glReadPixels() really slow, better solution to get OpenGL coordinates from mouse position?

Question

This is my code that I use to get my mouse position in the 3d scene:

void GetOGLPos(int x, int y, GLdouble &pX, GLdouble &pY, GLdouble &pZ){
 GLint viewport[4];
 GLdouble modelview[16];
 GLdouble projection[16];
 GLfloat winX, winY, winZ;

 glGetDoublev(GL_MODELVIEW_MATRIX, modelview);
 glGetDoublev(GL_PROJECTION_MATRIX, projection);
 glGetIntegerv(GL_VIEWPORT, viewport);

 winX = (float)x;
 winY = (float)viewport[3]-(float)y;
 glReadPixels(x, (int)winY, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &winZ);

 gluUnProject(winX, winY, winZ, modelview, projection, viewport, &pX, &pY, &pZ);
}

But I noticed a bad thing... 1-2 calls to that function per frame makes CPU usage 100%, 3 or more calls 200% CPU usage (I have 4 cores, 1-2 calls = 25%, 3 or more calls = 50%, I can't make it higher than 50% I think..)

Is there any other way to do this efficiently? I'm using 4 calls to that function every frame so I know which areas I should render for my scene (I take them from each screen corner).

Also I use this to know which place I am pointing with mouse, so I need it in real time, but I would like to use less CPU, because even just 1 call makes it 100% usage for single core systems.

EDIT

I've tried glPushName() method but its even slower, more likely slower in my GPU than in CPU. Also my CPU usage is only like 0-1% when I don't use a single glReadPixels() call in my program. Weird thing is that I get high CPU usage, but it doesn't make the program lag as you would expect with 100% usage... only problem comes when I use other programs while my program is on, then its laggy to use them.

Answer 1

It seems like you try to do picking in OpenGL.

Check out this tutorial, it should come with less performance penalty than your approach: http://gpwiki.org/index.php/OpenGL:Tutorials:Picking

This place mentions other ways of doing picking in OpenGL: http://www.opengl.org/resources/faq/technical/selection.htm