Stop spray and stay in its position - c++

Question

// #include loads up library files, the order can matter
// generally load glut.h last
#include <stdio.h> // this library is for standard input and output
#include <stdlib.h>
#include <time.h>
#include <math.h>
#include "glut.h"// this library is for glut the OpenGL Utility Toolkit
//this defines a constant for the array size
#define SPRAYSIZE 500
// the properties of a spray particle are defined in a struct
struct sprayParticle {
float x = 0; // current position x
float y = 0; // current position y
float startx = 0; // birth position x
float starty = 0; // birth position y
int startTime; // a birthtime in frames when it will be born
int startRange = 100; // the maximum time at which a birth can happen
bool started = false; // tracks whether the particle has benn born or not
float speed = 0.1;
float radius;
float startxd = 0; // starting direction vector x value
float startyd = 0; // startingdirection vestor y value
float xd = 0; // current direction vector x value
float yd = 0; // current direction vector x value
float alpha = 1.0; // transparency
};
int winWidth = 1000, winHeight = 1000;
int counter = 0;
time_t t;
sprayParticle spray[SPRAYSIZE];
float angle = 90; // the angle of the spray: 0 degrees is to the left,
// 90 degrees straight up, 180 to the right etc
float sprayWidth = 30;// the width of the spray in degrees
float sprayCenterX, sprayCenterY;
//variables for spray colour, set once per spray
float fr = 1; float fg = 1; float fb = 1;
// the gravity vector
float gx = 0;
float gy = -0.0005;
// the position of thepartcle ystem emitter, wher the rocket should be drawn
float rocketstartx = winWidth - 100, rocketstarty = winHeight - 100;
void init() {
glClearColor(0.0, 0.0, 0.0, 0.0); // set what colour you want the background to be
glMatrixMode(GL_PROJECTION); // set the matrix mode, we will look at this later
gluOrtho2D(0.0, winWidth, 0.0, winHeight);
}
void circle(double radius, double xc, double yc) {
int i;
double angle = 2 * 3.1415 / 20; // circle is drawn using 20 line.
double circle_xy[40][2];
circle_xy[0][0] = radius + xc;
circle_xy[0][1] = yc;
glBegin(GL_POLYGON);
for (i = 1; i < 20; i++) {
circle_xy[i][0] = radius * cos(i *angle) + xc;
circle_xy[i][1] = radius * sin(i * angle) + yc;
glVertex2f(circle_xy[i - 1][0], circle_xy[i - 1][1]);
glVertex2f(circle_xy[i][0], circle_xy[i][1]);
}
glEnd();
}
void normalise(int i) {
float mag;
mag = sqrt((spray[i].xd*spray[i].xd) + (spray[i].yd*spray[i].yd));
spray[i].xd = spray[i].xd / mag;
spray[i].yd = spray[i].yd / mag;
}
// we calculate the direction vector of the current particle from the global
variable angle and spread
void setDirectionVector(int i) {
float minAngle, maxAngle, range, newangle;
double newAngleInRadians; // variable
int rangeInt;
minAngle = angle - (sprayWidth / 2.0); // calc the minimum angle the particle could move along
maxAngle = angle + (sprayWidth / 2.0); // calc the maximum angle
range = maxAngle - minAngle;
rangeInt = (int)(range*100.0);
newangle = minAngle + ((float)(rand() % rangeInt) / 100.0); // generate a random angle between mi and max angles
newAngleInRadians = (double)(newangle / 360.0)*(2 * 3.1415); // convert it to radians
spray[i].xd = (float)cos(newAngleInRadians);// calc the diection vector x value
spray[i].yd = (float)sin(newAngleInRadians);// calc the diection vector y value
}
void initspray() {
for (int i = 0; i < SPRAYSIZE; i++) {
spray[i].x = winWidth / 2; // set current start x position
spray[i].y = 100;// set current start y position
spray[i].startx = spray[i].x; spray[i].starty = spray[i].y;// set start x and y position
spray[i].speed = 0.1 + (float)(rand() % 150) / 1000.0;// speed is 0.1 to 0.25
spray[i].startTime = rand() % spray[i].startRange;// set birth time
spray[i].radius = (float)(rand() % 15); // random radius
setDirectionVector(i);// set the current direction vector
spray[i].startxd = spray[i].xd; spray[i].startyd = spray[i].yd; // set start direction vector to current
}
// set colour of spray
fr = 0.5 + (float)(rand() % 500) / 1000.0;
fg = 0.5 + (float)(rand() % 500) / 1000.0;
fb = 0.5 + (float)(rand() % 500) / 1000.0;
}
void drawsprayParticle(int i) {
glLineWidth(2);
if (!spray[i].started) {
if (counter == spray[i].startTime) {
spray[i].started = true;
}
}
if (spray[i].started) {
glColor4f(fr, fg, fb, spray[i].alpha); // white particiles
circle(spray[i].radius, spray[i].x, spray[i].y);
spray[i].x = spray[i].x + (spray[i].xd*spray[i].speed);
spray[i].y = spray[i].y + (spray[i].yd*spray[i].speed);
// this produces a direction vector that is a little longer than 1
spray[i].yd = spray[i].yd + gy;
// so the normalise the vector to make length 1
normalise(i);
// reduce transparency
spray[i].alpha -= 0.00015;
}
if (spray[i].x<0 || spray[i].x>winWidth + 500 || spray[i].y<0 || spray[i].y>winHeight) {
spray[i].x = spray[i].startx; spray[i].y = spray[i].starty; //rocketstartx
spray[i].xd = spray[i].startxd; spray[i].yd = spray[i].startyd;
spray[i].alpha = 1.0;
}
}
void drawspray() {
// draw each spray particle
for (int i = 0; i < SPRAYSIZE; i++) {
drawsprayParticle(i);
}
// increment rocket position
rocketstartx += 0.2;
// if the rocket is oof the screen more nad 500 pixels to the right the rest it to 0
if (rocketstartx > winWidth + 500) { rocketstartx = 0; }
counter++;
}
// This is the display function it is called when ever you want to draw something
void display() {
glClear(GL_COLOR_BUFFER_BIT); // clear the screen using the background colour
glColor3f(1.0, 1.0, 1.0); // set colour to white
drawspray();
glFlush(); // force all drawing to finish
}
// This is the idle function it is called whenever the program is idle
void idle() {
display();
}
// As with many programming languages the main() function is the entry point for execution of the program
int main(int argc, char** argv) {
srand((unsigned)time(&t));
// initialise first spray work
initspray();
glutInit(&argc, argv); //perform the GLUT initialization
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGBA); // more initialisation
glutInitWindowSize(winWidth, winHeight); // set window position
glutInitWindowPosition(0, 0); // set window size
glutCreateWindow("Fire"); // create a display with a given caption for the title bar
glEnable(GL_BLEND); //Enable blending.
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
init(); // call init function defined above
glutIdleFunc(idle); // define what function to call when the program is idle
glutDisplayFunc(display); // define what function to call to draw
glutMainLoop();
// this line exits the program
return 0;
}
The original code above normally creates a fountain that sprays particles all over the screen, but I have changed the size and range of the spray, so I can create a flame. The problem is that I can't stop the spread of the particles and it keeps moving up. I want it to stay in its position.
This is how it works:
This is how I want it to work:

Answer 1

With the formula
spray[i].x = spray[i].x + (spray[i].xd*spray[i].speed);
spray[i].y = spray[i].y + (spray[i].yd*spray[i].speed);
the distance of the points to it origin linear increases. You have to decrease the speed by time to smoothly approach to a limit position.
e.g.
spray[i].speed *= 0.9992f;
Of course you have to rest the speed (spray[i].speed), when the point is "restarted" at its origin. If the speed of the point or the alpha value of the point falls below a threshold, then the point has to be "restarted", too:
void drawsprayParticle(int i) {
glLineWidth(2);
if (!spray[i].started) {
if (counter == spray[i].startTime) {
spray[i].started = true;
}
}
if (spray[i].started) {
glColor4f(fr, fg, fb, spray[i].alpha); // white particles
circle(spray[i].radius, spray[i].x, spray[i].y);
spray[i].x = spray[i].x + (spray[i].xd*spray[i].speed);
spray[i].y = spray[i].y + (spray[i].yd*spray[i].speed);
// this produces a direction vector that is a little longer than 1
spray[i].yd = spray[i].yd + gy;
// so the normalize the vector to make length 1
normalise(i);
// reduce transparency
spray[i].alpha -= 0.0003;
spray[i].speed *= 0.9992f;
}
if ( spray[i].x<0 || spray[i].x>winWidth + 500 ||
spray[i].y<0 || spray[i].y>winHeight ||
spray[i].alpha < 1.0f/256.0f ||
spray[i].speed < 0.04f ) {
spray[i].x = spray[i].startx; spray[i].y = spray[i].starty;
spray[i].xd = spray[i].startxd; spray[i].yd = spray[i].startyd;
spray[i].alpha = 1.0;
spray[i].speed = 0.1 + (float)(rand() % 150) / 1000.0;
}
}
This is an empirical algorithm, so you have to play around with the values, to get the effect of your need.
Additionally I increase the start range: int startRange = 1300;
Preview: