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the line_loop is created by opengl. I need to pick a vertex of the line segment and then drag it to somewhere in the 2D screen.

My thought is to pick up the vertex of the line by using Opengl picking method, and then the buffer storing the hit records is also created from glSelectBuffer. The problem is that how I can know which vertex is selected from information of the returned buffer? The buffer stores the name of the vertex. But it seems the vertex does not has a name in GL_RENDER mode?

Update: Is there any other convenient way to drag lines by mouse?

genpfault
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Sean
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  • I'm a bit confused, if you know where the mouse click point is, and where in screen-space your line end points are, surely you can work out which is the closest? – cmannett85 Aug 30 '11 at 06:52
  • @cbamber85, I wish to use Picking and selection methods of openGL. that is to say, I hope to derive which primitive to be selected just from the returned Buffer? – Sean Aug 30 '11 at 07:56
  • @cbamber85, do you mean I can compare the x, and y values of mouse point and all the line point to determine which point is selected? – Sean Aug 30 '11 at 07:58
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    Yeah, I know it's not glamourous but your line loop won't have that many points, will it? – cmannett85 Aug 30 '11 at 10:19

1 Answers1

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OpenGL is not a scene graph (gah, it seems every other question on OpenGL I answer begins with this statement). After you've drawn something, OpenGL no longer has any recollection of what you actually sent it. The old OpenGL selection mode is technically just testing if the geometry submitted is within the projections clip space range. On most OpenGL implementations selection mode falls back into software rendering mode, so you'll get a major performance hit.

There are several better ways to do selection (that's why selection mode has been removed from OpenGL after all). If it's just single vertices in specific geometry (like a selection rubberband) you're after, then you should perform the whole transformation of those points into normalized device coordinates yourself, sort them into some screen space spatial subdivision structure (2d Kd tree, quadtree, etc.) so that you can determine the point clicked on in O(log n) time – in contrast to the O(n) you'd have with selection mode, in which you have to "draw" the whole rubberband, so that all points are tested.

EDIT/Update

Since OpenGL is (just) a drawing API you also can't "drag around" things. You'll have to redraw them. Technically you should redraw the whole scene, or when starting a drag, draw the scene without the object about to be dragged into a texture (color and maybe depth), and then for each dragging step clear the view to the cached contents in the texture and then add the dragged objects in its updated position.

datenwolf
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