[gameprogrammer] Re: selection / picking on terrain geometry

oh yeah, the reason i think that's true is that they both rely on the same
hardware specs so at the core, they should have all the same functionality,
so long as we aren't talking about software only extensions



On 4/1/06, Alan Wolfe <alan.wolfe@xxxxxxxxx> wrote:
>
>  A total stab in the dark, I think DX must have the same picking that GL
> can do.  In my experience, I've seen everything in DX that can be done in
> GL, just going by a different name and different function calls to make it
> happen, but sometimes really really close looking code otherwise (:
>
>  On 4/1/06, Josh Stewart <aek@xxxxxxxxx> wrote:
> >
> > Thankyou everyone for your help,
> > Out of curiosity, does DirectX support any kind of picking / selection
> > like opengl ? (are there any directx buffs on this list? :)
> >
> > I am trying to write my graphics interface so that it is independant of
> > API, but if I provide any support to OpenGL picking system, I am trying
> > to think of the best way to make it generic enough so that a DirectX
> > implementation of my abstract Graphics could plugged on without much
> > fuss.
> >
> > From what I have read, directx favours "do it yourself" and doesnt
> > provide this.. If thats so, has anyone compared/measured the opengl
> > picking system vs their own implementation?
> >
> > -Josh
> >
> > >> From: Josh Stewart <aek@xxxxxxxxx>
> > >> Reply-To: gameprogrammer@xxxxxxxxxxxxx
> > >> To: gameprogrammer@xxxxxxxxxxxxx
> > >> Subject: [gameprogrammer] Re: selection / picking on terrain geometry
> >
> > >> Date: Sat, 01 Apr 2006 10:13:24 +0800
> > >>
> > >> Hi Shaun,
> > >> That sounds like a quick and effective way of doing it. My terrain is
> > >> heightmap generated and stored in a grid also (for now anyway). I
> > >> have figured out how to create the ray from the camera, but was still
> > >> looking for an optimal solution for searching all of the polys in the
> > >> terrain for an intersection. I was thinking I might need an octtree,
> > >> but thats something else I havent done yet :)
> > >> If you have some sample code that would be great.
> > >>
> > >> Much appreciated.
> > >> Josh
> > >>
> > >> Shaun Simpson wrote:
> > >>>
> > >>> Hey Josh ..
> > >>>
> > >>> for a university project a friend and i divised a test for working
> > >>> out terrain intersection for a 3d rts we were creating, our terrain
> > >>> was in a grid format (stored in a 2d array) with each point
> > >>> containing a height value, we worked out the intersection by
> > >>> translating the camera's coordinates onto the terrain and then
> > >>> firing a ray from the camera, by stepping forward along the ray at
> > >>> grid sized intervals we would check to see if it was lower than the
> > >>> height of the terrain if so that was our intersection tile., it is
> > >>> fairly accurate as long as you have small size tiles.. i still have
> > >>> have the code and can search out and forward the code if you wish,
> > >>> im not sure how applicable it is but for ourselves it was the
> > >>> simpliest solution and very cost effective.
> > >>>
> > >>> Shaun
> > >>> shaun_simpson_@xxxxxxxxxxx
> > >>>
> > >>>
> > >>>> From: Josh Stewart <aek@xxxxxxxxx>
> > >>>> Reply-To: gameprogrammer@xxxxxxxxxxxxx
> > >>>> To: gameprogrammer@xxxxxxxxxxxxx
> > >>>> Subject: [gameprogrammer] Re: selection / picking on terrain
> > geometry
> > >>>> Date: Fri, 31 Mar 2006 00:18:39 +0800
> > >>>>
> > >>>> I think that I will do this method with the ray/poly intersection
> > >>>> test.
> > >>>> I will have to think of a way to limit the testing to only terrain
> > >>>> polygons that are actually visible in the window (the terrain isn't
> > >>>> split up so non visible terrain isnt being culled atm)
> > >>>>
> > >>>> If I choose the method where I read the depth buffer and transform
> > >>>> that back to world coordinates, I think that I may have problems
> > >>>> clicking on the terrain if there is ever anything drawn over the
> > >>>> top of it (trees, particles, units), as  the depth buffer only
> > >>>> contains the last Z value of the item closest to the camera.
> > >>>>
> > >>>> Although maybe I can use some depth buffer hackery with
> > >>>> glDepthFunc(GL_GREATER), assuming the terrain is always the
> > >>>> furthest thing away, but that will surely mess with my rendering.
> > >>>>
> > >>>> Just have to find a good ray / poly intersection test that returns
> > >>>> the point of intersection. My maths book has a sample from Graphics
> >
> > >>>> Gems I ... I do wonder if there are newer methods though.
> > >>>>
> > >>>> Js
> > >>>>
> > >>>> Simon Mihevc wrote:
> > >>>>> You could do it by searching for an intersection betwen your
> > >>>>> terrain and a ray. Get the ray by projecting the mouse coordinates
> > >>>>> onto 2 different z planes, which gives the starting point and the
> > >>>>> direction vector for the ray. Then find the point on the terrain,
> > >>>>> that is closest to the ray for various ray lengths.
> > >>>>> Finding the point that is nearest to the ray is the problem here
> > >>>>> since there are a lot of points and there's no time to check them
> > >>>>> all. A bit of knowledge of linear algebra and algorithms helps
> > here.
> > >>>>>
> > >>>>>> I must be working on a project because, thats right folks, I have
> > >>>>>> another question :)
> > >>>>>> :)
> > >>>>>>
> > >>>>>> Assuming standard 3D RTS with 3D terrain and a 3rd person 45
> > >>>>>> degree camera (fixed or not, I havent decided yet)
> > >>>>>>
> > >>>>>> Using OpenGL selection / picking it seems that selection of units
> > >>>>>> in an RTS game, using a mouse, is relatively easy.
> > >>>>>> However I am having trouble figuring out the best way to click on
> >
> > >>>>>> a point on the screen where there are no units, and have the game
> > >>>>>> know the point on the terrain, where the user clicked.
> > >>>>>> The terrain is 1 large piece of geometry that is generated from
> > >>>>>> heightmap and optimised. I cannot just give it a name as clicking
> > >>>>>> anywhere will just return the selection stack with the name of
> > >>>>>> the terrain.
> > >>>>>> How can I find the point on the terrain where I was directing my
> > >>>>>> click.
> > >>>>>>
> > >>>>>> An example might be selecting some units and then clicking on an
> > >>>>>> empty space to "move here"
> > >>>>>>
> > >>>>>> Any help or suggestions, including further reading links to books
> > >>>>>> or sites that tackle the subject, is appreciated.
> > >>>>>> Thankyou
> > >>>>>>
> > >>>>>> Josh Stewart
> > >>>>>>
> > >>>>
> > >>
> > >
> >
> >
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> >
> >
> >
>

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