在 C 中将 float 16ths 转换为 4 位整数的最有效方法

Question

As part of an anti-aliasing routine I'm running on an STM32 (single precision FPU, if that matters), I'm looking to convert a float value 0 ≤ x < 1 to a 4 bit nibble representing opacity. Basically each LSB of the nibble corresponds to a 16th of the float, so 0 ≤ x < 0.0625 would be 0x0, 0.0625 ≤ x < 0.125 would be 0x1, so on. Are there any bit manipulation tricks I could use to make this operation fast/efficient?

Answer 1

For any floating-point value, x , such that 0 <= x < 1 , the value y = x * 16 will give y <= 0 < 16 . Casting this float value to an unsigned char will set the lower nibble of that byte to the number of 16 ^th s. Thus:

float x = 0.51;
unsigned char b = (unsigned char)(x * 16);

Then b will have the value 0x08 - representing eight sixteenths; and likewise for any other (valid) value of x .