[Mesa-dev] draw: Replace varray and vcache by vsplit

Chia-I Wu olvaffe at gmail.com
Fri Aug 13 07:04:50 PDT 2010


There are two primitive transformations in gallium draw module.  In
varray, primitives are "split"ted.  When a primitive has more vertices
than the middle end can handle, varray splits the primitive and calls
the middle end multiple times.

In vcache, primitives are "decompose"d.  More advanced primitives are
decomposed into one of point, line(_adj), or triangle(_adj).
Similarly, vcache may call the middle end multiple times to flush its
internal buffer.  In some cases, vcache passes the primitves through
without decomposing nor splitting, as can be seen in vcache_check_run.

The issue with vcache is that it has to decompose a primitive
differently depending on the provoking convention, as explained in


It becomes a problem when GS is active.

My proposal is to make vcache split instead of decompose.  Because
varray only splits and vcache has a pass-through path, the rest of the
workflow already has to support all primitive types.  Switching from
decompose to split does not require a big change to the rest of the

But then vcache will look a lot like varray, only with indexed
primitive support.  It leads me to a new frontend that replaces both
varray and vcache: vsplit


vsplit is based on varray.  It uses some code from vcache to support
indexed primitives.  When vcache decomposes, there are flags being set
to indicate that if the stipple counter should be reset or if some
edge of a triangle should be omitted in unfilled mode.  The segments
of a splitted primitive have flags for similar purposes too:

  DRAW_SPLIT_AFTER   More segments to come after this one
  DRAW_SPLIT_BEFORE  There are preceding segments

These flags are set by vsplit and the middle ends pass them to the
other stages.  Therefore, the run methods of middle ends are augmented
to take the flags.

To summarize, vsplit

 - fixes GS when (flatshade && flatshade_first) is on
 - never sends more vertices than the middle end claims to handle
 - is faster than vcache: split instead of decompose, no get_elt
 - no longer uses the higher bits of draw_elts for stipple/edge flags


olv at LunarG.com

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