Using a 3rd party plugin like the FILL: choke effect from RE:vision Effects can be effective. Ultimately, this effect on it's own isn't sufficient to get rid of the edges. Making some major adjustments to the blend will start to get rid of the green, but if we check the mask only box, we can see that this causes us to lose some key opaque areas. Looking at our effects window, we have a couple different options to try to fix this. You can see that we're left with this distinct green outline. This is a very basic keying tool, and we pulled the initial key in our previous segment. Let's start by demonstrating chroma key in premiere pro. Luckily, there are some tools we can use to help counteract the shortcomings of our footage.
Chroma Subsampling is a big issue when dealing with green screens, because the color detail that is lost degrades the edges between your subject and the screen. this ratio takes one sample from row a, and one sample from row b, again returning only 25 percent of the 4:4:4 method, and is most commonly found in the DV and DVCPRO formats. This ratio is commonly used in HDV, and MPEG formats including most DSLR's on the market today.
Essentially this is only 25 percent of the the chroma information of the 4:4:4 method. This ratio takes two samples from the top row, and none from the bottom, which means the bottom row is sharing the chroma information for the top row. Formats such as Panasonic DVCPROHD, and SONY XDCAM HD 4-2-2 use this method. Essentially, this is only 50 percent of the chroma information of the 4:4:4 sampling rate. 4:2:2 subsampling has two pixels on the top row that share chroma and two pixels on the bottom row. Typically, only expensive high end cameras record this way. In this example, each pixel gets sampled for Chroma information, resulting in 4:4:4, which means no chroma sub-sampling is being done. The "b" number represents the number of pixels in the bottom row that will be sampled. The "a" number represents the number of pixels in the top row that will be sampled for chroma. 'J" represents how many pixels wide the sampling pattern is. The common way of writing out the chroma subsampling ratio is J, a, b. The number of pixels that share the same data determines what the subsample rate is.
Engineers developed a way for multiple pixels in an image to share the same chroma data, with acceptable loss in perceived quality. In order to try to save space when recording images. Viewers are much more sensitive to luminance accuracy than chroma accuracy. However, if you remove the luminance from your pixels, the picture is black. If you remove the chroma from your pixels, you end up with a greyscale image. The pixels that make up your footage have information about Luma, or brightness, and chroma, or color. If the subject is placed too close to the wall, or a rim light isn't used, it can create some difficult issues in post.Ĭhroma sub-sampling of your footage can also contribute to the issue. One reason this can occur is due to bad lighting. After doing the initial work to pull a key, the subject is often left with a green outline, or jagged edges. When you see the sheer amount of work that goes into big-budget film green screen work, it's no wonder that sometimes our own results may not match up to our expectations. Using chroma key in premiere pro, ultra key in premiere pro, and keylight in after effects, we'll fine tune those edges to get a great looking key. In this segment, we talk about fixing those rough and discolored edges that are often left after pulling an initial key. Green Screen work is complex, even under ideal circumstances, and working with footage that hasn't been shot properly can be pretty tough.
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