[gameprogrammer] Re: selection / picking on terrain geometry

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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