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OpenGL Picking - how?

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What would be the best way (considering that I use OpenGL) to implement object picking? I used OpenGL's selection mode in the past, but I suspect there are 'better' methods out there. Also, while the OpenGL method only lets me select object (no less than 1 polygon), I'd like to be able to know where EXACTLY on the polygon the mouse is played (think about a RTS game where you click on the ground to move your units to a place. I want that effect) Also the precision of the OpenGL method is not really that good in my experience.

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Yeah, OpenGL picking is not good. If you want to do pixel-perfect picking, it's fairly simple, and the function gluUnProject is your best friend. Basically, make two calls to gluUnProject with your mouse coords: one with a z value of 0.0f, and the other with a z value of 1.0f. The two 3D coords which result from this are the start and end points of a ray that intersects your view frustum at the near and far clip planes exactly where the user clicked. Now you can use this ray to test against your geometry with some simple 3D math. Go with a coarse-grained test first, like testing the ray against geometry bounding boxes. If the ray intersects a bounding box, go ahead and test the polys of that geometry to see if the ray actually hit a poly. Finally, if a poly was hit (and if you really need to), you can determine the exact point of the ray/poly intersection. Google should be able to provide you with the 3D math you'll need if you're not familiar with it and there are also GDNet articles on it.

You know, I think this would be a good topic for an article, if one doesn't exist on here already... I might write one this week.

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Since object picking has been the topic of much discussion and confusion in these forums, I have written two sample programs that throughly demonstrates the procedure: one using the general approach of ray-object intersection and the other using OpenGL's selection buffer. The code hopefully contains enough comments to get people up and going. The first approach has two code paths. One that uses gluUnproject and one that manually performs all mathematical computations to fully demonstrate what's going on behind the scene. The latter is somehow heavy on math though and requires familiarity with the transformation pipeline.

Now, if anyone could provide some space to upload these to?! Perhaps someone with a GD premium account?

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I use a mixed approach with OpenGL SELECT for object determination and an unprojection to determine the 3D position from the 2D coordinates of the mouse cursor. I observed some wrong unprojection in z-direction.

Now I simple ignore the unprojected z coordinate and it works very well, but what is the right way to handle depth direction ?

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Is that the kind of picking i could use in a 3D RTS or should i use the other method described in the red book consisting on drawing with a different color for each unit ( with lighting disabled, a color would represent a unit id ) in the image buffer ( it could be in several passes if there are many units ) and comparing the color of the pixel under the cursor with the unit colors?

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Quote:
Is that the kind of picking i could use in a 3D RTS or should i use the other method described in the red book consisting on drawing with a different color for each unit ( with lighting disabled, a color would represent a unit id ) in the image buffer ( it could be in several passes if there are many units ) and comparing the color of the pixel under the cursor with the unit colors?


It is your call really. The main goal should just be to get it working. Using OpenGL selection buffer requires drawing all "selectable" entities twice, and it does suffer from precision problems and occasional missed hits. Doing the gluUnproject method means you don't need the additional drawing pass and the precision is much better (not bound to depth buffer precision), but you have to do the ray-object intersection manually. So which to choose depends on the complexity of the objects that need to be selectable, and your comfort level with 3D math and OpenGL in general. If you just want to get something working, go with OpenGL selection buffers. Once you find that it limits you, look into more sophisticated methods.

@Ashkan - that would probably make a really good article, you should contact the staff.

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