The BCC Chroma Key filter is designed to pull a perfect key including transparent areas on the first try by simply picking the backing color. In that sense it may be the easiest keyer on the market to use. It is based on the concept of dominant color. It automatically detects the dominant hue component of the sampled color, be it green, blue or orange, and works its magic based on that hue. It should not be confused with linear color keys or luma keys which define transparency proportional to a single color or luma value. The chromakey algorithm is far more intelligent than that.
To further convince ourselves that we pulled a successful key let's switch the preview to "Show Matte" in the Output popup. This is the ultimate test when setting up the keyer because it is easy to miss important print-through areas while looking at the composite image. Print-through areas will be much more apparent in the rendered output when you play them back because the moving background will crawl or animate "inside" your foreground object in an obvious way. So let's take a look at the matte.
Even though the key is near perfect (all transparent areas are solid black and all opaque areas are solid white) it can be further improved by decreasing the Density slider. This slider is changing the translucency of semi-transparent areas without affecting the opaque areas. After setting the value to 140 (make sure to hold all parameters for the duration of your clip) the glass becomes even more translucent. Now observe that the solid white areas of the matte remain white but semi-transparent gray areas become a little lighter.
But notice that we lost shadows that were present in the original shot. Let's see how to bring them back. Shadows will make the final composite much more natural and convincing. Most Chormakey shooting instructions suggest to minimize cast shadows by using more even lighting (with backlight and sidelight) but for BCC Chromakey shadows may be an added bonus. We will use the Lightness parameter to bring back the shadows. Let's set it to the value of 20. Voil. The shadows are back!