Path: chuka.playstation.co.uk!news
From: Kev <kevtoast@geocities.com>
Newsgroups: scee.yaroze.programming.3d_graphics
Subject: Calculating Normals
Date: Thu, 29 May 1997 12:55:50 +0100
Organization: PlayStation Net Yaroze (SCEE)
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Hi,

OK, I've finally got around to really playing with the Yaroze and I
like it.

I designing my own 3D world system (wow! original!) and I'm starting
off with the basics... I'm having a problem calculating normals for
TMDVERTs for use in my own custom GPU primitives.

This is the routine I'm using the calculate the normals for a triangle,
I'm a bit confused as it doesn't produce the correct results (when
compared to the normals produced by RSDTOOL for the same triangles):

/* calculate normal for flat shaded triangle
   IN: ptr to TriPoly structure
*/
void CalculateNormal(TriPoly *ptr)
{
    // functions used
    int GetSizeOfVector(TMDVERT *);
    void ScaleVectorToUnit(TMDVERT *);

    // temp storage
    TMDNORM normal;

    // convert vectors to unit vectors
    ScaleVectorToUnit(&(ptr->vert[0]));
    ScaleVectorToUnit(&(ptr->vert[1]));
    ScaleVectorToUnit(&(ptr->vert[2]));

    // calc normal
    normal.nx = (ptr->vert[1].vy - ptr->vert[0].vy) * (ptr->vert[0].vz -
ptr->vert[2].vz) -
                (ptr->vert[1].vz - ptr->vert[0].vz) * (ptr->vert[0].vy -
ptr->vert[2].vy);
    normal.ny = (ptr->vert[1].vz - ptr->vert[0].vz) * (ptr->vert[0].vx -
ptr->vert[2].vx) -
                (ptr->vert[1].vx - ptr->vert[0].vx) * (ptr->vert[0].vz -
ptr->vert[2].vz);
    normal.nz = (ptr->vert[1].vx - ptr->vert[0].vx) * (ptr->vert[0].vy -
ptr->vert[2].vy) -
                (ptr->vert[1].vy - ptr->vert[0].vy) * (ptr->vert[0].vx -
ptr->vert[2].vx);

    ...

}

The normal produced by this routine is quite different from that the
correct normal. Am I using the right method??? I'm sure the
ScaleVectorToUnit function and GetSizeOfVector functions are OK as
they are from Sony code anyway.

Can anyone help?

Cheers,

Kev.
-- 
http://www.geocities.com/SiliconValley/Pines/7428