Re: [pygame] Segmentation Fault in PolyPlay 1.3 Walker demo

[Lenard Lindstrom <len-l@xxxxxxxxx>]

Forget that last observation. The traceback doesn't show the arguments 
correctly. I have traced the problem to clipaaline. When pts[0] is 0 and 
pts[2] is 1 or pts[1] is 0 and pts[3] is 1 it copies pts[2] and pts[3] 
to pts[0] and pts[1]. This causes a divided by 0.0 later in drawaaline 
which leads to a memory access problem. It doesn't matter if the surface 
is 24 or 32 bit.

So for an (800x400) surface s these fail:

aaline(s, color, (0, 0), (1, 248))
aaline(s, color, (0, 0), (248, 1))

These work:

aaline(s, color, (0, 0), (2, 248))
aaline(s, color, (1, 0), (1, 248))
aaline(s, color, (0, 0), (248, 2))
aaline(s, color, (0, 1), (248, 1))

clipaaline has some kind of while loop thing going. I don't feel 
qualified enough to guess how it is supposed to work. I will leave it to 
someone with a better understanding of antialiasing to fix.


Lenard


Lenard Lindstrom wrote:
> To me the traceback shows drawaaline called with surf set to NULL. Am 
> I reading it incorrectly.
>
> Lenard
>
>
> René Dudfield wrote:
> > Cool.
> >
> > Here's the traceback:
> >
> >
> > 0x01546d05 in drawaaline (surf=0x0, color=0, x1=0, y1=0, x2=1, y2=248,
> > blend=1) at src/draw.c:1023
> > 1023                    DRAWPIX32(pixel, colorptr, brightness1, blend)
> > (gdb) where
> > #0  0x01546d05 in drawaaline (surf=0x0, color=0, x1=0, y1=0, x2=1,
> > y2=248, blend=1) at src/draw.c:1023
> > #1  0x0154849d in aaline (self=0x0, arg=0x21480) at src/draw.c:660
>
>
> Marcus von Appen wrote:
> > DRAWPIX32() seems to be the source as Rene tracked it down and after
> > giving it a short glance. My guess is that it does not take the
> > necessary shifts for 24bpp surfaces into account (in fact it does not do
> > any shifts, but assumes the 3 bytes follow up on each other and are
> > aligned properly).
> >
> >