[Pixman] [PATCH 10/13] Use floating point combiners for all operators that involve divisions

[Bill Spitzak]

Søren Sandmann wrote:
> Consider a DISJOINT_ATOP operation with the following pixels:
> 
> - source:	0xff (8 bits)
> - source alpha:	0x01 (8 bits)
> - mask alpha:	0x7b (8 bits)
> - dest:		0x00 (8 bits)
> - dest alpha:	0xff (8 bits)
> 
> When (src IN mask) is computed in 8 bits, the resulting alpha channel
> is 0 due to rounding:
> 
>      floor ((0x01 * 0x7b) / 255.0 + 0.5) = floor (0.9823) = 0
> 
> which means that since Render defines any division by zero as
> infinity, the Fa and Fb for this operator end up as follows:
> 
>      Fa = max (1 - (1 - 1) / 0, 0) = 0
> 
>      Fb = min (1, (1 - 0) / 1) = 1
> 
> and so since dest is 0x00, the overall result is 0.
> 
> However, when computed in full precision, the alpha value no longer
> rounds to 0, and so Fa ends up being
> 
>      Fa = max (1 - (1 - 1) / 0.0001, 0) = 1
> 
> and so the result is now
> 
>      s * ma * Fa + d * Fb
> 
>    = (1.0 * (0x7b / 255.0) * 1) + d * 0
> 
>    = 0x7b / 255.0
> 
>    = 0.4823

This example could be fixed by treating 0/0 as returning 0.