[Open-graphics] Re: float25 multiplier

[Timothy Normand Miller]

On 2/4/08, André Pouliot <andre.pouliot at gmail.com> wrote:
> I found that the bug was in the result of the multiplication. It was a
> rounding that's present in the software model and is not present in the
> rtl of the multiplication.
>
> Now the question is : do we care about the rounding? Adding the rounding
> to the fp multiplicator would add another stage with a 19 bits adder. I
> did a quick test with the ogsim program there seem to have some small
> variation on the edge of the triangle. I attach the resulting image for
> the 2 run of the program with and without the rounding enabled in the
> float25 class.
>
> Could someone tell me if we do without the rounding would the compound
> error would be to important  or we can neglect it?
>

I can't see any difference.  When it comes to pixel values, there are
no loops in OGA to compound errors, and the numbers get chopped to
8-bit values going out to memory.  However, it is coordinates I'm
concerned about.  I don't remember if we ever perform a multiply on a
float coordinate.  If we don't, then I suggest that we take careful
and prominent note of the fact that we don't do rounding in the
multiplies, and see if anyone notices a problem that could be fixed by
having it.


-- 
Timothy Normand Miller
http://www.cse.ohio-state.edu/~millerti
Open Graphics Project