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daviddoria

OpenGL Simple Picking

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I've seen code (namely: http://jerome.jouvie.free.fr/OpenGl/Tutorials/Tutorial27.php) that has zillions (ok, 50) lines for picking objects. The main part seems to be saving the projection matrix, setting the pick matrix to a very small window, and then resetting the projection matrix. If I have a very simply scene - maybe 10 objects - this all seems unnecessary. I thought I could just pick with the same view that I am displaying on the screen. I tried to do a very much simpler version:

void Pick(GLdouble x, GLdouble y)
{
   GLuint buffer[1024];
   const int bufferSize = sizeof(buffer)/sizeof(GLuint);

   GLint hits;
   
   GLint  min  = -1;
   GLuint minZ = -1;

   glSelectBuffer(bufferSize,buffer); 
   glRenderMode(GL_SELECT);           
   glInitNames();                     
	display();
   hits = glRenderMode(GL_RENDER);    

   /* Determine the nearest hit */

int i=0, j=0;
   if (hits)
   {
      for (i=0, j=0; i<hits; i++)
      {
         if (buffer[j+1]<minZ)
         {
            /* If name stack is empty, return -1                */
            /* If name stack is not empty, return top-most name */

            if (buffer[j]==0)
               min = -1;
            else
               min  = buffer[j+2+buffer[j]];

            minZ = buffer[j+1];
         }

         j += buffer[j] + 3;
      }
   }

cout << "Name: " << min << endl;

}
but it doesn't work - the picked values are all wrong. Is there any reason this wouldn't work? Dave

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Setting up the scene is just as relevant as processing the hits, so post that code also. And in what way does your present code not work? You should tell us what's wrong with it also. "Wrong values" is not a description of the problem.

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Here are the relevant functions: I don't know how else to describe the "wrong values" other than when I click a surface that I expect to be 1, it is not 1. Sometimes clicking no surface at all returns a hit when it clearly should not. (is there not a "code tag" on this forum?)

void display()
{
glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
glEnable(GL_DEPTH_TEST);
DrawCube();

glutSwapBuffers();
}

void DrawCube()
{
glPushName(1);

glLoadName(1);
glColor3f(1.0f, 0.0f, 0.0f); // Color Red
polygon(0,3,2,1);

glLoadName(2);
glColor3f(0.0f,1.0f,0.0f); // Color Green
polygon(2,3,7,6);

glLoadName(3);
glColor3f(0.0f, 0.0f, 1.0f); // Color Blue
polygon(0,4,7,3);

glLoadName(4);
glColor3f(1.0f,1.0f,0.0f); // Color Yellow
polygon(1,2,6,5);

glLoadName(5);
glColor3f(0.0f,1.0f,1.0f); // Color Cyan
polygon(4,5,6,7);

glLoadName(6);
glColor3f(1.0f,0.0f,1.0f); // Color Magenta
polygon(0,1,5,4);

}



GLfloat vertices[][3] = {{-1.0,-1.0,-1.0},{1.0,-1.0,-1.0},
{1.0,1.0,-1.0}, {-1.0,1.0,-1.0}, {-1.0,-1.0,1.0},
{1.0,-1.0,1.0}, {1.0,1.0,1.0}, {-1.0,1.0,1.0}};


void polygon(int a, int b, int c , int d)
{
glBegin(GL_QUADS);
glVertex3fv(vertices[a]);
glVertex3fv(vertices[b]);
glVertex3fv(vertices[c]);
glVertex3fv(vertices[d]);
glEnd();
}

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You're not setting the pick region anywhere, so the whole original viewport is the pick region at the moment. That means anything you see on the screen when rendering normally will record a hit. You can easily realize something is wrong with the pick region as the X and Y parameters (which I assume is the desired pick location) are never used. The tutorial you linked mentions the pick region.

When describing the problem, you have to provide as much details as you can; relevant or not. So you expect 1 and don't get 1... but what do you get? You get zero hits? You get a single hit with another hit than you expect? You get a hit with a number that you don't even use? You get a multiple hits when you expect a single hit? You get totally bogus values which doesn't even form a valid hit record?

All these questions can be described as you described the problem, but all of them provides other information that narrows the problem further, and all of them indicates different problems possibly with different solutions. You have to be aware that all we can use to help you is what you give us.

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so with gluUnProject, I just provide the window coordinate (i dont understand why it wants x, y, and Z?? instead of just x and y), the current projection matrix, the current modelview matrix, and the region that I want to select (a viewport?)

ie. if I want to see which object is at window coords 50,60 I would pass as the viewport parameter

glViewport(50, 60, 3, 3)

Thoughts?
Dave

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I would suggest you to look into color picking, which seems to be the contemporary method of opengl picking.
Basically you set up a FrameBufferObject for offscreen rendering, assign a picking id to each object and render the objects into the offscreen buffer using the picking id as color.
After the rendering you may read the buffer via glReadPixels at the position of the mouse cursor perform a lookup in a hashtable to retrieve the actual object, that has been picked.
The downside is you may not retrieve all objects in a position, only the topmost one, which should be the object nearest to the camera. But thats maybe what you want =)

I have done this and it works really well.

Good luck,
Frederick

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Quote:
Original post by daviddoria
so with gluUnProject, I just provide the window coordinate (i dont understand why it wants x, y, and Z?? instead of just x and y), the current projection matrix, the current modelview matrix, and the region that I want to select (a viewport?)

ie. if I want to see which object is at window coords 50,60 I would pass as the viewport parameter

glViewport(50, 60, 3, 3)
Nope. You render the scene to the depth buffer, and then feed gluUnproject() the x and y of the mouse, and the z from the depth-buffer under the mouse (use glReadPixels). You also feed it the current modelview and projection matrices, and the current viewport.

The huge advantage of this method is that rather than obtaining the object under the mouse, you will instead obtain the exact x, y, z point in the 3d scene under the mouse. You can compare this point with your objects to find the selected object, or you can use the exact position for texture painting, etc.

Also, if you pick an object using this method, and then move the mouse and pick again, using the exact same depth value, you will obtain the correct result of dragging the selected object (i.e. parallel to the screen, for any projection and transformation).

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hmmm so it seems gluUnProject is not really a replacement for the selection buffer because it gives back a coordinate in 3D space, not an object ID, which is what I am looking for in this case. I was reading that you have to ensure the monitor is in "truecolor" mode for color picking to work - is that really a problem?

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Quote:
Original post by daviddoria
hmmm so it seems gluUnProject is not really a replacement for the selection buffer because it gives back a coordinate in 3D space, not an object ID, which is what I am looking for in this case.
Is it hard to look up an object from a position in your architecture? A simple bounds query should do the trick - I delegate it to my collision library.

Quote:
I was reading that you have to ensure the monitor is in "truecolor" mode for color picking to work - is that really a problem?
If it isn't in true colour mode, then you run the danger that your colours will be clamped/scaled, and when you read them back they won't match.

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