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Convert a number range to another range, maintaining ratio

So I have a function that returns values within 0 and 255 and I need to convert these values to something between -255 and 255
So 200 would be roughly 145, 150 would be roughly 45 and so on..
I have looked at Convert a number range to another range, maintaining ratio but the formulas there won’t work.
Any other formula I could use?

Answer 1

Try this:

int Adjust( int num )
{
    return num * 2 - 255;
}