[Pixman] [RFC] mmx: add and use expand_4xpacked565

[Søren Sandmann]

Matt Turner <mattst88 at gmail.com> writes:

> Given a pixel with only the red component of these values, the results
> are off-by-one.
>
> 0x03 -> 0x19 (0x18)
> 0x07 -> 0x3A (0x39)
> 0x18 -> 0xC5 (0xC6)
> 0x1C -> 0xE6 (0xE7)
>
> (Same for blue, and green has many more cases)
>
> It uses
> R8 = ( R5 * 527 + 23 ) >> 6;
> G8 = ( G6 * 259 + 33 ) >> 6;
> B8 = ( B5 * 527 + 23 ) >> 6;
>
> I don't guess there's a way to tweak this to produce the same results
> we get from expand565, is there?

Maybe I'm missing something, but this certainly produces the correct
result:

    r8 = (r5 * 8 + r5 / 4) = r5 * (8 + 0.25) = r5 * (32 + 1) / 4 
       = (r5 * 33) >> 2

and similar for green, where the result would be

    g8 = (g6 * 65) >> 4


Søren