C-RGBA转换未返回所需的RGB结果

Question

Okay, I have a method that I am having a difficult time with returning the desired results. So I made a test result, and it is returning a result that I was not expecting.

Here is an example -->

int color = 0x00FFFF00;

return 0xFF & (color >> 24) | 0xFF & (color >> 16) | 0xFF & (color >> 8);

From what I know, this should return:

0x00FFFF

However, it actually returns:

0x0000FF

Can someone please explain what is happening? I would also like to know how I could properly convert the 32-bit RGBA integer to the 24-bit RGB integer, thank you.

Answer 1

I also caught what I did wrong. I was not shifting the red and green elements of the integer cast to match the RGB integer --->

int color = 0x00FFFF00;

// Returns 0x0000FF
return 0xFF & (color >> 24) | 0xFF & (color >> 16) | 0xFF & (color >> 8);

Here is the fixed code --->

unsigned int color = 0x00FFFF00; // Unsigned Integer to avoid shifting the Two's Complement flag into the RGB value as clearlight pointed out.

// Returns 0x00FFFF
return (0xFF & (color >> 24)) << 16 | (0xFF & (color >> 16)) << 8 | 0xFF & (color >> 8);

Answer 2

Try this:

#include <stdio.h>
int main() {

        unsigned int x = 0x00FFFF00;
        printf("0%8.8x", (x >> 8) & 0x00FFFFFF);
}

$ cc -o shft shft.c
$ ./shft
0x0000ffff

It's better if you make color an unsigned int , otherwise you can get the sign bit shifted in from the left, but in this case we're masking off the shifted in bytes, but be careful of that.