Re: Question about GdkColor

[Cédric Lucantis <omer no-log org>]

Le Vendredi 17 Mars 2006 13:43, Eduardo M KALINOWSKI a écrit :
> yeajchao wrote:
> > Hello all
> >     I am chinese,I am sorry for my terriblly poor English !
> >
> >     I have confused about GdkColor
> >
> >     In general,the RGB color mode ,the red or green or blue's value is
> > from 0 to 255
> >    But ,the GdkColor ,the value  is from 0 to 65535
> >
> >   My question is ,how to map (0--255) to (0--65535)
> >
> >   For example ,i have a general color (245,222,179)
> >    map it to GdkColor ,what is the value ?
>
> Let's use some basic maths:
>
> 245     x
> --- = -----
> 255   65535
>
> Solving for x, we discover that
>
> x = 245*65535 / 255 = 62965
>
> And that's the value for red. Similar calculations will provide the values
> for green and blue.
>
> BTW, using hexadecimal base this "conversion" gets simpler. Try for
> yourself.

A range of [0-255] is stored on 8 bits, and [0-65535] on 16 bits. So a
bit shift is simpler and faster:

chan_16bits = chan_8bits << 8
chan_8bits  = chan16bits >> 8

Note this is not absolutely correct because the 8 bit value 0xff (255)
will become 0xff00 (and should be 0xffff), but this won't probably be
noticiable (because most video cards are limited to 8 bits channels,
and anyway most human people wouldn't see the difference:). If you
want a perfect mapping, you can do this:

chan_16bits = (chan_8bits << 8) | chan_8bits

I'm not sure it's really 'perfect', but I use it and it works ;)

Bye
-- 
Cédric Lucantis