MaxVertexBlendMatrices, multi-matrix vertex blending and D3DX

[Paul TBBle Hampson]

This is a bit of an old topic, but it relates to two active bugs, 8357
(affecting Everquest) and 14608 (affecting Warhammer Online)

The rough summary of the situation is that these two games use d3dx9_30
and d3dx9_34 respectively, and at least the former uses
ConvertToIndexedBlendedMesh.

Anyway, they both show an odd behaviour where if the device caps report
MaxVertexBlendMatrices <= 2 (or <2 maybe) then the d3dx apparently
generates odd behaviour and unrecognised vertex entries, the upshot
being that the character's vertices all end up on the ground at the
model's world transform.

If MaxVertexBlendMatrices is > 2 then the vertices come out fine, and
everything seems to work.

A d3d trace from Warhammer Online indicates that D3DRS_VERTEXBLEND is
only ever set to 0 (only tested up to the character select screen, which
is the first screen where the bug is visible) both with and without
the patch.

From what I understand from here and on MSDN, indexed multimatrix
blending in the fixed function pipeline is generally emulated except on
certain older ATI Radeon cards, so I guess that D3DX
ConvertToIndexedBlended actually ends up emulating the index blending
without actually using the D3DRS_VERTEXBLEND renderstates.

The current workaround is to patch Wine to report MaxVertexBlendMatrices
4, but that will cause bad behaviour in programs that do use the
multimatrix vertex blending, as I understand.

Anyway, what I'm wondering is would it be worth throwing together a
patch that implements the multimatrix blending in drawStridedSlow for
cards that don't support ARB_VERTEX_BLEND (ie all of them under Linux)

If this is sensible, any pointers towards the right thing to do would be
greatly appreciated, this isn't an area I'm massively experienced in.

The only other option I'm aware of is to await the shader-based fixed
function pipeline replacement which might contain an implementation of
the D3DRS_VERTEXBLEND renderstates.

The last time I'm aware of this being discussed was [1], in February
last year.

[1] http://www.winehq.org/pipermail/wine-devel/2008-February/062433.html

-- 
-----------------------------------------------------------
Paul "TBBle" Hampson, B.Sc, LPI, MCSE
Very-later-year Asian Studies student, ANU
The Boss, Bubblesworth Pty Ltd (ABN: 51 095 284 361)
Paul.Hampson at Pobox.com

Of course Pacman didn't influence us as kids. If it did,
we'd be running around in darkened rooms, popping pills and
listening to repetitive music.
 -- Kristian Wilson, Nintendo, Inc, 1989

License: http://creativecommons.org/licenses/by/2.5/au/
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