RGB888 to RGB565 / Bit Shifting

Question

I want to combine three characters into a short using bit shifting. This is for implementing the RGB565 color palette (where there are 5 bits for red, 6 for green, 5 for blue).

Here is my example program, i'm just missing a step in the middle i think where i need to do some anding.

#include <stdio.h>

int main( ){
        unsigned char r, g, b;
        unsigned short rgb;

        r = 255;        // 0xFF 1111 1111
        g = 100;        // 0x64 0110 0100
        b = 50;         // 0x32 0011 0010

        r = r >> 3;     // 0x31 0001 1111
        g = g >> 2;     // 0x19 0001 1001
        b = b >> 3;     // 0x06 0000 0110

        //r = r & something; //
        //g = g & something; //
        //b = b & something; //

        // Desired result:
        //          R      G     B
        // 0xFB26 11111 011001 00110
        rgb = r | g | b;

        printf( "r 0x%x g 0x%x b 0x%x, rgb 0x%08x\n", r, g, b, rgb );
}

You can see my desired result at the end. Thanks for the help!

Answer 1

rgb = ((r & 0b11111000) << 8) | ((g & 0b11111100) << 3) | (b >> 3);

We shift r left by 11 bits, g left by 5 bits and bitwise OR these with b shifted right by 3 bits. (NB: this assumes the values have already been correctly masked, if needed, to remove any unwanted bits.)