Search TipsUsing Overlay Tools and Screen Correct
Screen Correct compensates for anomalies in the backing area such as uneven lighting, smudges, seams, variation in the backing color, green set pieces, and unwanted shadows cast by set pieces, boom arms, etc. This technique assumes that an exact copy of the problematic green screen element with the subject matter omitted, usually called a Clean Plate, is supplied.
Although this technique gives great results by differentiating between foreground elements and backing flaws, you often haven’t got the necessary reference materials. In that case, you can create a synthetic Clean Plate with Ultimatte.
To achieve the best results, use a reference Clean Plate as an input to Ultimatte in addition to allowing Ultimatte to generate a synthetic plate. In this way, the reference plate allows for the best shadow separation, while the synthetic plate is used to reduce artifacts such as film grain or video noise (which is rarely accurately reproduced when the clean plate is shot during the time of principal photography). Switch the view to screen by clicking the overlay dropdown menu and selecting screen.
With screen correct selected, use the add overlay tool to scrub on areas that represent the green screen, including shadows. Usually, it’s best to press Alt+Ctrl/Cmd while scrubbing, so that you are picking the color in the input image at that pixel, instead of the color of the processed image.
The overlay is used in the final process to fully eliminate these areas from the final composite. Continue in this manner until the foreground subject and its shadows are surrounded with an overlay. Make sure the overlay does not cover any foreground detail that is to be retained. If the overlay does cover foreground subject matter, then use the remove overlay tool to scrub on those areas that should not be covered by the overlay. Repeat these two processes until the overlay covers the green screen areas and no subject matter. View the composite in the Viewer to see the screen-corrected composite with anomalies in the backing area removed. To learn which controls were automatically set by Ultimatte, click the Screen Correct tab and note the position of the controls.
Note: When scrubbing on the image using add overlay, the RGB value of the points selected are accumulated in a keep list and removeoverlay points are accumulated in a drop list. If both lists have equivalent values, a conflict may occur, resulting in no visible change in the overlay. If a conflict occurs, try using Undo (Ctrl/Cmd+X) which flushes the last set of points added to the appropriate list. If that doesn’t help, you can also press the Reset button on the ScreenCorrect tab to clear the lists and start over.
Under some circumstances, it may be difficult to completely cover the screen area with the overlay. This does not mean that the screen cannot be removed in that area, but that other controls may need to be adjusted to help reduce anomalies in those areas that were not covered by the overlay. It may be advantageous to resample the screen using the Screen tool above the Viewer in the area where covering the screen with the overlay is unsuccessful, but be careful about selecting in shadow areas if the final composite requires the retention of shadows.
Additionally, it may be difficult in some circumstances to remove the overlay from very dark areas of foreground detail that may be close to values in dark or shadow areas of the screen. If the overlay does include some dark foreground areas, these areas may be recovered by enabling the Shadows controls.
Note: Since Ultimatte produces a synthetic clean plate using an interpolation process, it is important to exclude non-video areas of the image, such as the black blanking lines that may appear in video or the black areas in letterbox images. You can use other nodes, such as Crop, to exclude these areas from the overlay and final composites. Otherwise the synthetic clean plate may contain black (or other colors) which can result in an inaccurate clean plate being generated.
Adjusting overlay controls
You can choose your overlay view, color and output mode in the properties panel, on the Ultimatte tab.
• overlay - The overlay control is used to show the calculated overlay on the Viewer and it’s helpful for debugging purposes. It helps tune the screen correct controls and also the add/remove overlay tools. In this way you can see immediately if something is wrong and needs to be adjusted. In the overlay dropdown menu, change
• off - to not view the overlay.
• screen - to view the subject in clear, and the preliminary matte area blended with the overlay color.
• subject - show the subject blended with overlay color, and the preliminary matte area in clear.
• show image as monochrome - check this to make the input image appear in grayscale so that the overlay areas can be more easily distinguished.
• overlay color - use this to change the color of the overlay. You can adjust the alpha channel to modify the opacity of the overlay.
Adjusting the screen correct controls
• screen tolerance - This is used to adjust the color range or tolerance to be included or excluded from the screen overlay.
• shrink - Use this control to shrink the screen overlay.
• darks(reds smaller) - Use this control to include or exclude dark areas from the screen overlay in those areas where the blue value (when using green screen) is greater than the red value in the original foreground image.
• darks(reds larger) - Use this control to include or exclude dark areas from the screen overlay in those areas where the red value is greater than the blue value (when using green screen) in the original foreground image.
• brights(reds smaller) - Use this control to include or exclude bright areas from the screen overlay in those areas where the blue value (when using green screen) is greater than the red value in the original foreground image.
• brights(reds larger) - Use this control to include or exclude bright areas from the screen overlay in those areas where the red value is greater than the blue value (when using green screen) in the original foreground image.
• orphans - Use this to include or exclude neighboring "rogue" pixels from the screen overlay.
