[Scummvm-devel] colorToRGB() implementation

[yotam barnoy]

Elegant and very impressive!


On Wed, Mar 17, 2010 at 1:21 PM, Marcus Comstedt <marcus at mc.pp.se> wrote:

>
> yotam barnoy <yotambarnoy at gmail.com> writes:
>
> > If it's efficiency we're after, then the multiplication is a small part
> of
> > the cost - division is much more expensive (about 4 times more on MIPS).
> >
> > The problem is that we need to multiply by more correct ratios than our
> > shifting simplifications allow. For example to convert a 5 bit channel to
> 8
> > bits, we need to multiply by 255/31 = 8.225 instead of 8, which is what
> we
> > currently do.
>
> Don't get too hooked up on these fractions.  There's a much simpler
> way.  What we're doing now is inserting 0 bits in the "loss"
> positions.  What we should do, to get the correct result, is to repeat
> the most significant bits in those positions.  So in the case of 5 bit
> to 8 bit, we should take the top 3 bits of the 5 bit value and insert
> to the bottom 3 bit positions.
>
>  r = (r << 3)|(r >> 2);
>
> No multiplications, divisions or conditionals.  What you get is
> actually a multiplication with binary 100.01, or 8.25 in decimal.
> This is more than your computed 8.225, but due to the fact that we
> always get rounding downwards, we actually get the right result anyway
> (the 8.225 value assumes rounding towards nearest integer).
>
>
>  // Marcus
>
>
>
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