RGB565 to 24 bit

[@rt]

Hi Guys
I have some source RGB565 images converted to display on 24 bit platforms which look ok:
https://imgur.com/a/WlmNaVu

But it occurs to me I’m not quite correct. For Red & Blue channels the maximum value is 31, and for Green, 63.
None of these values are evenly divisible by 255, so I’ve just multiplied R&B by 8, and green by 4.
The maximum values for this scheme (as in the photo linked above) would then be 248 for Red & Blue, and 252 for Green.

Without floating point math (which would be easy), how are you supposed to do this conversion?
Images looks ok, but I suspect I’m leaving something out because a plain white image would be slightly green, etc.
Cheers.

[oPossum]

Use fixed point.

RB: n * 2114 >> 8
G:  n * 1040 >> 8

[@rt]

Very nice, thanks

[DJohn]

You don't even need to multiply.  Just repeat the bits that you've got until the target is full.  So 5 bit abcde becomes 8 bit abcdeabc.  In software that takes enough masking and shifting that (on a processor with a fast multiply) it might not be worth it.  In hardware it's trivial.

[@rt]

I have a timer set up already, and could time the routine, but it’s a Windows supercomputer, and doesn’t matter for static images.

I can’t tell the difference at all, even side by side, but the peace of mind is priceless.
https://imgur.com/a/TdeWi4p