[Mesa-dev] [PATCH 09/10] vbo: fix GL_LINE_LOOP stray line bug

Charmaine Lee charmainel at vmware.com
Sat Oct 17 18:31:42 PDT 2015

>From: Brian Paul <brianp at vmware.com>
>Sent: Friday, October 16, 2015 2:25 PM
>To: mesa-dev at lists.freedesktop.org
>Cc: Charmaine Lee; Jose Fonseca; Sinclair Yeh
>Subject: [PATCH 09/10] vbo: fix GL_LINE_LOOP stray line bug

>When long GL_LINE_LOOP primitives don't fit in one vertex buffer they
>have to be split across buffers.  The code to do this was basically correct
>but drivers had to pay special attention to the _mesa_prim::begin,end flags
>in order to draw the sections of the line loop properly.  Apparently, the
>only drivers to do this were those using the old 'tnl' module for software
>vertex processing.

>Now we convert the split pieces of GL_LINE_LOOP prims into GL_LINE_STRIP
>primitives so that drivers don't have to worry about the special begin/end
>flags.  The only time a driver will get a GL_LINE_LOOP prim is when the
>whole thing fits in one vertex buffer.

>Most fixes bug 81174, but not completely.  There's another bug somewhere
>in the src/gallium/auxiliary/draw/ code.  If the piglit lineloop test is
>run with -count 4096, rendering is correct, but with -count 4097 there are
>stray lines.  4096 is a magic number in the draw code (search for "4096").

>Also note that this does not fix long line loops in display lists.  The
>next patch fixes that.

>Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=81174
> src/mesa/vbo/vbo_context.h   |  5 ++++-
> src/mesa/vbo/vbo_exec_api.c  | 38 +++++++++++++++++++++++++++++++++++++-
> src/mesa/vbo/vbo_exec_draw.c | 12 ++++++++++++
> 3 files changed, 53 insertions(+), 2 deletions(-)

>diff --git a/src/mesa/vbo/vbo_context.h b/src/mesa/vbo/vbo_context.h
>index 1e85335..28f43b4 100644
>--- a/src/mesa/vbo/vbo_context.h
>+++ b/src/mesa/vbo/vbo_context.h
>@@ -205,7 +205,10 @@ vbo_get_default_vals_as_union(GLenum format)
> static inline unsigned
> vbo_compute_max_verts(const struct vbo_exec_context *exec)
> {
>-   return (VBO_VERT_BUFFER_SIZE - exec->vtx.buffer_used) /
>+   /* Subtract one so we're always sure to have room for an extra
>+    * vertex for GL_LINE_LOOP -> GL_LINE_STRIP conversion.
>+    */
>+   return (VBO_VERT_BUFFER_SIZE - exec->vtx.buffer_used - 1) /
>           (exec->vtx.vertex_size * sizeof(GLfloat));

Shouldn't this be 
   (VBO_VERT_BUFFER_SIZE - exec->vtx.buffer_used) / (exec->vtx.vertex_size *sizeof(GLfloat)) - 1;

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