Hi Gamdev.net,
I have a question that I'm hoping can be answered. I'm starting work on implementing shadow volumes in my game engine and I've been researching the topic a little here and there. I came across a PDF (located here: http://fabiensanglard.net/doom3_documentation/37730-293752.pdf) that disects part of the Shadow Volume code in the Doom 3 source. I tend to learn by getting something to work and then fiddiling with it until I completely understand it, so I'm trying to implement so I can master the concept before throwing it out and rewriting it, but I'm struggling with it a bit because the code seems to use quaternions for light rotations. This wouldn't be much of an issue except that everything that Direct3D 9 uses utilizes simply an x, y, and z value. There is a theta value...
Question 1) When I issue, to Direct3D 9, a statement to set a light's position or direction, are each of the components (x, y, and z) considered separate rotational vectors that have been formed by other values calculated internally by Direct 3D? Hard to explain but if you goto http://www.cs.princeton.edu/~gewang/projects/darth/stuff/quat_power.html and search in the document for the section Euler to Quaternion you'll see what I mean. I mean are the x, y, and z values some independant set of rotation values for each axis (Like some single value that encompasses the y and z rotations for x, the x and z rotations for y, and the x, and y rotations for z), or are x, y and z really just that and I'm overthinking them? If they really are just simple values to be treated, for example, as x is to be accompanied with y and z make up the entire rotation at face-value then how would one get a w value to use with the implementation presented in that paper I linked at the begining of the post? And how would this algorithm work with point lights? In that paper, the part that I'm referring to is on page 15 in Appendix A:
/*
Calculating Triangle Facing
Copyright (C) 2005 Id Software, Inc.
Written by J.M.P. van Waveren
This code is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This code is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
*/
struct Vec4 {
float x, y, z, w;
};
struct Plane {
float a, b, c, d;
};
void CalculateFacing( const Plane *planes, const int numTriangles, const Vec4 &light, byte *facing ) {
int i;
for ( i = 0; i < numTriangles; i++ ) {
facing[i] = planes[i].a * light.x +
planes[i].b * light.y +
planes[i].c * light.z +
planes[i].d * light.w > 0.0f;
}
facing[numTriangles] = 1; // for dangling edges to reference
}
