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boingx
05-24-2013, 10:42 AM
hello guys :D
this is my first post since im new to this forums. i have this program source whose algorithm im not able to understand;
like how the planes fly and flip backwards in this when they reach border.
i will be really grateful if someone can explain me this in some detail since im still a noob and haven fully understood many things in this.

/* This program is to draw planes and add or remove them.
It also allows to gave motion to those planes. The planes are created in various colors,
more colors can be added to the program by modifying the code a little*/

#include<GL/glut.h>
#include <stdlib.h>
#include <stdio.h>
#ifndef WIN32
#include <unistd.h>
#else
#define random rand
#define srandom srand
#endif
#include <math.h>
#define REMOVE_PLANE 2
#define MOTION_ON 3
#define MOTION_OFF 4
#define QUIT 5

/* Some <math.h> files do not define M_PI... */
#ifndef M_PI
#define M_PI 3.14159265
#endif
#ifndef M_PI_2
#define M_PI_2 1.57079632
#endif

GLboolean moving = GL_FALSE;

#define MAX_PLANES 15

struct {
float speed; /* zero speed means not flying */
GLfloat red, green, blue;
float theta;
float x, y, z, angle;
} planes[MAX_PLANES];

#define v3f glVertex3f /* v3f was the short IRIS GL name for
glVertex3f */

void
draw(void)
{
GLfloat red, green, blue;
int i;

glClear(GL_DEPTH_BUFFER_BIT);
/* paint black to blue smooth shaded polygon for background */
glDisable(GL_DEPTH_TEST);
glBegin(GL_POLYGON);
glColor3f(0.0, 0.0, 0.0);
v3f(-20, 20, -19);
v3f(20, 20, -19);
glColor3f(0.0, 0.0, 1.0);
v3f(20, -20, -19);
v3f(-20, -20, -19);
glEnd();
/* paint planes */
glEnable(GL_DEPTH_TEST);
for (i = 0; i < MAX_PLANES; i++)
if (planes[i].speed != 0.0) {
glPushMatrix();
glTranslatef(planes[i].x, planes[i].y, planes[i].z);
glRotatef(290.0, 1.0, 0.0, 0.0);
glRotatef(planes[i].angle, 0.0, 0.0, 1.0);
glScalef(1.0 / 3.0, 1.0 / 4.0, 1.0 / 4.0);
glTranslatef(0.0, -4.0, -1.5);
glBegin(GL_TRIANGLE_STRIP);
/* left wing */
v3f(-7.0, 0.0, 2.0);
v3f(-1.0, 0.0, 3.0);
glColor3f(red = planes[i].red, green = planes[i].green,
blue = planes[i].blue);
v3f(-1.0, 7.0, 3.0);
/* left side */
glColor3f(0.6 * red, 0.6 * green, 0.6 * blue);
v3f(0.0, 0.0, 0.0);
v3f(0.0, 8.0, 0.0);
/* right side */
v3f(1.0, 0.0, 3.0);
v3f(1.0, 7.0, 3.0);
/* final tip of right wing */
glColor3f(red, green, blue);
v3f(7.0, 0.0, 2.0);
glEnd();
glPopMatrix();
}
glutSwapBuffers();
}

void
tick_per_plane(int i)
{
float theta = planes[i].theta += planes[i].speed;
planes[i].z = -9 + 4 * cos(theta);
planes[i].x = 4 * sin(2 * theta);
planes[i].y = sin(theta / 3.4) * 3;
planes[i].angle = ((atan(2.0) + M_PI_2) * sin(theta) - M_PI_2) * 180 / M_PI;
if (planes[i].speed < 0.0)
planes[i].angle += 180;
}

void
{
int i;

for (i = 0; i < MAX_PLANES; i++)
if (planes[i].speed == 0) {

#define SET_COLOR(r,g,b) \
planes[i].red=r; planes[i].green=g; planes[i].blue=b;

switch (random() % 6) {
case 0:
SET_COLOR(1.0, 0.0, 0.0); /* red */
break;
case 1:
SET_COLOR(1.0, 1.0, 1.0); /* white */
break;
case 2:
SET_COLOR(0.0, 1.0, 0.0); /* green */
break;
case 3:
SET_COLOR(1.0, 0.0, 1.0); /* magenta */
break;
case 4:
SET_COLOR(1.0, 1.0, 0.0); /* yellow */
break;
case 5:
SET_COLOR(0.0, 1.0, 1.0); /* cyan */
break;
}
planes[i].speed = ((float) (random() % 20)) * 0.001 + 0.02;
if (random() & 0x1)
planes[i].speed *= -1;
planes[i].theta = ((float) (random() % 257)) * 0.1111;
tick_per_plane(i);
if (!moving)
glutPostRedisplay();
return;
}
}

void
remove_plane(void)
{
int i;

for (i = MAX_PLANES - 1; i >= 0; i--)
if (planes[i].speed != 0) {
planes[i].speed = 0;
if (!moving)
glutPostRedisplay();
return;
}
}

void
tick(void)
{
int i;

for (i = 0; i < MAX_PLANES; i++)
if (planes[i].speed != 0.0)
tick_per_plane(i);
}

void
animate(void)
{
tick();
glutPostRedisplay();
}

void
visible(int state)
{
if (state == GLUT_VISIBLE) {
if (moving)
glutIdleFunc(animate);
} else {
if (moving)
glutIdleFunc(NULL);
}
}

/* ARGSUSED1 */
void
keyboard(unsigned char ch, int x, int y)
{
switch (ch) {
case ' ':
if (!moving) {
tick();
glutPostRedisplay();
}
break;
case 27: /* ESC */
exit(0);
break;
}
}

void
{
switch (item) {
break;
case REMOVE_PLANE:
remove_plane();
break;
case MOTION_ON:
moving = GL_TRUE;
glutIdleFunc(animate);
break;
case MOTION_OFF:
moving = GL_FALSE;
glutIdleFunc(NULL);
break;
case QUIT:
exit(0);
break;
}
}

int
main(int argc, char *argv[])
{
glutInit(&argc, argv);
/* use multisampling if available */
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH | GLUT_MULTISAMPLE);
glutCreateWindow("My life as Paper planes");
glutDisplayFunc(draw);
glutKeyboardFunc(keyboard);
glutVisibilityFunc(visible);
/* setup OpenGL state */
glClearDepth(1.0);
glClearColor(0.0, 0.0, 0.0, 0.0);
glMatrixMode(GL_PROJECTION);
glFrustum(-1.0, 1.0, -1.0, 1.0, 1.0, 20);
glMatrixMode(GL_MODELVIEW);
/* add three initial random planes */
srandom(getpid());
/* start event processing */
glutMainLoop();
return 0; /* ANSI C requires main to return int. */
}

tonyo_au
05-24-2013, 09:18 PM
glTranslatef(planes[i].x, planes[i].y, planes[i].z);[/COLOR]
glRotatef(290.0, 1.0, 0.0, 0.0);
glRotatef(planes[i].angle, 0.0, 0.0, 1.0);

this positions to plane for drawing

tick_per_plane
this moves that plane

boingx
05-24-2013, 09:21 PM
but what about his part (which is inside tick_plane()):

void
tick_per_plane(int i)
{
float theta = planes[i].theta += planes[i].speed;
planes[i].z = -9 + 4 * cos(theta);
planes[i].x = 4 * sin(2 * theta);
planes[i].y = sin(theta / 3.4) * 3;
planes[i].angle = ((atan(2.0) + M_PI_2) * sin(theta) - M_PI_2) * 180 / M_PI;
if (planes[i].speed < 0.0)
planes[i].angle += 180;
}

tonyo_au
05-24-2013, 09:40 PM
This just positions the plane with a function that is dependent on theta. Since each component uses sin/cos of the angle theta, they will cycle since sine/cosine are repeating functions. The maths shows you how they vary.