[gl4java-usergroup] RE: Picking problem

[Alban C. <aco...@wa...>]

>Hi all,
>
>I've been playing with picking and started with the Red Books picking
>example. (the one with a grid of different color squares and the square
>changes color when you click it.)
>
>However, when I call the picking code, and use:
>
>hits =3D gl.glRenderMode(GL_RENDER);
>
>hits is always 0. I've searched the archives and no one else seems to
>have this particular problem, so I must be doing something rather dumb.
>I've included the two source files. Any help would be greatly
>appreciated.
>
>--Mark

Hi Mark,

I personnaly had no problems implementing picking and it works
perfectly, though I've had the same behaviour (hits return 0) when
implementing the picking on quads.

This is not a gl4java bug nor a MS JVM bug, but a general OpenGL
problem.
Strangely, OpenGL in ANY implementation is unable to pick quads in ortho
mode. I assume this could be because picking implementation could be
tightly dependant of the depth buffer, and the depth buffer could be
somewhat limited/uncompatible in ortho mode.=20

To be able to pick quads (or any 2D elements), you have to implement
color picking, as stated in the OpenGL faq
http://www.opengl.org/developers/faqs/technical/selection.htm, section
20.010

I did it and it works perfectly, though the help on its implementation
in the OpenGL faq is very uncomplete and confuse.
The strategy is to draw your quads in an offscreen buffer that cannot be
seen by the user, assigning each quad  a unique color that you record as
an integer value in an hashtable, associated with your quad name. (I
personnaly took pure red, as an integer value between 0 and 255 wich
allows me to have 256 unique colors. To be able to pick more objects,
you can add a green and a blue value, allowing you to pick up to 16
Million objects). In order to avoid your color to be corrupted by
openGL, you have to disable any component that might affect your color :
gl.glDisable(gl.GL_BLEND);
gl.glDisable(gl.GL_FOG);
gl.glDisable(gl.GL_LIGHTING);
gl.glDisable(gl.GL_TEXTURE_1D);
gl.glDisable(gl.GL_TEXTURE_2D);
gl.glDisable(gl.GL_TEXTURE_3D);
gl.glShadeModel(gl.GL_FLAT);

Whenever you perform a pick, you read the color of the pixel located
under the mouse using glReadPixels(). Once you have the color, you can
find out wich object it relates to by retrieving the name of the quad
associated as a pair with the integer value of the color in your
hashtable.

But before implementing color picking, you should try drawing 3D objects
with your current implementation (such as a cube, and not in ortho mode)
and your picking should work fine with these.

Regards,

Alban Cousini=E9