extract bits from 32 bit float numbers in C

问题

32 bits are represented in binary using the IEEE format. So how can I extract those bits? Bitwise operations like & and | do not work on them! what i basically want to do is extract the LSB from 32 bit float images in opencv thanx in advance!

回答1:

uint32_t get_float_bits(float f) {
    assert(sizeof(float) == sizeof(uint32_t)); // or static assert
    uint32_t bits;
    memcpy(&bits, &f, sizeof f);
    return bits;
}

As of C99, the standard guarantees that the union trick works (provided the sizes match), and implementations have generally guaranteed it even before they were required to. Personally I don't know what people see in it, I prefer this.

If you just want the LSB, and you know the endian-ness, you can access just one byte of the float directly, without any memcpy, or union, or violation of strict aliasing.

int lsb = ((unsigned char*)&f)[0] & 1; // little-endian
int lsb = ((unsigned char*)&f)[sizeof(float)-1] & 1; // big-endian

回答2:

You can use a union to pull values out safely (demo):

union fi_t
{
    unsigned int i;
    float f;
};

fi_t fi;
fi.f = 1.5;
unsigned int i = fi.i;

(just never typecast, this will invoke dreaded ftol, which may use SSE2 to convert to integer form, or FISTP, which won't yield the IEEE bits you are after)

回答3:

The old trick:

float num = 0.5;
uint32_t binary_representation = *(uint32_t *)&num;

回答4:

#include<stdio.h>
union abc
{
    float fo;
    unsigned int no;
};
int main()
{   
    union abc test;
    test.fo=36.5;
    unsigned int x=test.no;
    for( int i = 0; i < sizeof(float)*8; i++ )
    {
        printf("%d", x & 0x1);
        x = x >> 1;
    }

    return 0;
}

this was a way to extract the bits of the float!

来源：https://stackoverflow.com/questions/11136408/extract-bits-from-32-bit-float-numbers-in-c

标签

OpenCV

floating-point

bit-manipulation

ieee-754