conor conor - 27 days ago 14
C Question

RGB888 to RGB565 / Bit Shifting

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 Source
rgb = (r << 11) | (g << 5) | b;

We shift r left by 11 bits, g left by 5 bits and OR these with (unshifted) b.