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picking

Started by Ultraseamus Jun 10, 2006 at 3:05 AM 4 replies 600+ views

Original Post

Jun 10, 2006 at 3:05 AM

am using the NeHe tutorial with some modifactions, with it i got picking too work, but then i need to be able to place objects after i picked them up, so for that i needed to use glOrtho(0,640,480,0,-1,1); so that i could use the mouse coords to place the object. So now i have decided that i may as well just draw everything with glOrtho(0,640,480,0,-1,1); but now picking does not work correctly, no matter where i click on the screen it detects a collision with my object, here is my selection code: void Selection(void) { GLuint buffer[512]; GLint hits; GLint viewport[4]; glGetIntegerv(GL_VIEWPORT, viewport); glSelectBuffer(512, buffer); (void) glRenderMode(GL_SELECT); glInitNames(); glPushName(0); glMatrixMode(GL_PROJECTION); glPushMatrix(); glLoadIdentity(); gluPickMatrix((GLdouble) mouse_x, (GLdouble) (viewport[3]-mouse_y), 1.0f, 1.0f, viewport); gluPerspective(45.0f, (GLfloat) (viewport[2]-viewport[0])/(GLfloat) (viewport[3]-viewport[1]), 0.1f, 100.0f); glMatrixMode(GL_MODELVIEW); DrawTargets(); glMatrixMode(GL_PROJECTION); glPopMatrix(); glMatrixMode(GL_MODELVIEW); hits=glRenderMode(GL_RENDER); if (hits > 0) { int choose = buffer[3]; x+=0.02f; pickedObject = choose; if (!object[choose].hit) { object[choose].x += 0.5; } } }

Bob III

Jun 11, 2006 at 3:39 AM

Why are you using a perspective transform for selection when you're drawing everything in ortho?

I'm tired and can't rule out that there's a reason (why am I checking the forum then? ehh)

Ultraseamus

Jun 11, 2006 at 4:31 PM

Yes, i know what i have there is not correct, that is just how i was doing picking before, I am learning openGL as i go so i sterted by drawing everything in a way it would work with this method of picking. I am just not sure exactly how to change the code to work with ortho, I have tried a few things that i thought would work, but i still have the same problem of always regestering a hit.

Bob III

Jun 11, 2006 at 11:01 PM

To correct this problem you would just replace gluPerspective with glOrtho, I'm pretty sure.

Is the modelview matrix in the right state when you DrawTargets?

If you arn't, check glGetError() - maybe you missed a matrix op somewhere.

Ultraseamus

Jun 12, 2006 at 12:07 AM

hmm, well i had already tried just replacing gluPerspective with glOrtho, but i still get the same problem. Here is my code for DrawTargets:

void DrawTargets(void)
{
glLoadIdentity();
for (int loop=0; loop<1; loop++)
{
glLoadName(loop);
glPushMatrix();
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
glOrtho(0,640,480,0,-1,1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glPushMatrix();
glTranslatef(object[loop].x,object[loop].y,object[loop].distance);
Object(30,30,0);
glPopMatrix();
}
}

I googled the problem, and it looks liek there are a few other people out there with the same issue, where it always registers a hit, but i have not yet found a good solution short of completly re-coding my program. I suspect i am missing some small detail, but i have not been able to figure it out yet. Thanks for any help you can offer.

Bob III

Jun 13, 2006 at 12:34 AM

You overwrote the selection matrix you computed in the projection matrix setup by loading identity and making another ortho transform.

And you glPushMatrix two times more than you pop.

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