Category : C Source Code
Archive   : GRAFGEM3.ZIP
Filename : 3D.C

/* rolling-ball-gems3.c */
/*
* Take a 3D graphics object "obj" and the incremental mouse
* motion data "(dx,dy,state)" and rotate the object
* using the rolling ball algorithm. A generic interactive
* graphics interface is assumed that maps onto common protocols
* such as Xlib in an obvious way.
*
* It is assumed that an object "obj" of type "Polyhedron" has
* already been drawn once on the display, and that it carries
* a 4x4 frame matrix "obj->frame[i][j]" specifying the position
* and orientation at which it is to be drawn.
*
* Make3DRot is assumed to construct a 4x4 rotation matrix from
* the angle and axis parameters as shown in the text and in
* GGI "Rotation Tools," by M. Pique, p.466.
*
* Make3DTranslation is assumed to store an (x,y,z) position in a 4x4
* matrix in such a way that it can be extracted by Tx = obj->frame[3][0]; .
*
* CombineMatrices3D performs a left to right matrix multiplication,
* leaving the result in the location specified by the last argument.
*
* CopyMatrix3D copies a matrix into another.
*
* Entry: display - generic graphics device specification.
* obj - the object (typically a wire frame) to be rotated.
* dx,dy - the distance the mouse moved in screen coordinates
* since the previous event was processed.
* state - state of the mouse such as button presses.
*
* Exit: Object "obj" erased, rotated, and redrawn.
*/

#include
#include "defs.h"

ModifyObject(display, obj, dx, dy, state)
Display *display;
Polyhedron *obj;
int dx, dy, state;
{double Tx, Ty, Tz, n[3], dr, denom, cos_theta, sin_theta;
double Matrix3D[4][4], TmpMat[4][4], TransToOrigin[4][4], Rmat[4][4];
static double Radius=100.0;

/* Example of interactive method: apply rolling ball to
* object's orientation as long as mouse Button1 is
* held down.
*/
if (state == Button1)
{
/* Obtain current object position from its frame. */
Tx = obj->frame[3][0];
Ty = obj->frame[3][1];
Tz = obj->frame[3][2];

/* Compute the rolling ball axis and angle from the incremental mouse
* displacements (dx,dy) and compute corresponding rotation matrix RMat.
* See text for full form of Rmat returned by Make3DRot.
* NOTE: For window systems using a left-handed screen
* coordinate system, the formula (-dy,dx,0) given
* in the text for the rotation axis direction must
* be changed to (+dy,dx,0) to give the desired effect!
* We explicitly use this coordinate system in the example
* code because so many systems possess this reversal.
*/
dr = sqrt((double)(dx*dx + dy*dy));
denom = sqrt(Radius*Radius + dr*dr);
cos_theta = Radius/denom;
sin_theta = dr/denom;
n[0] = (double)(dy)/dr; /* Change sign for right-handed coord system. */
n[1] = (double)(dx)/dr;
n[2] = 0.0;
Make3DRot(cos_theta, sin_theta, n, Rmat);

/* Translate current object frame to origin. */
Make3DTranslation(-Tx, -Ty, -Tz, TransToOrigin);
CombineMatrices3D(TransToOrigin, obj->frame, Matrix3D);

/* Rotate about origin. */
CombineMatrices3D(Rmat, Matrix3D, TmpMat);

/* Translate rotated temporary frame back to original object position. */
Make3DTranslation(Tx, Ty, Tz, TransToOrigin);
CombineMatrices3D(TransToOrigin, TmpMat, Matrix3D);

/* Erase current object. */
SetFunction(display, ERASE);
DrawObject(display, obj);

/* Install new frame in object, draw rotated object. */
CopyMatrix3D(Matrix3D, obj->frame);
SetFunction(display, DRAW);
DrawObject(display, obj);

}}