[Mono-list] new color.cs for inclusion in CVS
Wed, 6 Mar 2002 18:42:26 -0600
> I think the following would be a nice addition to the following
> function... color.a = ( argpacked & 0xff000000 ) >> 24;
> color.r = ( argpacked & 0xff0000 ) >> 16;
> color.g = ( argpacked & 0xff00 ) >> 8;
> color.b = ( argpacked & 0xff );
Be wary of casts and operators... Your innocent-looking first line does the
1. Cast argpacked to an Int64, a signed data type capable of holding the
result of the AND.
2. Cast 0xFF000000 from a UInt32 to an Int64.
3. Perform the AND.
4. Perform the shift.
5. Cast down to a byte.
I've found judicial use of the "unchecked" operator to be helpful. I have a
class that does colors, and it extracts values using code like this:
color.a = (byte)(argpacked >> 24);
color.r = (byte)(argpacked >> 16);
color.g = (byte)(argpacked >> 8);
color.b = (byte)(argpacked);
This produces very nice IL, and avoids unnecessary casting and overflow
Also, I see your point about local variables being unassigned. So really,
both cases should be equally optimal. But one's shorter! :P