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@ -17,7 +17,7 @@ So, **we can just use an integer data type as a fixed point data type**, there i
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We may look at it this way: we still use integers but we use them to count smaller fractions than 1. For example in a 3D game where our basic spatial unit is 1 meter our variables may rather contain the number of centimeters (however in practice we should use powers of two, so rather 1/128ths of a meter). In the example in previous paragraph we count 1/4ths (we say our **scaling factor** is 1/4), so actually the number represented as `00000100` is what in floating point we'd write as `1.0` (`00000100` is 4 and 4 * 1/4 = 1), while `00000001` means `0.25`.
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This has just one consequence: **we have to [normalize](normalize.md) results of multiplication and division** (addition and subtraction work just as with integers, we can normally use the `+` and `-` operators). I.e. when multiplying, we have to divide the result by the inverse of the fractions we're counting, i.e. by 4 in our case (1/(1/4) = 4). Similarly when dividing, we need to MULTIPLY the result by this number. This is because we are using fractions as our units and when we multiply two numbers in those units, the units multiply as well, i.e. in our case multiplying two numbers that count 1/4ths give a result that counts 1/16ths, we need to divide this by 4 to get the number of 1/4ths back again (this works the same as e.g. units in physics, multiplying number of meters by number of meters gives meters squared.) For example the following integer multiplication:
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This has just one consequence: **we have to [normalize](normalization.md) results of multiplication and division** (addition and subtraction work just as with integers, we can normally use the `+` and `-` operators). I.e. when multiplying, we have to divide the result by the inverse of the fractions we're counting, i.e. by 4 in our case (1/(1/4) = 4). Similarly when dividing, we need to MULTIPLY the result by this number. This is because we are using fractions as our units and when we multiply two numbers in those units, the units multiply as well, i.e. in our case multiplying two numbers that count 1/4ths give a result that counts 1/16ths, we need to divide this by 4 to get the number of 1/4ths back again (this works the same as e.g. units in physics, multiplying number of meters by number of meters gives meters squared). For example the following integer multiplication:
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`00001000` * `00000010` = `00010000` (8 * 2 = 16)
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@ -29,7 +29,7 @@ SIDE NOTE: in practice you may see division replaced by the shift operator (inst
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With this normalization we also have to **think about how to bracket expressions to prevent rounding errors and [overflows](overflow.md)**, for example instead of `(x / y) * 4` we may want to write `(x * 4) / y`; imagine e.g. *x* being `00000010` (0.5) and *y* being `00000100` (1.0), the former would result in 0 (incorrect, rounding error) while the latter correctly results in 0.5. The bracketing depends on what values you expect to be in the variables so it can't really be done automatically by a compiler or library (well, it might probably be somehow handled at [runtime](runtime.md), but of course, that will be slower). There are also ways to prevent overflows e.g. with clever [bit hacks](bit_hack.md).
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The normalization is basically the only thing you have to think about, apart from this everything works as with integers. Remember that **this all also works with negative number in [two's complement](twos_complement.md)**, so you can use a signed integer type without any extra trouble.
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The normalization and bracketing are basically the only things you have to think about, apart from this everything works as with integers. Remember that **this all also works with negative number in [two's complement](twos_complement.md)**, so you can use a signed integer type without any extra trouble.
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Remember to **always use a power of two scaling factor** -- this is crucial for performance. I.e. you want to count 1/2th, 1/4th, 1/8ths etc., but NOT 1/10ths, as might be tempting. Why are power of two good here? Because computers work in binary and so the normalization operations with powers of two (division and multiplication by the scaling factor) can easily be optimized by the compiler to a mere [bit shift](bit_shift.md), an operation much faster than multiplication or division.
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@ -55,7 +55,7 @@ printf("%f\n",a);
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Equivalent code with fixed point may look as follows:
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```
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#define UNIT 1024 // our "1.0" value
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#define UNIT 1024 // our "1.0" value
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int
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a = 21 * UNIT,
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