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From: Lewis_Evans@Playstation.sony.com
Newsgroups: scee.yaroze.beginners
Subject: World---->Local convertion
Date: 2 Mar 1998 12:40:20 -0000
Organization: Sony Computer Entertainment Europe - 119.SS5
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From: Lewis_Evans@Playstation.sony.com
To: news@playstation.co.uk

If you want to express the coordinates of a world point in a local
coordinate system,
you can just use the technique as the SCEE demos 'flying' and 'dungeon';
those
demos involve ships hunting and shooting at each other, to do this they
express the target ship's position in their own coord system.
This is done by using the fact that a coordinate system's matrix is just a
set
of 3 vectors specifying the local x,y and z axes.

Here's a couple of functions, nearly the same, but the different ways of
accessing the matrix elements mean that one does sub->super conversion
and the other does super->sub.

     Lewis



     // eg know position or vector in world space,
     // want to find it in object coordinate terms

     // NOTE: Only takes account of rotation
     // NOT displacement as well

void ExpressSuperPointInSub (VECTOR* superPoint,
               GsCOORDINATE2* subSystem, VECTOR* subPointOutput)
{
     MATRIX* matrix;

     matrix = &subSystem->coord;

     setVECTOR(subPointOutput,
                    ((superPoint>vx * matrix->m[0][0]
                    + superPoint>vy * matrix->m[0][1]
                    + superPoint>vz * matrix->m[0][2]) >> 12),

                    ((superPoint>vx * matrix->m[1][0]
                    + superPoint>vy * matrix->m[1][1]
                    + superPoint>vz * matrix->m[1][2]) >> 12),

                    ((superPoint>vx * matrix->m[2][0]
                    + superPoint>vy * matrix->m[2][1]
                    + superPoint>vz * matrix->m[2][2]) >> 12) );
}





     // eg know position or vector in object space,
     // want to find it in world coordinate terms

     // NOTE: Only takes account of rotation
     // NOT displacement as well

void ExpressSubPointInSuper (VECTOR* subPoint,
               GsCOORDINATE2* subSystem, VECTOR* superPointOutput)
{
     MATRIX* matrix;

     matrix = &subSystem->coord;

     setVECTOR(superPointOutput,
                    ((subPoint>vx * matrix->m[0][0]
                    + subPoint>vy * matrix->m[1][0]
                    + subPoint>vz * matrix->m[2][0]) >> 12),

                    ((subPoint>vx * matrix->m[0][1]
                    + subPoint>vy * matrix->m[1][1]
                    + subPoint>vz * matrix->m[2][1]) >> 12),

                    ((subPoint>vx * matrix->m[0][2]
                    + subPoint>vy * matrix->m[1][2]
                    + subPoint>vz * matrix->m[2][2]) >> 12) );
}


To:       Lewis Evans
cc:
From:     news @ playstation.co.uk
Subject:  World---->Local convertion:scee.yaroze.beginners




From: "Emanuele Diotallevi" <g.mork@flashnet.it>
Newsgroups: scee.yaroze.beginners
Subject: World---->Local convertion
I'm programming collisions with Bounding boxes ,but i must find the local
coord. of a point in the world,so lw matrix calculates world coord from
local cord ,i thinked that i must calculate the inverse matrix of lw matrix
calculating the (in italian we say " Determinante ") :
LW^(-1) =((LW^a)^t)/Det.(LW)???
If there's someone that have a function that calculates the A^(-1) matrix
please help me!