Fastest way to cast int to UInt32 bitwise?

Question

i have some low level image/texture operations where 32-bit colors are stored as UInt32 or int and i need a really fast bitwise conversion between the two.

e.g.

 int color = -2451337;  

 //exception
 UInt32 cu = (UInt32)color;

any ideas?

thanks and regards

Answer 1

int color = -2451337;
unchecked {
    uint color2 = (uint)color;
    // color2 = 4292515959
}

Answer 2

BitConverter.ToUInt32(BitConverter.GetBytes(-2451337), 0)

Answer 3

Those using a language like VB, which don't have a really convenient way of disabling overflow checks during the conversion, could use something like:

    Shared Function unsToSign64(ByVal val As UInt64) As Int64
        If (val And &H8000000000000000UL)  0 Then Return CLng(val Xor &H8000000000000000UL) Xor &H8000000000000000 Else Return CLng(val)
    End Function
    Shared Function signToUns64(ByVal val As Int64) As UInt64
        If val < 0 Then Return CULng(val Xor &H8000000000000000) Xor &H8000000000000000UL Else Return CULng(val)
    End Function

or

    Shared Function unsToSign(ByVal val As UInt64) As Int64
        Return CLng(val And &H7FFFFFFFFFFFFFFFUL) + (CLng(-((val And &H8000000000000000UL) >> 1)) << 1)
    End Function
    Shared Function signToUns(ByVal val As Int64) As UInt64
        Return CULng(val And &H7FFFFFFFFFFFFFFF) + (CULng(-((val And &H8000000000000000) >> 1)) << 1)
    End Function

Versions for 32 bits would be very similar. I'm not sure which approach would be faster. The shifts are a little silly, but they avoid the need for 'if' tests.