[Mesa-dev] [Bug 81174] Gallium: GL_LINE_LOOP broken with more than 512 points

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=81174

--- Comment #16 from Brian Paul <brianp at vmware.com> ---
I'm digging into this bug because it pertains to an issue with a particular app
and the VMware gallium driver.

The VBO code for splitting GL_LINE_LOOP is actually correct, I believe, but our
implementations of vbo_context::draw_prims(), such as st_draw_vbo() and
brw_draw_prims() are subtly broken.  And has been broken since day one!

The issue comes from the two 'begin' and 'end' flags in the _mesa_prim
structure.  These flags indicate whether the primitive's vertices start at a
glBegin() and whether the prim's vertices end at a glEnd().  Suppose we have a
long GL_LINE_LOOP that gets broken into three pieces.  Here are the flags for
the three _mesa_prims that we draw:

_mesa_prim[0].begin = 1
_mesa_prim[0].end = 0
_mesa_prim[1].begin = 0
_mesa_prim[1].end = 0
_mesa_prim[2].begin = 0
_mesa_prim[2].end = 1

For all three drawing calls, the 0th vertex in the primitive's vertex buffer
will be a copy of the first glVertex() issued after glBegin.  If N is the
number of vertices in the _mesa_prim:

For _mesa_prim[0] we should draw the line segments from v[0] .. V[N-1]
For _mesa_prim[1] we should draw the line segments from v[1] .. V[N-1]
For _mesa_prim[2] we should draw the line segments from v[1] .. V[N-1] and an
extra line from V[N-1] to v[0]

You can see this in the old 'tnl' code's t_vb_rendertmp.h code for
GL_LINE_LOOP.

Our implementations of draw_prims() ignore those flags and always draw
V[0].V[N-1] so we get the stray lines that people are seeing.

Furthermore, draw_prims() is supposed to look at the 'begin' flag to know when
to reset the line stipple counter.  We don't do that in the state tracker
either.

I've posted a patch series that fixes this.  The basic idea is when we have to
split a GL_LINE_LOOP, draw the pieces with GL_LINE_STRIP instead so that
drivers don't need to worry about the 'begin' and 'end' flags (except where the
stipple counter matters).  Drivers will only get a GL_LINE_LOOP when all the
vertices live in one vertex buffer.

Unfortunately, after fixing the VBO code, there's still a bug somewhere in the
gallium 'draw' code.  See comments in the patch series for more information.

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