaking. We are using an extreme case here just to demonstrate how good a job ULTRA can do with a difficult shot.
In the Sessions tab, inside the Sample folder, you’ll find a session entitled Keyer Setup Sample. If you would like to compare your results, save your session and open this session.
- Start a New Session.
- In the Browse tab, find the clip called “Suzy – Tight – in wind.”You can play this clip simply by right-clicking on it in the folder and selecting Play. This clip has Suzy doing the infamous “hair flip” on a wrinkled green screen. This type of clip is a notorious “torture test” for any chroma keyer
Fig 1: Raw Suzy Clip
- Drag and drop the clip into the Input Clip icon.
- This particular clip of Suzy has a nice “Full Frame” at the end of the shot, so in the source preview monitor, scrub to the end of the clip.
- Go to the Keyer tab, and click the Set Key button.
- Go to the Backgrounds tab.
- Find the picture entitled “Clouds 02″ and drag and drop it into the background icon.
Fig 2: Starting Key
The starting key isn’t bad, and for some applications we might be done already. But we really want to show off how well ULTRA can make this shot look, so we’re going to get rid of that last bit of green in Suzy’s hair. This residual green may be hard to see against the current background. By using a pure white, pure black, and checkerboard pattern, you can detect and eliminate imperfections in the key. These useful backgrounds are built into the Background icon for your convenience. Right-click on the Background icon and try the different options to see which one best shows the green tinge. In general, black and white work well for checking edging or background imperfections, while the checkerboard backdrop is great for detecting unwanted transparency.
- Set the background to white by right-clicking the Background Icon and choosing Assign White.
- Select the Keyer tab.
- Adjust the alpha curve slider to clean up the sections of the hair that have disappeared along with the background, or have “keyed out.” Our sample session has a value of .402.You can see a graphic representation of Alpha Curve in the little box in the lower right corner. At this point, the hair may have a bit of a “halo.” Don’t worry – we’ll clean that up in the next few steps.
- Assign checkerboard to the background.
- Look for any transparency in Suzy. Adjust the transparency slider appropriately. You want the transparency to be as high as possible without causing Suzy to be see-through.A good section of the image to zoom in on in the first frame is Suzy’s left cheek. Look at the area where her hair falls over her cheek, and see if you detect any of the checkerboard showing through. Between .640 and .700 you’ll start to detect some of the checkerboard showing through. Our sample file has a value of .643.
TIP: Remember that you can zoom in or out of the preview monitor by dragging while holding down the right-hand mouse button (AKA right-dragging.)
- Use the Enable check box to flip back and forth from Keyer on and Keyer off, so that you can compare the original image to the keyed image.
- Set the background back to clouds by right-clicking on the input clip icon, and choosing Reassign Last Source.With the clouds back in place, you may notice a little bit of “noise” in the edging of the key. To clean this out, you can adjust the Start Threshold of the keyer.
- Adjust the Start Threshold up a bit. We use a value of .234 for this scene.TIP: When attempting to make fine-tune adjustments, click on the slider, and then use the left/right arrows on your keyboard to make adjustments.
Next, we’ll tweak the spill suppression settings to remove the green from not only Suzy’s hair, but her skin tone as well. The way spill suppression operates is by adding the opposite color of the background to areas of the keyed image that contain spill from the background.
- Start by lowering the Color Curve just a hair. To achieve good color suppression with skin tones, you’ll want the Color Curve graphic in the lower right corner to have just a bit of a curve to it instead of a straight line. A value of .475 should be enough.
- Take the Spill Suppression slider and move it all the way to the right.
- Take the Desaturation slider and try to dial in just the areas that need suppressing. In this case, pay special attention to Suzy’s hair and skin tone. At this step, you want to eliminate all of the green, even overcompensate with too much red. You’ll dial down the red in the next step. Our test session has a value of .643.
- Now, lower the suppression slider until the green spill is gone, but the rest of the colors in the image are true. The test session value is .832.
Fig 4:Color Supression Results
Try to avoid using the matte post processing settings unless they are absolutely necessary. These sliders take the key edges and further shrink them down, which can cause subtle parts of the key to disappear, like hair, transparent objects etc. In this case the wrinkles in the green screen are still showing on the left side of the image (assign Black to the background, and you’ll really see it.)
- Raise the Shrink Matte slider until the wrinkles on the image are less noticeable. Try a value of .240.Ideally, the color scheme of the background image and the key source should be matched so the two images look like they were shot under the same lighting conditions, but that rarely happens in the real world. In order to properly match the two, we will need to make some changes to both the background image and the keyed source to blend them together. Keep in mind that each tab adjusts whichever icon is active, indicated by a green LED. First, let’s adjust the background.
- Click on the LED next to the Background icon.
- Enable Pre-processing in the Colors tab.
- Boost the brightness up to roughly .334.
- Lower the saturation down to roughly .141
- Lower the Contrast to .158.
- Set red, green and blue to .244, .254, and .213, respectively.Essentially, we are painting in a touch more green to the image – not enough to make the eye register “green,” but it helps match the color of the camera to the background and makes that super-blue sky shot (probably processed more than once already) seem more realistic.
Fig 4: Correcting Background
Now, we need to match Suzy to the outdoor background. For this step, we’ll be using the post-processing color correction.
- Select the Input Clip icon so the green LED is highlighted.
- Enable Post Processing in the Colors tab.Part of shooting outdoors is seeing fairly harsh highlights on the talent. In this step we can use the highlights on Suzy’s forehead and cheeks as a reference point to “bring out” these highlights. We also want to eliminate some of the “overspill” from the spill suppression by reducing the red in the image.
- Reduce the brightness of the image a bit. A setting of .244 is good.
- Lower the saturation down to the .150 – .200 range.
- Raise the contrast slider to accentuate the lighting highlights on Suzy’s forehead and cheekbones. We want to make these look less like studio lights and more like natural sunlight. A value of .234 works very well.
- Finally, remove just a bit of red from the image. We actually only dropped the red value to .248 in our sample session. The best judge is to use Suzy’s pink shirt, and try and match the original value with the corrected value.
Fig 5: Correcting Talent Colors
You should now be ready to save out your clip!