How to convert 24 bit RGB to 8 bit RGB

Question

I was wondering what is the best way to convert a 24-bit RGB color (8 bits per color) into an 8-bit color (2bits Blue, 3 Green, 3 Red). I'd like C code for doing this.

Answer 1

8 bit RGB is normally an indexed (palettized) color format, see Palette (computing).

The way you described it though, getting 8 bpp out of 24 bpp is pretty straightforward - you would need to strip least significant bits and merge the bits into single byte value, per pixel. AFAIK it is not a standard or well-known pixel color representation.

Answer 2

Not in C but in javascript.

encodedData = (Math.floor((red / 32)) << 5) + (Math.floor((green / 32)) << 2) + Math.floor((blue / 64));

https://stackoverflow.com/a/25258278/2130509

Answer 3

To convert 8bit [0 - 255] value into 3bit [0, 7], the 0 is not a problem, but remember 255 should be converted to 7, so the formula should be Red3 = Red8 * 7 / 255.

To convert 24bit color into 8bit (RRRGGGBB),

8bit Color = (Red * 7 / 255) << 5 + (Green * 7 / 255) << 2 + (Blue * 3 / 255)