Attempt on branch-free sRGB conversion by LUT without excessive range

srgb-lut.c

#include <stdio.h>
#include <math.h>

#define LUT_LOG2_SIZE 8
#define LUT_SIZE ((1 << LUT_LOG2_SIZE) + 1)
float lut[LUT_SIZE];

float convert(float f)
{
	union {
		float f;
		unsigned int i;
	} u = { f };
	/* perform max(f, 0) */
	unsigned int mask = ~((signed int)u.i >> 31);
	u.i &= mask;
	/* bring 0..1.0 range to a fixed exponent */
	u.f += 1.0f;
	/* clear everything if greater than 1.0 */
	mask = -((u.i >> 23) == 127);
	u.i &= mask;
	/* remote non-mantissa bits */
	int idx = u.i & ((1 << 23) - 1);
	/* set implicit 1 bit, so 1.0 doesn't become 0.0 */
	idx |= ~mask & (1 << 23);

	return lut[idx >> (23 - LUT_LOG2_SIZE)];
}

int main(int argc, char* argv[])
{
	int i;
	float f;

	for (i = 0; i < LUT_SIZE; ++i) {
		f = i * (1.0f / (LUT_SIZE - 1));
		lut[i] = pow(f, 2.2f);
	}
	for (f = -1.0f; f < 2.0f; f += 0.2)
		printf("%f\n", convert(f));
	for (f = 2.0f; f < 4.0f; f += 0.4)
		printf("%f\n", convert(f));
	return;
}