Path: chuka.playstation.co.uk!news
From: "Mario Wynands" <mario@sidhe.co.nz>
Newsgroups: scea.yaroze.programming.3d_graphics,scee.yaroze.programming.3d_graphics
Subject: Billboarding - fast method?
Date: Wed, 6 May 1998 15:53:21 +1200
Organization: Sidhe Interactive
Lines: 53
Message-ID: <6iomsu$jpr33@chuka.playstation.co.uk>
NNTP-Posting-Host: p9-max5.well.ihug.co.nz
X-Newsreader: Microsoft Outlook Express 4.72.2106.4
X-MimeOle: Produced By Microsoft MimeOLE V4.72.2106.4
Xref: chuka.playstation.co.uk scea.yaroze.programming.3d_graphics:237 scee.yaroze.programming.3d_graphics:539

I am once again revisiting the topic of billboarding (thats having objects
always face the camera when they are drawn).

Given that I want to be able to use this effect for rendering (amongst other
things) particle objects, I am looking for the fastest solution possible
that doesn't directly affect the coordinate system of the object itself.

At the moment I am trying to do the appropriate transformation at draw time
by modifying the screen transformation matrix along the lines of -


void DrawTMD(GsDOBJ2 *dobj, GsOT *ot, int billboard)
{

 MATRIX screen_matrix;

 GsGetLs(dobj->coord2, &screen_matrix);

 if (billboard)
 {
  screen_matrix.m[0][0] = ONE;
  screen_matrix.m[0][1] = 0;
  screen_matrix.m[0][2] = 0;
  screen_matrix.m[1][0] = 0;
  screen_matrix.m[1][1] = ONE;
  screen_matrix.m[1][2] = 0;
  screen_matrix.m[2][0] = 0;
  screen_matrix.m[2][1] = 0;
  screen_matrix.m[2][2] = ONE;
 }

 GsSetLsMatrix(&screen_matrix);   // perspective, etc)
 GsSortObject4(dobj, ot, 14 - ot->length - OT_SHIFTMOD, (u_long
*)getScratchAddr(0));

}



This kind of works, but by zeroing out all the rotations I am also the
killing perspective transformation meaning the object is slightly stretched.
I am using a reference point camera system so I don't really want to redo
the perspective correction if I can help it.

Has anybody stumbled across a way to leverage the Yaroze libraries to
perform billboarding at draw time?


Thanks

Mario