* Ported to X11 and xscreensaver by jwz, 27-Feb-2002.
*
* http://astronomy.swin.edu.au/~pbourke/modelling/fluid/
+ *
+ * Some physics improvements by Steven Barker <steve@blckknght.org>
*/
-/* cjb notes
- *
- * Future ideas:
+/* Future ideas:
* Specifying a distribution in the ball sizes (with a gamma curve, possibly).
* Brownian motion, for that extra touch of realism.
+ *
+ * It would be nice to detect when there are more balls than fit in
+ * the window, and scale the number of balls back. Useful for the
+ * xscreensaver-demo preview, which is often too tight by default.
*/
#include <math.h>
float *r; /* ball radiuses */
float *m; /* ball mass, precalculated */
- float e; /* coefficient of friction, I think? */
+ float e; /* coeficient of elasticity */
float max_radius; /* largest radius of any ball */
Bool random_sizes_p; /* Whether balls should be various sizes up to max. */
state->accy = get_float_resource ("gravity", "Gravity");
if (state->accy < -1.0 || state->accy > 1.0) state->accy = 0.01;
- state->e = get_float_resource ("friction", "Friction");
+ state->e = get_float_resource ("elasticity", "Elacitcity");
if (state->e < 0.2 || state->e > 1.0) state->e = 0.97;
state->tc = get_float_resource ("timeScale", "TimeScale");
{
int a, b;
float d, vxa, vya, vxb, vyb, dd, cdx, cdy;
- float ma, mb, vela, velb, vela1, velb1;
+ float ma, mb, vca, vcb, dva, dvb;
float dee2;
check_window_moved (state);
}
/* For each ball, compute the influence of every other ball. */
- for (a=1; a <= state->count; a++)
- if (a != state->mouse_ball)
- for (b=1; b <= state->count; b++)
- if (a != b)
- {
- d = ((state->px[a] - state->px[b]) *
- (state->px[a] - state->px[b]) +
- (state->py[a] - state->py[b]) *
- (state->py[a] - state->py[b]));
- dee2 = (state->r[a] + state->r[b]) *
- (state->r[a] + state->r[b]);
- if (d < dee2)
- {
- state->collision_count++;
- d = sqrt(d);
- dd = state->r[a] + state->r[b] - d;
- /* A pair of balls that have already collided in this
- * current frame (and therefore touching each other)
- * should not have another collision calculated, hence
- * the fallthru if "dd ~= 0.0".
- */
- if ((dd < -0.01) || (dd > 0.01))
- {
- cdx = (state->px[b] - state->px[a]) / d;
- cdy = (state->py[b] - state->py[a]) / d;
-
- /* Move each ball apart from the other by half the
- * 'collision' distance.
- */
- state->px[a] -= 0.5 * dd * cdx;
- state->py[a] -= 0.5 * dd * cdy;
- state->px[b] += 0.5 * dd * cdx;
- state->py[b] += 0.5 * dd * cdy;
-
- ma = state->m[a];
- mb = state->m[b];
- vxa = state->vx[a];
- vya = state->vy[a];
- vxb = state->vx[b];
- vyb = state->vy[b];
-
- vela = sqrt((vxa * vxa) + (vya * vya));
- velb = sqrt((vxb * vxb) + (vyb * vyb));
-
- vela1 = vela * ((ma - mb) / (ma + mb)) +
- velb * ((2 * mb) / (ma + mb));
- velb1 = vela * ((2 * ma) / (ma + mb)) +
- velb * ((mb - ma) / (ma + mb));
-
- vela1 *= state->e; /* "air resistance" */
- velb1 *= state->e;
+ for (a=1; a <= state->count - 1; a++)
+ for (b=a + 1; b <= state->count; b++)
+ {
+ d = ((state->px[a] - state->px[b]) *
+ (state->px[a] - state->px[b]) +
+ (state->py[a] - state->py[b]) *
+ (state->py[a] - state->py[b]));
+ dee2 = (state->r[a] + state->r[b]) *
+ (state->r[a] + state->r[b]);
+ if (d < dee2)
+ {
+ state->collision_count++;
+ d = sqrt(d);
+ dd = state->r[a] + state->r[b] - d;
+
+ cdx = (state->px[b] - state->px[a]) / d;
+ cdy = (state->py[b] - state->py[a]) / d;
+
+ /* Move each ball apart from the other by half the
+ * 'collision' distance.
+ */
+ state->px[a] -= 0.5 * dd * cdx;
+ state->py[a] -= 0.5 * dd * cdy;
+ state->px[b] += 0.5 * dd * cdx;
+ state->py[b] += 0.5 * dd * cdy;
+
+ ma = state->m[a];
+ mb = state->m[b];
+
+ vxa = state->vx[a];
+ vya = state->vy[a];
+ vxb = state->vx[b];
+ vyb = state->vy[b];
+
+ vca = vxa * cdx + vya * cdy; /* the component of each velocity */
+ vcb = vxb * cdx + vyb * cdy; /* along the axis of the collision */
+
+ /* elastic collison */
+ dva = (vca * (ma - mb) + vcb * 2 * mb) / (ma + mb) - vca;
+ dvb = (vcb * (mb - ma) + vca * 2 * ma) / (ma + mb) - vcb;
+
+ dva *= state->e; /* some energy lost to inelasticity */
+ dvb *= state->e;
+
#if 0
- vela1 += (frand (50) - 25) / ma; /* brownian motion */
- velb1 += (frand (50) - 25) / mb;
+ dva += (frand (50) - 25) / ma; /* q: why are elves so chaotic? */
+ dvb += (frand (50) - 25) / mb; /* a: brownian motion. */
#endif
- state->vx[a] = -cdx * vela1;
- state->vy[a] = -cdy * vela1;
- state->vx[b] = cdx * velb1;
- state->vy[b] = cdy * velb1;
- }
- }
- }
- /* Force all balls to be on screen.
- */
+ vxa += dva * cdx;
+ vya += dva * cdy;
+ vxb += dvb * cdx;
+ vyb += dvb * cdy;
+
+ state->vx[a] = vxa;
+ state->vy[a] = vya;
+ state->vx[b] = vxb;
+ state->vy[b] = vyb;
+ }
+ }
+
+ /* Force all balls to be on screen.
+ */
for (a=1; a <= state->count; a++)
{
if (state->px[a] <= (state->xmin + state->r[a]))
"*random: True",
"*gravity: 0.01",
"*wind: 0.00",
- "*friction: 0.8",
+ "*elasticity: 0.97",
"*timeScale: 1.0",
"*doFPS: False",
"*shake: True",
{ "-count", ".count", XrmoptionSepArg, 0 },
{ "-gravity", ".gravity", XrmoptionSepArg, 0 },
{ "-wind", ".wind", XrmoptionSepArg, 0 },
- { "-friction", ".friction", XrmoptionSepArg, 0 },
+ { "-elasticity", ".elasticity", XrmoptionSepArg, 0 },
{ "-fps", ".doFPS", XrmoptionNoArg, "True" },
{ "-no-fps", ".doFPS", XrmoptionNoArg, "False" },
{ "-shake", ".shake", XrmoptionNoArg, "True" },
projects / xscreensaver / blobdiff