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I'm trying to, in JOGL, pick from a large set of rendered quads (several thousands). Does anyone have any recommendations?

To give you more detail, I'm plotting a large set of data as billboards with procedurally created textures.

I've seen this post OpenGL GL_SELECT or manual collision detection? and have found it helpful. However it can take my program up to several minutes to complete a rendering of the full set, so I don't think drawing 2x (for color picking) is an option.

I'm currently drawing with calls to glBegin/glVertex.../glEnd. Given that I made the switch to batch rendering on the GPU with vao's and vbo's, do you think I would receive a speedup large enough to facilitate color picking?

If not, given all of the recommendations against using GL_SELECT, do you think it would be worth me using it?

I've investigated multithreaded CPU approaches to picking these quads that completely sidestep OpenGL all together. Do you think a OpenGL-less CPU solution is the way to go?

Sorry for all the questions. My main question remains to be, whats a good way that one can pick from a large set of quads using OpenGL (JOGL)?

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viperld002
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    How can it take several minutes to render the full set if the set is a few thousand quads? Isn't the throughput at millions if not billions of triangles per second nowadays? – whoplisp Aug 02 '11 at 19:24
  • You are correct. I've since modified my rendering technique to work properly (through display lists) and it now renders my procedural results at real-time rates. – viperld002 Sep 26 '11 at 22:35

4 Answers4

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The best way to pick from a large number of quad cannot be easily defined. I don't like color picking or similar techniques very much, because they seem to be to impractical for most situations. I never understood why there are so many tutorials that focus on people that are new to OpenGl or even programming focus on picking that is just useless for nearly everything. For exmaple: Try to get a pixel you clicked on in a heightmap: Not possible. Try to locate the exact mesh in a model you clicked on: Impractical.

If you have a large number of quads you will probably need a good spatial partitioning or at least (better also) a scene graph. Ok, you don't need this, but it helps A LOT. Look at some tutorials for scene graphs for further information's, it's a good thing to know if you start with 3D programming, because you get to know a lot of concepts and not only OpenGl code.

So what to do now to start with some picking? Take the inverse of your modelview matrix (iirc with glUnproject(...)) on the position where your mouse cursor is. With the orientation of your camera you can now cast a ray into your spatial structure (or your scene graph that holds a spatial structure). Now check for collisions with your quads. I currently have no link, but if you search for inverse modelview matrix you should find some pages that explain this better and in more detail than it would be practical to do here.

With this raycasting based technique you will be able to find your quad in O(log n), where n is the number of quads you have. With some heuristics based on the exact layout of your application (your question is too generic to be more specific) you can improve this a lot for most cases.

An easy spatial structure for this is for example a quadtree. However you should start with they raycasting first to fully understand this technique.

iuiz
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  • Thank you for your advice. I indeed ended up partitioning my application's space to group geometry and accelerate the processing of picking from these groups. – viperld002 Sep 26 '11 at 22:37
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I just implemented color picking but glReadPixels is slow here (I've read somehere that it might be bad for asynchron behaviour between GL and CPU).

Another possibility seems to me using transform feedback and a geometry shader that does the scissor test. The GS can then discard all faces that do not contain the mouse position. The transform feedback buffer contains then exactly the information about hovered meshes.

You probably want to write the depth to the transform feedback buffer too, so that you can find the topmost hovered mesh. This approach works also nice with instancing (additionally write the instance id to the buffer)

I haven't tried it yet but I guess it will be a lot faster then using glReadPixels. I only found this reference for this approach.

S.L. Barth is on codidact.com
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danielb
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Never faced such problem, but in my opinion, I think the CPU based picking is the best way to try.

If you have a large set of quads, maybe you can group quads by space to avoid testing all quads. For example, you can group the quads in two boxes and firtly test which box you

neodelphi
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I'm using the solution that I've borrowed from DirectX SDK, there's a nice example how to detect the selected polygon in a vertext buffer object.

The same algorithm works nice with OpenGL.

Mihai Iorga
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shadeglare
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