Re: [AGG] Blending colors, associated alpha, etc

[Marc V. O. <mar...@ea...>]

Hi,


on 6/18/04 4:26 AM, Stephan Assmus at sup...@gm... wrote:

> I also think it is really worth looking into what Marc Van Olmen wrote.
> I knew of that trick but never really put it to use because at the time
> I read about it, I didn't know my way arround that level of byte
> juggling.
> 
> Where I read about it, it was called Packed Multiply. The idea is
> similar to SIMD, only for portable C code. Of course it works only for
> uint8 calulations. I'm going to explain it in my words, because for me,
> the code Marc posted takes a second look to understand it. Basically
> you put 2 uint8 values in the first and third byte of a uint32. Then
> you do the multiplication by a uint8 value, now your two values have 16
> bit results in the first half and second half of the uint32, to scale
> them correctly, you shift by one byte to the right and mask out the
> lower half of the upper value, which is now in the upper half of the
> lower value. Now you're back to two uint8 on byte one and three of the
> uint32. I think that should work on both big endian and little endian

This also works with a signed value that is the weirdest thing about it. I
don't exactly understand completely why it works also with signed because I
always assume that a signed number was just one bit being used for a sign
but it is more then that all bits are reversed until the last significant
bit , it took me a day to grasp this.
Because (Source-destintation)*alpha + destintation, where
(source-destination) can be negative.