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

[Søren Sandmann]

Søren Sandmann <sandmann at cs.au.dk> 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

I should maybe expand a bit more on this: Pixman uses bit replication
when it goes from lower bit depths to higher ones. That is, a five bit
value:

        abcdef

is expanded to 

        abcdefabc

which corresponds to a left-shifting r5 by 3 and adding r5 right-shifted
by 2. This is the computation that is turned into a multiplication and a
shift in the formula above.

A more correct way to expand would be

        floor ((r5 / 31.0) * 255.0 + 0.5)

but this is fairly expensive (although it can be done with integer
arithmetic and without divisions, and may actually be equivalent to your
formula -- I haven't checked).


Søren