GLSLで使いたいのでビット演算を使わずにfloatをunsigned char 4つにパック・アンパックする

float_pack.cpp

#include <cmath>
#include <limits>
#include <iostream>

struct ivec4 {
	unsigned char x, y, z, w;
};

float fract(float x) {
	return x - std::floorf(x);
}

float sign(float x) {
	if (x > 0.0f) {
		return 1.0f;
	}
	else if (x == 0.0f) {
		return 0.0f;
	}
	else {
		return -1.0f;
	}
}

unsigned char step(float a, float b) {
	return a <= b ? 1 : 0;
}

ivec4 packn(float val) {
	float a = fract(val);
	float b = fract(val * 256.0f);
	float c = fract(val * 256.0f * 256.0f);
	ivec4 ret;
	ret.y = unsigned char(a * 256.0f);
	ret.z = unsigned char(b * 256.0f);
	ret.w = unsigned char(c * 128.0f);
	return ret;
}

float unpackn(ivec4 packed) {
	return (packed.y / 256.0f) + (packed.z / 256.0f / 256.0f) + (packed.w / 128.0f / 256.0f / 256.0f);
}

ivec4 pack(float val) {
	float s = sign(val);
	val *= s;
	float log = std::log2f(val);
	float exp = std::floorf(log);
	float pow = std::powf(2.0f, exp);
	float m = val / pow;
	ivec4 packed = packn(m - 1.0f);
	packed.x = unsigned char(exp + 127.0f);
	packed.w += step(s, 0.0) * 128;
	return packed;
}

float unpack(ivec4 packed) {
	float e = float(packed.x - 127.0f);
	float f = std::floorf(float(packed.w / 128.0f));
	float s = sign(f - 0.5f);
	packed.w -= unsigned char(f * 128);
	float m = unpackn(packed) + 1.0f;
	return m * std::powf(2.0f, float(e)) * -s;
}

int main() {
	std::cout << std::fixed;

	float val = 0.114514f;
	ivec4 packed = packn(val);
	std::cout << "val = " << val << " unpacked = " << unpackn(packed) << std::endl;

	val = -121.0001f;
	packed = pack(val);
	std::cout << "val = " << val << " unpacked = " << unpack(packed) << std::endl;

	val = 114514.1919f;
	packed = pack(val);
	std::cout << "val = " << val << " unpacked = " << unpack(packed) << std::endl;

	val = 456.4322f;
	packed = pack(val);
	std::cout << "val = " << val << " unpacked = " << unpack(packed) << std::endl;

	val = -0.000012f;
	packed = pack(val);
	std::cout << "val = " << val << " unpacked = " << unpack(packed) << std::endl;

	val = 1.0f;
	packed = pack(val);
	std::cout << "val = " << val << " unpacked = " << unpack(packed) << std::endl;

	val = 0.0f;
	packed = pack(val);
	std::cout << "val = " << val << " unpacked = " << unpack(packed) << std::endl;

	getchar();
	return 0;
}