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adding mass-ratio to collisions for different sized particles

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This commit is contained in:
Damian Schneider
2025-12-06 16:31:09 +01:00
parent 6f6ac066c9
commit a421cfeabe
2 changed files with 26 additions and 10 deletions
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26
wled00/FXparticleSystem.cpp
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@@ -849,6 +849,8 @@ void ParticleSystem2D::handleCollisions() {
if (pidx >= usedParticles) pidx = 0; // wrap around
}
uint32_t massratio1 = 0; // 0 means dont use mass ratio (equal mass)
uint32_t massratio2 = 0;
for (uint32_t i = 0; i < binParticleCount; i++) { // go though all 'higher number' particles in this bin and see if any of those are in close proximity and if they are, make them collide
uint32_t idx_i = binIndices[i];
for (uint32_t j = i + 1; j < binParticleCount; j++) { // check against higher number particles
@@ -856,12 +858,18 @@ void ParticleSystem2D::handleCollisions() {
if (perParticleSize && advPartProps != nullptr) { // using individual particle size
collDistSq = (PS_P_MINHARDRADIUS << 1) + ((((uint32_t)advPartProps[idx_i].size + (uint32_t)advPartProps[idx_j].size) * 52) >> 6); // collision distance, use 80% of size for tighter stacking (slight overlap)
collDistSq = collDistSq * collDistSq; // square it for faster comparison
// calculate mass ratio for collision response
uint32_t mass1 = 1 + ((uint32_t)advPartProps[idx_i].size * advPartProps[idx_i].size); // +1 to avoid division by zero
uint32_t mass2 = ((uint32_t)advPartProps[idx_j].size * advPartProps[idx_j].size);
uint32_t totalmass = mass1 + mass2;
massratio1 = (mass2 << 8) / totalmass; // massratio 1 depends on mass of particle 2, i.e. if 2 is heavier -> higher velocity impact on 1
massratio2 = (mass1 << 8) / totalmass;
}
int32_t dx = (particles[idx_j].x + particles[idx_j].vx) - (particles[idx_i].x + particles[idx_i].vx); // distance with lookahead
if (dx * dx < collDistSq) { // check x direction, if close, check y direction (squaring is faster than abs() or dual compare)
int32_t dy = (particles[idx_j].y + particles[idx_j].vy) - (particles[idx_i].y + particles[idx_i].vy); // distance with lookahead
if (dy * dy < collDistSq) // particles are close
collideParticles(particles[idx_i], particles[idx_j], dx, dy, collDistSq);
collideParticles(particles[idx_i], particles[idx_j], dx, dy, collDistSq, massratio1, massratio2);
}
}
}
@@ -871,7 +879,7 @@ void ParticleSystem2D::handleCollisions() {
// handle a collision if close proximity is detected, i.e. dx and/or dy smaller than 2*PS_P_RADIUS
// takes two pointers to the particles to collide and the particle hardness (softer means more energy lost in collision, 255 means full hard)
void WLED_O2_ATTR ParticleSystem2D::collideParticles(PSparticle &particle1, PSparticle &particle2, int32_t dx, int32_t dy, const uint32_t collDistSq) {
void WLED_O2_ATTR ParticleSystem2D::collideParticles(PSparticle &particle1, PSparticle &particle2, int32_t dx, int32_t dy, const uint32_t collDistSq, uint32_t massratio1, uint32_t massratio2) {
int32_t distanceSquared = dx * dx + dy * dy;
// Calculate relative velocity note: could zero check but that does not improve overall speed but deminish it as that is rarely the case and pushing is still required
int32_t relativeVx = (int32_t)particle2.vx - (int32_t)particle1.vx;
@@ -899,11 +907,11 @@ void WLED_O2_ATTR ParticleSystem2D::collideParticles(PSparticle &particle1, PSpa
int32_t dotProduct = (dx * relativeVx + dy * relativeVy); // is always negative if moving towards each other
if (dotProduct < 0) {// particles are moving towards each other
// integer math used to avoid floats.
// integer math is much faster than using floats (float divisions are slow on all ESPs)
// overflow check: dx/dy are 7bit, relativV are 8bit -> dotproduct is 15bit, dotproduct/distsquared ist 8b, multiplied by collisionhardness of 8bit. so a 16bit shift is ok, make it 15 to be sure no overflows happen
// note: cannot use right shifts as bit shifting in right direction is asymmetrical for positive and negative numbers and this needs to be accurate! the trick is: only shift positive numers
// Calculate new velocities after collision
int32_t surfacehardness = 1 + max(collisionHardness, (int32_t)PS_P_MINSURFACEHARDNESS); // if particles are soft, the impulse must stay above a limit or collisions slip through at higher speeds, 170 seems to be a good value
int32_t surfacehardness = max(collisionHardness, (int32_t)PS_P_MINSURFACEHARDNESS >> 1); // if particles are soft, the impulse must stay above a limit or collisions slip through at higher speeds, 170 seems to be a good value
int32_t impulse = (((((-dotProduct) << 15) / distanceSquared) * surfacehardness) >> 8); // note: inverting before bitshift corrects for asymmetry in right-shifts (is slightly faster)
#if defined(CONFIG_IDF_TARGET_ESP32C3) || defined(ESP8266) // use bitshifts with rounding instead of division (2x faster)
@@ -913,11 +921,19 @@ void WLED_O2_ATTR ParticleSystem2D::collideParticles(PSparticle &particle1, PSpa
int32_t ximpulse = (impulse * dx) / 32767;
int32_t yimpulse = (impulse * dy) / 32767;
#endif
// if particles are not the same size, use a mass ratio. mass ratio is set to 0 if particles are the same size
if (massratio1) {
particle1.vx -= (ximpulse * massratio1) >> 7; // mass ratio is in fixed point 8bit, multiply by two to account for the fact that we distribute the impulse to both particles
particle1.vy -= (yimpulse * massratio1) >> 7;
particle2.vx += (ximpulse * massratio2) >> 7;
particle2.vy += (yimpulse * massratio2) >> 7;
}
else {
particle1.vx -= ximpulse; // note: impulse is inverted, so subtracting it
particle1.vy -= yimpulse;
particle2.vx += ximpulse;
particle2.vy += yimpulse;
}
if (collisionHardness < PS_P_MINSURFACEHARDNESS && (SEGMENT.call & 0x07) == 0) { // if particles are soft, they become 'sticky' i.e. apply some friction (they do pile more nicely and stop sloshing around)
const uint32_t coeff = collisionHardness + (255 - PS_P_MINSURFACEHARDNESS);
// Note: could call applyFriction, but this is faster and speed is key here

2
wled00/FXparticleSystem.h
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@@ -201,7 +201,7 @@ private:
//paricle physics applied by system if flags are set
void applyGravity(); // applies gravity to all particles
void handleCollisions();
[[gnu::hot]] void collideParticles(PSparticle &particle1, PSparticle &particle2, const int32_t dx, const int32_t dy, const uint32_t collDistSq);
void collideParticles(PSparticle &particle1, PSparticle &particle2, const int32_t dx, const int32_t dy, const uint32_t collDistSq, uint32_t massratio1, uint32_t massratio2);
void fireParticleupdate();
//utility functions
void updatePSpointers(const bool isadvanced, const bool sizecontrol); // update the data pointers to current segment data space