Get mouse click coordinates in OpenGL during animation? - c

Question

I want to make a game in which when somebody clicks on the moving ball, it bursts. I have added the codes for animation and the mouse click event, but when the animation is going on, the click function is not working. When I tried it without the animation, it worked properly. I want to know why is this happening.
#include<stdio.h>
#include<GL/glut.h>
#include<unistd.h>
#include<math.h>
int x, y;
float mx, my;
float i, j;
void mouse(int button, int state, int mousex, int mousey)
{
if(button==GLUT_LEFT_BUTTON && state==GLUT_DOWN)
{
mx = mousex;
my = mousey;
printf("%f %f\n",mx,my);
glutPostRedisplay();
}
}
void init()
{
glClearColor(0.0, 0.0, 0.0, 1.0);
glColor3f(0.0, 1.0, 0.0);
glPointSize(1.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0, 1560, 0, 840);
}
int randValue()
{
int i = rand();
int num = i%1000;
return num;
}
void blast(int x, int y)
{
glBegin(GL_POLYGON);
glColor3f(1.0f,0.0f,0.0f);
glVertex2i(x-100, y-100);
glVertex2i(x, y-100);
glVertex2i(x-22, y-20);
glVertex2i(x-100, y-30);
glVertex2i(x-30, y-40);
glVertex2i(x-150, y-80);
glVertex2i(x-20, y);
glVertex2i(x, y-40);
glVertex2i(x-66, y-125);
glVertex2i(x-34, y-32);
glVertex2i(x-32, y-55);
glVertex2i(x-32, y);
glVertex2i(x-60, y-57);
glVertex2i(x-75, y-69);
glVertex2i(x-100, y);
glEnd();
glFlush();
}
void display()
{
int j = 0, k = 0, l = 1;
while(1)
{
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(0.0, 1.0, 0.0);
glBegin(GL_POINTS);
for (i = 0;i < 6.29;i += 0.001)
{
x = 100 * cos(i);
y = 100 * sin(i);
glVertex2i(x / 2 + j, y / 2 + k);
if((x / 2 + j) >= 1560 || (y / 2 + k) >= 840)
{
glEnd();
glFlush();
glClear(GL_COLOR_BUFFER_BIT);
blast(x / 2 + j, y / 2 + k);
sleep(2);
j = randValue();
k = 0;
}
}
j = j + 3;
k = k + 5;
glEnd();
glFlush();
}
}
int main (int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(1360, 768);
glutInitWindowPosition(0, 0);
glutCreateWindow("{Project}");
init();
glutDisplayFunc(display);
glutMouseFunc(mouse);
glutMainLoop();
}

Answer 1

Your code has an infinite loop inside the display function, thus you never give the control back to GLUT. GLUT already has an infinite loop like that inside glutMainLoop.
Instead you shall render only ONE frame in display, post glutPostRedisplay and return:
void display()
{
glClear(GL_COLOR_BUFFER_BIT);
// ... draw the frame here ...
// for exmaple:
i += 0.001;
float x = 100 * cos(i);
float y = 100 * sin(i);
glColor3f(0.0, 1.0, 0.0);
glBegin(GL_POINTS);
glVertex2f(x, y);
glEnd();
glFlush();
glutPostRedisplay();
}
Then your mouse function will be called and you'll be able to update the state as necessary.

Answer 2

There are two problems here:
OpenGL has no support for an input device by itself, you normally use OpenGL to present information but you have something else attached to the window where you present the info that is what gives you mouse access. this involves to know which is the other environment you are using that offers you a pointing device into the screen area.
if you have the window mouse coordinates you need to map well on the window you present your OpenGL output, but you have to convert them back to some point in your scene, but probably your ball is not there. There's some ambiguity when passing from a plane image representing a 3D scene to a point in that scene in 3D, as you have all points in the Z axis sharing the same screen coordinates in 2D screen. so you have to trace back to the possible position of the ball from the point of view (the camera), based on the window coordinates of the mouse. This is a geometrical problem that involves the inverse transformation of a projection, that is always singular.
you can solve this without having to guess, as you know where your ball is, you can redo the transformation that made it to appear in the two dimensional window, and then compare coordinates based on those. OpenGL allows you to know the actual transformation it is doing to represent your scene, and you can use it to see where in the screen your ball is represented (you don't need to do this for every vertex of the ball, only for the center, for example) and then check if your shot has gone close enough to hit the ball. You should consider also if some other object upper in the Z axis is in the way, so you don't kill anybody behind a wall.