Chroma Key in Avid
“Chroma key” is the workhorse of the digital effects world, used to combine foreground and background footage into one seamless shot. The TV weatherman, for example, is shot in front of a green or blue screen that is “keyed out,” or made invisible, and then combined with the desired background layer, such as an animated weather map or a forecast graphic. Avid Media Composer provides an effect called “SpectraMatte,” an advanced chroma keyer that makes it easy to get a high-quality key.
1. Drop the background footage onto the timeline on track V1, and the foreground footage above it on track V2. When you apply the key to the foreground footage, the colors you key out will become transparent, making the background footage visible.
2. Open the “Effects” palette and go to the “Key” category. Drag the “SpectraMatte” effect onto the foreground clip on track V2.
3. In the “Effect Editor” window for SpectraMatte, click on the “Color Preview” window in the “Key Color” parameter category. SpectraMatte keys out blue by default, so you will need to select the color you want to key out in your foreground clip. The “Color Preview” window is a small colored box that indicates the current color of the key. When you click the box, the mouse icon will turn into a dropper, indicating that it is now a color selector.
4. Click an area of the foreground image that best represents the color you want to key out. If you are keying out a green screen backdrop, click an area that’s closest to the true color of the screen, not a shadow or highlight. SpectraMatte will switch to keying out the color you select, and it will disappear, allowing the background to show through.
5. Check your key by clicking the “Bypass” button. This turns the effect on and off, making it easy to see exactly what you’re keying out. You can get more information about the key by clicking the “Show Alpha” button in the “Matte Analysis” section of the “Effect Editor” window. “Show Alpha” reveals the matte that the keyer has created. Black areas are fully transparent, and white areas are fully opaque. Shades of gray, usually found around edges between the black and white portions, indicate a blend of foreground and background.
6. Click the “Spectragraph” button to change the view to a color scope that allows you to fine-tune the key. The spectragraph shows three layers of color. The black wedge is the part of the color spectrum that’s keyed out. The middle colored layer shows the entire color spectrum in the component CbCr color space, and the top layer is a vectorscope that shows the distribution of colors in the foreground image. Pixels on the vectorscope that fall into the black wedge on the color spectrum are within the key area, and are keyed out in the image. This allows you to see exactly what color ranges in the image fall into the key. Pixels near the center of the spectragraph have a low saturation, and pixels near the edge have a high saturation.
7. Adjust the controls under the “Chroma” category to change the shape and position of the wedge, adjusting the key. The “Tolerance” slider changes the angle of the point of the wedge, allowing you to change the range of hues that are keyed out around the color you selected. If the area you want to key is very uniform in color, the “Tolerance” control will not have much effect. The “Saturation” slider moves the wedge closer or farther from the center of the color wheel, changing the range of saturation that is keyed out, but not the hue. The “Softness” sliders blur or sharpen the edge of the wedge, and “Alpha Offset” changes the overall size of the wedge.
8. Adjust areas of key color that have spilled onto the foreground subject by adjusting the sliders in the “Spill” category of the “Effect Editor.” If your subject is standing in front of a green screen, you often get a green spill around the edges of the subject caused by light reflecting off the screen onto the subject. SpectraMatte removes spill by changing the hue of colors that are near the key color. On the spectragraph, the vectorscope may show pixels close in hue to the key color that do not fall into the wedge. Adjusting the “Spill Angle Offset” slider changes the hue of these pixels so that they no longer have the spill color. Adjusting the slider too far often affects too wide a range of colors, changing the color of the whole foreground image, so make small changes until it looks right to you. The “Spill Saturation” slider changes the saturation range of pixels that will be affected by spill suppression.
9. Remove unwanted shadows that are messing up your key by using the “Suppress Shadow” slider. The “Suppress Shadow” slider increases the saturation values used to calculate the key, without actually affecting the image. In other words, SpectraMatte will treat shadowy, desaturated areas of the background as if they had a higher saturation. You can see the effect graphically on the vectorscope, as adjusting the slider moves the pixels further from the center of the color wheel.
10. Soften or harden the edges of the key with the “Matte Blur” slider. A subtle blur around the edges often makes a matte look more natural, and can help hide flaws in the key.
11. Invert the key, removing foreground subject and keeping the background. This changes SpectraMatte into a 3D warp effect. Pressing the “Promote” button in the bottom right corner of the “Effect Editor.” Open the “Foreground” category, and click the “Invert” button near the bottom of the menu.