Convert numbers within a range to numbers within another range [duplicate]

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Possible Duplicate:

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?

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public static int ConvertRange(
    int originalStart, int originalEnd, // original range
    int newStart, int newEnd, // desired range
    int value) // value to convert
{
    double scale = (double)(newEnd - newStart) / (originalEnd - originalStart);
    return (int)(newStart + ((value - originalStart) * scale));
}

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Try this:

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

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General solution for arbitrary range...

var val1 = 200;
var min1 = 0;
var max1 = 255;
var range1 = max1 - min1;

var min2 = -255;
var max2 = 255;
var range2 = max2 - min2;

var val2 = val1*range2/range1 + min2;

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public int ConvertRange(
           int originalStart, int originalEnd,
           int newStart, int newEnd,
           int value)
{

  int originalDiff = originalEnd - originalStart;
  int newDiff = newEnd - newStart;
  int ratio = newDiff / originalDiff;
  int newProduct = value * ratio;
  int finalValue = newProduct + newStart;
  return finalValue; 

}

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Adjusted = original / 255 * 510 - 255

145 = 200 / 255 * 510 - 255
 45 = 145 / 255 * 510 - 255