| Chromakey overview.
Primatte contributes to a process called 'chromakey'. But what exactly is chromakey? Knowing will help you to determine whether Primatte is right for your photography work.
Chromakey is the process used to insert a new background behind a model or object. The subject is photographed against a background of a single color (or a relatively narrow range of colors). Then that single color is removed digitally. Primatte performs this extraction directly inside Photoshop as a plugin.
How it works.
The backing screen is typically blue or green, the two colors that contrast most to human flesh tones. The background color should NOT be present in the subject.
This allows the digital extraction tool to detect a difference between foreground and background. To photograph a woman wearing blue jeans, for instance, use a green screen.
When the solid color is digitally extracted, the masking process leaves transparency (blank space) around your subject. Once the color has been removed, you can add in a new background. How? Simply copy a new image into your Photoshop file. |
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In this photoshoot, our model has been positioned against a blue screen. Within Primatte, anything blue is identified as black and targeted for removal.
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Chromakey terms.
Chromakeying is often referred to as 'blue screen' or 'green screen' photography, because of the frequency of those colors being used as backdrops.
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A term common in Photoshop is 'masking'. To 'create a mask' means to isolate a part of an image for the purpose of editing. We use this word in Primatte's interface.
Two synonyms for mask are 'key' and 'matte'. You may hear the expression 'pull a key' or 'keying'. This simply means creating a mask. |
Behind the scenes.
So, what really happens in Primatte when you click and drag with a tool? Well, the heart of Primatte is its proprietary mathematical algorithm. This is called the 'Polyhedral Slicing Algorithm', and it operates in 3D RGB Color Space. Huh?
Basically, Primatte separates all of the pixels in your photograph into four categories. It evaluates the pixels that have been put in each of these categories. Then it decides what to do with these pixels; that is, keep them or delete them. |
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An simple but important point to understand is that Primatte works 'non-contiguously'. That is, the plugin does not simply evaluate the area of the image that you've sampled. That's how standard Photoshop tools work. Instead, Primatte evaluates and operates on the entire photograph, which is what makes it so powerful.
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