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    <p>To follow up on the answer already given, squares, cubes and triangles are convex shapes so you can perform ray-object intersection quite easily, even directly from the geometry rather than from the mathematical description of the perfect object.</p> <p>You're going to need to be able to calculate the distance of a point from the plane and the intersection of a ray with the plane. As a simple test you can implement yourself very quickly, for each polygon on the convex shape work out the intersection between the ray and the plane. Then check whether that point is behind all the planes defined by polygons that share an edge with the one you just tested. If so then the hit is on the surface of the object — though you should be careful about coplanar adjoining polygons and rounding errors.</p> <p>Once you've found a collision you can easily get the length of the ray to the point of collision. The object with the shortest distance is the one that's in front.</p> <p>If that's fast enough then great, otherwise you'll probably want to look into partitioning the world or breaking objects down to their silhouettes. Convex objects are really simple — consider all the edges that run between one polygon and the next. If only exactly one of those polygons is front facing then the edge is part of the silhouette. All the silhouettes edges together can be projected to a convex 2d shape on the view plane. You can then test touches by performing a 2d point-in-polygon from that.</p> <p>A further common alternative that eliminates most of the maths is picking. You'd render the scene to an invisible buffer with each object appearing as a solid blob in a suitably unique colour. To test for touch, you'd just do a <code>glReadPixels</code> and inspect the colour. </p> <p>For the purposes of glu on the iPhone, you can grab <a href="http://www.mesa3d.org/glu.html" rel="nofollow">SGI's implementation</a> (as used by MESA). I've used its tessellator in a shipping, production project before.</p>
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    1. COHi Tommy thanks for answering my question. Yesterday I had implemented touch detection in my scene by using the ray sphere intersection method. However I am unable to keep up with the math, if the objects in my scene start rotating. I am now looking at the picking approach. I guess my first question is how to render my scene to the hidden buffer
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    2. COThe easiest approach — assuming you're at least double buffered — is to render the scene as per usual, to the normal frame buffer, then do the `glReadPixels`, then a quick `glClear` and draw the scene in the usual way. Don't flip the buffers until the version you actually want the user to see is drawn and your specially coloured version will remain invisible. Or, while debugging, disable the normal draw to quickly and visually ensure you're getting uniquely coloured objects, each in a single colour.
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    3. COI was sort of following the basic approach of rendering then clearing and then rendering the scene... But this produced a very distinct flash... I solved the flash problem by not issuing the call to presentRenderBuffer when I was drawing the scene for touch detection. Now touch detection works correctly. I guess now the next challenge is to use vertex arrays
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