i would normally just use the following to round while shifting down

c = (c + (1<<7)) >> 8;

mathematically, this is like 'floor(x + 0.5)', which rounds x to the
nearest integer

and it saves one whole AND operation and a temporary register! ;)
ok, so it's pretty minimal, but like Dan said, sometimes you gotta be
frugal ;)

i think it also makes it better for Arm code optimisation as [IIRC] you
can do the above in one instruction [though it's been a while so you
might need to correct me on that].


alex:)



Dan Posluns wrote:
> On Saturday, November 8, 2003, at 04:57 AM, gbadev@yahoogroups.com
> wrote:
>
>
>>BTW, do to weird round off, when we rotate/scale a multi-part sprite
>>around, the sprite tends to break up a little. Have you guys
>>figured out the trick to fixing this?
>
>
> If it is, in fact, round-off that is causing your error, you can make
> sure that your fixed-point calculations are rounded to the nearest
> value instead of truncated when you shift off any of the fractional
> portion.
>
> For example, say I multiply two 24.8 numbers:
>
> c = a * b; // c is now 16.16
>
> In order to reduce it back to 24.8, I would normally do the following:
>
> c >>= 8;
>
> To make sure the number is rounded instead of just truncated, I can
> mask out everything except the 7th bit of the number, and then add it
> to the original number before shifting:
>
> c += (c & (1 << 7)); // (1 << 7) is (0000 0000 0000 0000 0000 0000 1000
> 0000)b
> c >>= 8;
>
> If you round off your calculations as you go instead of just
> truncating, I find it tends to significantly reduce those minor screen
> errors. Now the only trick is figuring out which calculations can
> benefit from the rounding process and which calculations it is wasted
> cycles for. (This shouldn't have to be a consideration if you are only
> doing it once per frame, but if like me you are doing these
> calculations in a tight inner loop you need to be much more frugal with
> your cycles.)
>
> Dan.
>
> --
> Dan Posluns, B. Eng. & Scty. (Software Engineering and Society)
> dan@... - ICQ: 35758902
> http://www.danposluns.com
>
> "Only when the last tree has been cut down, only when the last river
> has been poisoned, only when the last fish has been caught - only then
> will you realize that money cannot be eaten."
> - Cree prophecy
>



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