Detect what the user clicks on

OpenGL OpenGL: Basic Coding
1

I am drawing a bunch of spheres with:


void DrawPoint(Point P, double rad, int Color)
{
	glPushMatrix();
	        glColor3f(1,1,1); //white
		glTranslatef(P.getX(), P.getY(), P.getZ());
		glutSolidSphere(rad, 20, 20);
	glPopMatrix();
}

I am storing these in a display list.

Once the spheres are displayed and i am rotating and zooming and all that good stuff (in the glut main loop), I would like the user to click on one of them and report which one he clicked on. Is this possible?

Thanks!

David

2

Yes, but you need to use the selection mechanism, and you will have to change the hit target name between spheres.

You will get a list of hits in the buffer which you will have to determine which is closest.

Colors and readback may be simpler.

http://www.opengl.org/resources/faq/technical/selection.htm

3

A better solution would be to to render each sphere with another color. And read the pixel with glReadPixel from the backbuffer.

4

Thanks for the replies guys. I found a super easy way to do it though. There is a function set called ZPR (Zoom Pan Rotate - http://www.nigels.com/glt/gltzpr/)..)

  1. it is great for what I am doing (plotting stuff that i want to look at, nothing like gaming haha)

  2. all you have to do to select an object in a display list is:


int main()
{
//...other setup stuff...
zprSelectionFunc(SelecFunc);
zprPickFunc(pick);
//...more stuff...
}



void pick(GLint name)
{
   printf("Pick: %d
", name);
   fflush(stdout);
}



void SelecFunc()
{
	DrawSpheres();
	glCallList(TestList);
}

This should be global


GLuint TestList;



void DrawSpheres()
{
	TestList = glGenLists(1);
	glNewList(TestList,GL_COMPILE);
	   DrawSphere(0,0,0,1,0,0,5);
	   DrawSphere(1,0,0,0,1,0,6);
	   DrawSphere(0,1,0,0,0,1,7);
   glEndList();

}



void DrawSphere(double x, double y, double z, double r, double g, double b, GLuint nam)
{
    glPushMatrix();  
		glPushName(nam);
         glColor3f(r,g,b);
         glTranslatef(x,y,z);
		 glutSolidSphere(1, 20, 20);
      glPopName();
    glPopMatrix();
}

Hope this helps others! By the way, why is something to the effect of ZPR not built right into glut?? It seems to be SUPER useful!

-Dave

5

Well, that ZPR is just a wrapper for the selection mode. See dorbies reply.

Please note that this feature is very useful for small scenes but can get very slow for bigger ones.

CatDog