Invert pixel color vectors of your images.
Yes - each color channel value is subtracted from its maximum, mathematically identical to how a traditional film negative represents colors.
For simple black-text-on-white documents, inverting produces white-text-on-black, which some people find easier to read in low light - for full-color photos, inversion is more of an artistic effect than a readability aid.
Yes - inverting an already-inverted image restores the original colors, since the operation is its own exact reverse.
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