Question

I’ve got an 16bit bitmap image with each colour represented as a single short (2 bytes), I need to display this in a 32bit bitmap context. How can I convert a 2 byte colour to a 4 byte colour in C++?

The input format contains each colour in a single short (2 bytes).

The output format is 32bit RGB. This means each pixel has 3 bytes I believe?

I need to convert the short value into RGB colours.

Excuse my lack of knowledge of colours, this is my first adventure into the world of graphics programming.

Answer 1

Common formats include BGR0,
RGB0, 0RGB, 0BGR. In the code below I have assumed 0RGB. Changing this
is easy, just modify the shift amounts in the last line.

unsigned long rgb16_to_rgb32(unsigned short a)
{
/* 1. Extract the red, green and blue values */

/* from rrrr rggg gggb bbbb */
unsigned long r = (a & 0xF800) >11;
unsigned long g = (a & 0x07E0) >5;
unsigned long b = (a & 0x001F);

/* 2. Convert them to 0-255 range:
There is more than one way. You can just shift them left:
to 00000000 rrrrr000 gggggg00 bbbbb000
r <<= 3;
g <<= 2;
b <<= 3;
But that means your image will be slightly dark and
off-colour as white 0xFFFF will convert to F8,FC,F8
So instead you can scale by multiply and divide: */

r = r * 255 / 31;
g = g * 255 / 63;
b = b * 255 / 31;
/* This ensures 31/31 converts to 255/255 */

/* 3. Construct your 32-bit format (this is 0RGB): */
return (r << 16) | (g << 8) | b;

/* Or for BGR0:
return (r << 8) | (g << 16) | (b << 24);
*/
}