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パーティクルの速度と色の透明度をリンクさせてみました。
Velocity is linked with color alpha.
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						#pragma once #include "ofMain.h" #include "ofxBox2d.h" class ofApp : public ofBaseApp { public: 	void setup(); 	void update(); 	void draw(); 	void keyPressed(int key) {} 	void keyReleased(int key) {} 	void mouseMoved(int x, int y) {} 	void mouseDragged(int x, int y, int button) {} 	void mousePressed(int x, int y, int button) {} 	void mouseReleased(int x, int y, int button) {} 	void mouseEntered(int x, int y) {} 	void mouseExited(int x, int y) {} 	void windowResized(int w, int h) {} 	void dragEvent(ofDragInfo dragInfo) {} 	void gotMessage(ofMessage msg) {} 	ofxBox2d box2d; 	vector<pair<shared_ptr<ofxBox2dCircle>, ofColor>> circles; 	int force_field_radius; };  | 
					
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						#include "ofApp.h" //-------------------------------------------------------------- void ofApp::setup() { 	ofSetFrameRate(60); 	ofSetWindowTitle("openFrameworks"); 	ofBackground(39); 	// Initialize ofxBox2d 	this->box2d.init();     this->box2d.setGravity(0, 0); 	this->box2d.createBounds(); 	this->box2d.setFPS(60); 	this->box2d.registerGrabbing(); 	// Add Circle 	for (int i = 0; i < 256; i++) { 		float radius = 12; 		shared_ptr<ofxBox2dCircle> circle = make_shared<ofxBox2dCircle>(); 		circle->setPhysics(1.0, 0.85, 0.5); 		circle->setup(this->box2d.getWorld(), ofRandom(ofGetWidth()), ofRandom(ofGetHeight()), radius); 		ofColor color; 		color.setHsb(ofRandom(255), 239, 239); 		this->circles.push_back(make_pair(circle, color)); 	} 	this->force_field_radius = 150; } //-------------------------------------------------------------- void ofApp::update() { 	// Field of Force on Center 	for (int i = 0; i < this->circles.size(); i++) { 		shared_ptr<ofxBox2dCircle> circle = get<0>(this->circles[i]); 		ofPoint point = get<0>(this->circles[i])->getPosition(); 		float distance = point.distance(ofPoint(ofGetWidth() / 2, ofGetHeight() / 2)); 		if(distance < this->force_field_radius) { 			ofPoint p = ofPoint(this->force_field_radius * cos(ofGetFrameNum() * DEG_TO_RAD), this->force_field_radius * sin(ofGetFrameNum() * DEG_TO_RAD)); 			circle->addForce(p, ofMap(distance, 0, this->force_field_radius, 15, 1)); 		} 	} 	// Compute box2d 	this->box2d.update(); } //-------------------------------------------------------------- void ofApp::draw() { 	ofPushMatrix(); 	ofTranslate(ofGetWidth() / 2, ofGetHeight() / 2); 	// Draw Filed of Force 	ofNoFill(); 	ofSetColor(239); 	ofSetLineWidth(0.5); 	ofBeginShape(); 	for (int deg = 0; deg < 360; deg += 1) { 		ofVertex(this->force_field_radius * cos(deg * DEG_TO_RAD), this->force_field_radius * sin(deg * DEG_TO_RAD)); 	} 	ofEndShape(true); 	ofFill(); 	ofBeginShape(); 	for (int deg = ofGetFrameNum(); deg < ofGetFrameNum() + 360; deg += 120) { 		ofVertex(this->force_field_radius * cos(deg * DEG_TO_RAD), this->force_field_radius * sin(deg * DEG_TO_RAD)); 	} 	ofEndShape(true); 	ofSetColor(39); 	ofBeginShape(); 	for (int deg = ofGetFrameNum(); deg < ofGetFrameNum() + 360; deg += 120) { 		ofVertex(this->force_field_radius * 0.5 * cos(deg * DEG_TO_RAD), this->force_field_radius * 0.5 * sin(deg * DEG_TO_RAD)); 	} 	ofEndShape(true); 	ofPopMatrix(); 	// Draw Circles 	ofSetLineWidth(3); 	for (int i = 0; i < this->circles.size(); i++) { 		shared_ptr<ofxBox2dCircle> circle = get<0>(this->circles[i]); 		ofColor circle_color = get<1>(this->circles[i]); 		float radius = circle->getRadius(); 		ofPoint point = circle->getPosition(); 		ofSetColor(circle_color, ofMap(circle->getVelocity().length(), 3, 12, 0, 255)); 		ofDrawCircle(point, radius); 		ofDrawLine(point, point - get<0>(this->circles[i])->getVelocity()); 	} } //-------------------------------------------------------------- int main() { 	ofSetupOpenGL(720, 720, OF_WINDOW); 	ofRunApp(new ofApp()); }  |