[Plib-users] R: R: R: Matrices and sgCoord
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[Plib-users] R: R: R: Matrices and sgCoord
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From: Paolo L. <p.l...@ci...> - 2007-07-25 11:51:43
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Dimistris,=20
> -----Messaggio originale-----
> Da: pli...@li...=20
> [mailto:pli...@li...] Per conto=20
> di Dimitris Mexis
> Inviato: mercoled=EC 25 luglio 2007 12.39
> A: PLIB Users
> Oggetto: Re: [Plib-users] R: R: Matrices and sgCoord
>=20
> No there was not in your code but in plib I have found these=20
> functions and I don't understand them :
> void orthoXform ( sgMat4 m ) in Sphere code, and in sg.h
> void sgXformVec3 ( sgVec3 dst, sgMat4 mat ) ;
> void sgXformVec3 ( sgVec3 dst, sgVec3 src, sgMat4 mat ) ;
>=20
> void sgXformPnt3 ( sgVec3 dst, sgMat4 mat ) ;
> void sgXformPnt3 ( sgVec3 dst, sgVec3 src, sgMat4 mat ) ;
>=20
> void sgFullXformPnt3 ( sgVec3 dst, sgMat4 mat ) ; void=20
> sgFullXformPnt3 ( sgVec3 dst, sgVec3 src, sgMat4 mat ) ;
>=20
> void sgXformVec4 ( sgVec4 dst, sgMat4 mat ) ;
> void sgXformVec4 ( sgVec4 dst, sgVec4 src, sgMat4 mat ) ;
>=20
> void sgXformPnt4 ( sgVec4 dst, sgMat4 mat ) ;
> void sgXformPnt4 ( sgVec4 dst, sgVec4 src, sgMat4 mat ) ;
>=20
> void sgFullXformPnt4 ( sgVec4 dst, sgMat4 mat ) ; void=20
> sgFullXformPnt4 ( sgVec4 dst, sgVec4 src, sgMat4 mat ) ;
>=20
> ?????
Ok, these are all functions, not class methods. Just try to read =
'transform'
in place of 'xform', and you'll get a plenty of ways to transform a =
point or
a vector, 3- or 4-dims, by a transformation matrix.
For example, sgXformVec3 will multiply a sgVec3 by the 3x3 rotation =
section
of a sgMat4, whilst sgXformPnt3 does what sgXformVec3 does + adds the
matrix's translation row to the resuling vector (i.e. treats it as an =
actual
'point').
The 'Full' versions transform vecs and pnts 'homogeneously'.
Paolo
> ----- Original Message ----
> From: Paolo Leoncini <p.l...@ci...>
> To: PLIB Users <pli...@li...>
> Sent: Wednesday, July 25, 2007 1:30:17 PM
> Subject: [Plib-users] R: R: Matrices and sgCoord
>=20
> Dimitris,=20
>=20
> > -----Messaggio originale-----
> > Da: pli...@li...
> > [mailto:pli...@li...] Per conto di=20
> > Dimitris Mexis
> > Inviato: mercoled=EC 25 luglio 2007 11.47
> > A: PLIB Users
> > Oggetto: Re: [Plib-users] R: Matrices and sgCoord
> >=20
> > Thank you Paolo, for the clarification, so it is right to=20
> handle the=20
> > last row, and it wasn't accident.
> > I liked your second option of working with the coords.
> > By the way...What is an xform? What do the function do in=20
> plib about=20
> > sgxform*?
> > They are not matrices right? ( plib has sgMat for them...no?)
>=20
> 'xform' is a way 3D graphics programmers use to refer to=20
> 2D/3D transformations in variable names ('x' stands for=20
> 'trans'). There should be also a class method in SG (IIRC=20
> XformVec3 and XformPnt) with this prefix.
> Using it makes me feeling a very english/american 3D programmer.
>=20
> There's no sgxform in the code I wrote, unless an error were=20
> introduced when the e-mail systems formatted my message.
>=20
> > Thank you
>=20
> Greetings,
>=20
> Paolo
>=20
>=20
> > ----- Original Message ----
> > From: Paolo Leoncini <p.l...@ci...>
> > To: PLIB Users <pli...@li...>
> > Sent: Wednesday, July 25, 2007 12:33:55 PM
> > Subject: [Plib-users] R: Matrices and sgCoord
> >=20
> > Dimitris,
> >=20
> > > -----Messaggio originale-----
> > > Da: pli...@li...
> > > [mailto:pli...@li...] Per conto di=20
> > > Dimitris Mexis
> > > Inviato: marted=EC 24 luglio 2007 16.26
> > > A: pli...@li...
> > > Oggetto: [Plib-users] Matrices and sgCoord
> > >=20
> > > The following code has been a success in order to make the
> > object move
> > > at a constant speed.
> > >=20
> > > sgCoord stigma;
> > >=20
> > > void GLObject::setStigma( sgMat4 & objDst){
> > > sgSetCoord( & stigma, objDst);
> > > }
> > >=20
> > > GLObject::move(){
> > > //The rotation matrix for moving around trireme.
> > > sgMat4 RotateMatrix =3D {
> > > 1, 0, 0, 0,
> > > 0, 1, 0, 0,
> > > 0, 0, 1, 0,
> > > 0,( getSpeed()), 0, 1
> > > };
> > > =20
> > > sgMakeCoordMat4( SHIPSRC, &trireme->getStigma());
> > sgMultMat4( SHIPDST,
> > > SHIPSRC, RotateMatrix); setStigma( SHIPDST);
> > >=20
> > > What I don;t understand is that I took the usual matrix=20
> and instead=20
> > > tweaking the last column element by the second row, I
> > inversed it and
> > > put the getSpeed() which is <>0 in the bottom row! Why? this?
> >=20
> > I hope to have understood correctly your perplexity.
> >=20
> > First, SSG has a different coord reference system wrt OpenGL.=20
> > Both have the same handness, so x+ is along right in both,=20
> yet OpenGL=20
> > is y+ -up, z+ -going outside the screen, whilst SSG is z+ -up, y+=20
> > -going into the screen.
> >=20
> > Second, SG's sgMat4 is a 4x4 homogeneous matrix for specifying=20
> > projection, viewing, modelling and/or object position. For old,=20
> > nostalgic programmers, it's a-la-IrisGL.
> > The fourth row is the translation.
> >=20
> > Then, when your ship moves on the sea plane, it actually=20
> moves on the=20
> > x-y plane (z=3D0), and hence elements of a sgMat4 ship_xform=20
> to change=20
> > are ship_xform [3][0], ship_xform[3][1] (ship_xform[3] can=20
> be pointed=20
> > to by a
> > sgVec3 to set a transformation vector xyz).
> >=20
> > That's the reason why you put getSpeed() for setting=20
> > RotateMatrix[3][1]. In a un-rotated viewing, you should see=20
> the ship=20
> > entering/exiting into/out the screen.
> >=20
> > > I made then the matrix multiplications, and took the result to=20
> > > sgCoord.
> > > Is this right approach? Ok it works, but it worths?
> >=20
> > You don't specify what you do with sgCoord stigma after the=20
> last set.=20
> > If you need to move it I assume your ship is a
> >=20
> > ssgTransform *ship;
> >=20
> > So you'll probably call
> >=20
> > ship->setTransform( /* sgCoord* */ stigma ); // one of=20
> the several=20
> > ship->forms
> > to set a xform
> >=20
> >=20
> > Alternatively I could have done this way:
> >=20
> > void move_ship( ssgTransform *ship )
> > {
> > sgMat4 ship_xform;
> > ship->getTransform( ship_xform );
> > sgVec3 ship_pos =3D { 0, getSpeed(), 0 }; // don't know if=20
> getSpeed=20
> > is directly callable, or an object method
> > sgAddVec3( ship_xform[3], ship_pos ); // just increment=20
> > the position
> > coords, resembles sgMultMat4 for the translation part only
> > ship->setTransform( ship_xform ); // set the updated=20
> > matrix back to
> > the object
> > }
> >=20
> > Greetings,
> >=20
> > Paolo Leoncini
> >=20
> >=20
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