How Photoshop Translates RGB Color to Gray
By Scott Valentine

You’re probably well aware that there exists no less than 1.274 gazillion ways to convert your photo from color to grayscale, and that you should know at least one-third of them so you can handle any situation. But what if you want to add your own custom technique? You’ll need to know something about how Photoshop “sees” color and how it manages the change from color to gray.

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