OpenGL GL_SELECT or manual collision detection?

Question

As seen in the image

I draw set of contours (polygons) as GL_LINE_STRIP. Now I want to select curve(polygon) under the mouse to delete,move..etc in 3D .

I am wondering which method to use:

1.use OpenGL picking and selection. ( glRenderMode(GL_SELECT) )

2.use manual collision detection , by using a pick-ray and check whether the ray is inside each polygon.

Answer 1

umanga, Cant see how to reply inline... maybe I should sign up :)

First of all I must apologize for giving you the wrong algo - i did the back face culling one. But the one you need is very similar which is why I got confused... d'oh.

Get the camera position to mouse vector as said before.

For each contour, loop through all the coords in pairs (0-1, 1-2, 2-3, ... n-0) in it and make a vec out of them as before. I.e. walk the contour.

Now do the cross prod of those two (contour edge to mouse vec) instead of between pairs like I said before, do that for all the pairs and vector add them all up.

At the end find the magnitude of the resulting vector. If the result is zero (taking into account rounding errors) then your outside the shape - regardless of facing. If your interested in facing then instead of the mag you can do that dot prod with the mouse vector to find the facing and test the sign +/-.

It works because the algo finds the amount of distance from the vector line to each point in turn. As you sum them up and you are outside then they all cancel out because the contour is closed. If your inside then they all sum up. Its actually Gauss's Law of electromagnetic fields in physics...

See:http://en.wikipedia.org/wiki/Gauss%27s_law and note "the right-hand side of the equation is the total charge enclosed by S divided by the electric constant" noting the word "enclosed" - i.e. zero means not enclosed.

You can still do that optimization with the bounding boxes for speed.