Klaus Peichl wrote:
>
>
> Thomas Winischhofer wrote:
>
> > Apparently, after looking into the 3.3.6 code, the first generations of
> > S3 do support Bresenham lines after all.
> >
> > What I see from comparing the code is that the 3.3.6 version does a lot
> > of stuff to reduce the error term to 12 bit which is the hardware's
> > maximum.
> >
> > The 4.x code, although demanding a 12bit Bresenham error term from XAA,
> > simply adds the given minor and error term and writes the result to the
> > hardware register, without checking it for overflows. I don't know what
> > values this function is given, but in case the values are somewhat big,
> > this may result in a completely wrong error term. The result may very
> > well look like what the original poster saw, namely lines with a wrong
> > slope.
> >
> > Furthermore, I think the way the error term, e1 and e2 values are
> > calculated differ:
> >
> > 3.3:
> >
> > (deltas: adx = x2 - x1; ady = y2 - y1)
> >
> > if (adx > ady) {
> > axis = X_AXIS; <-- x is major axis; adx=major > ady=minor
> > e1 = ady << 1;
> > e2 = e1 - (adx << 1);
> > e = e1 - adx;
> > } else {
> > axis = Y_AXIS; <-- y is major axis; adx=minor < ady=major
> > e1 = adx << 1;
> > e2 = e1 - (ady << 1);
> > e = e1 - ady;
> > ...
> > }
> >
> > Simply put, if I understood that right, this does
> >
> > e1 = minor;
> > e2 = minor - major;
> >
> > 4.x:
> >
> > Here it looks like this:
> >
> > e1 = major;
> > e2 = minor - major;
> >
> > Would be interesting to know what impact this difference has. If
> > anybody's interested, I could compile the driver with that change and
> > send out the binary.
>
> That would be great!
> Will the binary be a replacement for /usr/bin/X11/XFree86
> or will I have to change something else to test it?
>
The default XFree86 binary is a module loader, using drivers in
/usr/X11R6/lib/modules/drivers/ by default. Thomas would likely provide
you a s3.o module you could swap with your existing driver.
--
Kevin
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