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clamping values

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I'm looking at dot3 bumpmapping and at one point during the info I'm looking at it says that an integer between [0,255] must be clamped to [-1.0,1.0].
float clamp(unsigned char i)
	float temp;

	temp = (i / 255.0f);	// first, clamp the value to [0,1]
	temp *= 2;		// multiply by 2
	temp -= 1.0f;		// subtract 1

	return temp;
This is what I did, and it initially looks to be fine (works with values 0,65,127,191,255). I was wondering if there was a generalised forumla for clamping a value between [a,b] to [c,d]. I took a look on mathworld, but I'm not really sure what to search for.

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[a,b] ->f [c,d]

f(x) = (x-a) // move [a,b] to [0,b-a] so we may scale by means of multiplication
* (d-c)/(b-a) // scale [0,b-a] to [0,d-c]
+ c // move from [0,d-c] to [c,d]

I'm not sure word "clamp" fits here. It's more like "remap".
Unless you're changing data type in the meantime, so it's clamped to some min/max automatically.

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