# Thread: Identifying the Visible points

1. ## Identifying the Visible points

I have a 3d model made of around 3000 vertices, i set the camera and view the model by projecting it onto an image using the model view matrices , i also use the gluproject function to find where the vertices of the 3d model are projected onto the image.

out of the 3000 and odd vertices i would like to know , the list of points visible for the given camera matrix specified.

EG:- if a 3d model of a sphere consists of 1000 points then , only around 500 points are visible from any camera position , i would like to know if there is any opengl routine/code to identify this set of points

2. ## Re: Identifying the Visible points

No gl calls here, only geometry.

Use a octree (or a k-dtree or any spatial structure you like) to memorize the points then read this
http://www.lighthouse3d.com/opengl/viewfrustum/

3. ## Re: Identifying the Visible points

In my case all the 3d points are in the view frustum, i would like to get the list of points that form the front faces of the 3d model.

4. ## Re: Identifying the Visible points

I don't know of a call that'd give you that list.. A classic trick is to draw all faces using a distinct color for each, reading back the back-buffer and searching for colors. This won't give you the vertices though, only (sometimes partially) visible faces.

If you're after the forward-facing ones instead of visible ones (which is the same in your sphere case), just dot the normal with the camera vector, whichever is positive is forward facing.

5. ## Re: Identifying the Visible points

Ops.. sorry, I misunderstand.

Then is more complex, you have two cases:
-Vertex belonging to back facing faces.
-Vertex occluded by other geometry.

First case is simple, as Fred say, you simply have to make a dot product with the camera.
For occluded vertex you can use the occlusion query extension to read the number of pixel generated by the draw command, I don't know if it works with points. You have to make a query for every vertex, can be rather slow.
It's quite old but it's faster than reading the buffer. The opengl 3.0 conditional rendering is based on this.

Here an example:
http://www.codesampler.com/oglsrc/og...cclusion_query

Note: Don't work with transparent object.

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