// Copyright 2013 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "goose.h"
namespace {
// The maximum speed of a goose. Measured in meters/second.
const double kMaxSpeed = 2.0;
// The maximum force that can be applied to turn a goose when computing the
// aligment. Measured in meters/second/second.
const double kMaxTurningForce = 0.05;
// The neighbour radius of a goose. Only geese within this radius will affect
// the flocking computations of this goose. Measured in pixels.
const double kNeighbourRadius = 64.0;
// The minimum distance that a goose can be from this goose. If another goose
// comes within this distance of this goose, the flocking algorithm tries to
// move the geese apart. Measured in pixels.
const double kPersonalSpace = 32.0;
// The distance at which attractors have effect on a goose's direction.
const double kAttractorRadius = 320.0;
// The goose will try to turn towards geese within this distance (computed
// during the cohesion phase). Measured in pixels.
const double kMaxTurningDistance = 100.0;
// The weights used when computing the weighted sum the three flocking
// components.
const double kSeparationWeight = 2.0;
const double kAlignmentWeight = 1.0;
const double kCohesionWeight = 1.0;
} // namespace
Goose::Goose() : location_(0, 0), velocity_(0, 0) {
}
Goose::Goose(const Vector2& location, const Vector2& velocity)
: location_(location),
velocity_(velocity) {
}
void Goose::SimulationTick(const std::vector<Goose>& geese,
const std::vector<Vector2>& attractors,
const pp::Rect& flock_box) {
Vector2 acceleration = DesiredVector(geese, attractors);
velocity_.Add(acceleration);
// Limit the velocity to a maximum speed.
velocity_.Clamp(kMaxSpeed);
location_.Add(velocity_);
// Wrap the goose location to the flock box.
if (!flock_box.IsEmpty()) {
while (location_.x() < flock_box.x())
location_.set_x(location_.x() + flock_box.width());
while (location_.x() >= flock_box.right())
location_.set_x(location_.x() - flock_box.width());
while (location_.y() < flock_box.y())
location_.set_y(location_.y() + flock_box.height());
while (location_.y() >= flock_box.bottom())
location_.set_y(location_.y() - flock_box.height());
}
}
Vector2 Goose::DesiredVector(const std::vector<Goose>& geese,
const std::vector<Vector2>& attractors) {
// Loop over all the neighbouring geese in the flock, accumulating
// the separation mean, the alignment mean and the cohesion mean.
int32_t separation_count = 0;
Vector2 separation;
int32_t align_count = 0;
Vector2 alignment;
int32_t cohesion_count = 0;
Vector2 cohesion;
for (std::vector<Goose>::const_iterator goose_it = geese.begin();
goose_it < geese.end();
++goose_it) {
const Goose& goose = *goose_it;
// Compute the distance from this goose to its neighbour.
Vector2 goose_delta = Vector2::Difference(
location_, goose.location());
double distance = goose_delta.Magnitude();
separation_count = AccumulateSeparation(
distance, goose_delta, &separation, separation_count);
align_count = AccumulateAlignment(
distance, goose, &alignment, align_count);
cohesion_count = AccumulateCohesion(
distance, goose, &cohesion, cohesion_count);
}
// Compute the means and create a weighted sum. This becomes the goose's new
// acceleration.
if (separation_count > 0) {
separation.Scale(1.0 / static_cast<double>(separation_count));
}
if (align_count > 0) {
alignment.Scale(1.0 / static_cast<double>(align_count));
// Limit the effect that alignment has on the final acceleration. The
// alignment component can overpower the others if there is a big
// difference between this goose's velocity and its neighbours'.
alignment.Clamp(kMaxTurningForce);
}
// Compute the effect of the attractors and blend this in with the flock
// cohesion component. An attractor has to be within kAttractorRadius to
// effect the heading of a goose.
for (size_t i = 0; i < attractors.size(); ++i) {
Vector2 attractor_direction = Vector2::Difference(
attractors[i], location_);
double distance = attractor_direction.Magnitude();
if (distance < kAttractorRadius) {
attractor_direction.Scale(1000); // Each attractor acts like 1000 geese.
cohesion.Add(attractor_direction);
cohesion_count++;
}
}
// If there is a non-0 cohesion component, steer the goose so that it tries
// to follow the flock.
if (cohesion_count > 0) {
cohesion.Scale(1.0 / static_cast<double>(cohesion_count));
cohesion = TurnTowardsTarget(cohesion);
}
// Compute the weighted sum.
separation.Scale(kSeparationWeight);
alignment.Scale(kAlignmentWeight);
cohesion.Scale(kCohesionWeight);
Vector2 weighted_sum = cohesion;
weighted_sum.Add(alignment);
weighted_sum.Add(separation);
return weighted_sum;
}
Vector2 Goose::TurnTowardsTarget(const Vector2& target) {
Vector2 desired_direction = Vector2::Difference(target, location_);
double distance = desired_direction.Magnitude();
Vector2 new_direction;
if (distance > 0.0) {
desired_direction.Normalize();
// If the target is within the turning affinity distance, then make the
// desired direction based on distance to the target. Otherwise, base
// the desired direction on MAX_SPEED.
if (distance < kMaxTurningDistance) {
// Some pretty arbitrary dampening.
desired_direction.Scale(kMaxSpeed * distance / 100.0);
} else {
desired_direction.Scale(kMaxSpeed);
}
new_direction = Vector2::Difference(desired_direction, velocity_);
new_direction.Clamp(kMaxTurningForce);
}
return new_direction;
}
int32_t Goose::AccumulateSeparation(double distance,
const Vector2& goose_delta,
Vector2* separation, /* inout */
int32_t separation_count) {
if (distance > 0.0 && distance < kPersonalSpace) {
Vector2 weighted_direction = goose_delta;
weighted_direction.Normalize();
weighted_direction.Scale(1.0 / distance);
separation->Add(weighted_direction);
separation_count++;
}
return separation_count;
}
int32_t Goose::AccumulateAlignment(double distance,
const Goose& goose,
Vector2* alignment, /* inout */
int32_t align_count) {
if (distance > 0.0 && distance < kNeighbourRadius) {
alignment->Add(goose.velocity());
align_count++;
}
return align_count;
}
int32_t Goose::AccumulateCohesion(double distance,
const Goose& goose,
Vector2* cohesion, /* inout */
int32_t cohesion_count) {
if (distance > 0.0 && distance < kNeighbourRadius) {
cohesion->Add(goose.location());
cohesion_count++;
}
return cohesion_count;
}
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