How to convert packed integer (16.16) fixed-point to float?
How to convert a "32-bit signed fixed-point number (16.16)" to a float?
Is (fixed >> 16) + (fixed & 0xffff) / 65536.0 ok? What about -2.5? And -0.5?
Or is fixed / 65536.0 the right way?
(PS: How does signed fixed-point "-0.5" looks like in memory anyway?)
Answer
I assume two's complement 32 bit integers and operators working as in C#.
How to do the conversion?
fixed / 65536.0
is correct and easy to understand.
(fixed >> 16) + (fixed & 0xffff) / 65536.0
Is equivalent to the above for positive integers, but slower, and harder to read. You're basically using the distributive law to separate a single division into two divisions, and write the first one using a bitshift.
For negative integers fixed & 0xffff doesn't give you the fractional bits, so it's not correct for negative numbers.
Look at the raw integer -1 which should map to -1/65536. This code returns 65535/65536 instead.
Depending on your compiler it might be faster to do:
fixed * (1/65536.0)
But I assume most modern compilers already do that optimization.
How does signed fixed-point "-0.5" looks like in memory anyway?
Inverting the conversion gives us:
RoundToInt(float*65536)
Setting float=-0.5 gives us: -32768.