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The contrast is how the colors differ with different lighting. Contrast only should be high in lower lights or if its for certain effects.
Chroma key photography is where the subject is photographed in front of a solid-color background -- usually green or blue -- that can easily be removed in post-processing using a program like...
Contrast is the difference between the lights and darks of an image. The greater the difference between lights and darks, the higher the contrast.
silver
you dont...never