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OpenGL Object Clipping - Me or OpenGL?

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Ok, I'm going to have tons of objects/particles, etc. in my game... and ALL of them must be accounted for. Now, as it stands, I am translating to all these objects and drawing them, regardless of whether they are on screen or not. For example, if the camera is at (0,0), and one of my objects is at (10000, 10000) (way off screen), I still tell it to translate to (10000, 10000) and call the display list. Now, is OpenGL going to clip that polygon automagically for me, or will I get a big performance boost if I test if the object is offscreen MYSELF, and then draw the object based on those tests? Thanks for any help you can provide [edited by - neonfaktory on December 25, 2002 9:10:13 PM]

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You will get a large boost if you test if the object are onscreen yourself. When i did that on the object and traingle level it doubled my frame rate. If you need to know how to do that then goto and look for their frustum calling tutorial for opengl, and if thats still not a large enough boost, you might want to look into octrees.

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always let opengl do clipping
BUT i think u meant to say culling (not clipping)
in that case do gross culling yourself, if something is not on screen dont send it to opengl, for large scenes this will result in a large performance boost

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It''s actually a bunch of objects on the same 2D coordinate plane. You can move the camera around on that plane (X,Y) and also zoom in and out on the Z axis. I just wanted some simple test to determine if my objects are withing the viewing range or not... I could probably do it with a simple calc based on the FOV angle... couldn''t I? Or should I just go with the Culling EQs that you can find in most culling tuts? Thanks for the help so far

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Using a quadtree, you could keep track of all objects that are within a given cell of the quadtree, and if you can calculate that a given cell is not in the view at all, you can ignore all the objects inside the cell, which removes a lot of testing at the object level.

If a cell is in view, then you test all the objects in the cell (if you wish) or just submit them to OpenGL and let the API handle the culling.

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The API performs its culling just by having the geometry sent to it. You submit polygons, and the API culls it automatically.

Otherwise you are performing your own culling calculations, and if the cull test fails, you just don''t send that polygon to the API.

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There is a little bit of confusion here...

Clipping clips your polygons against the view frustum, cutting them if they need to be cut, so that the rasterizer works properly (think of the old days, when you would write sw renderers).

Culling is removing polygons from the scene, so that you don''t send them to the graphics card at all.

OpenGL is going to CLIP your polygons anyway, so don''t clip them. If you can easily CULL polygons that have no chance of being seen (in your example, your object at (10000,10000)), then do it.

Other people have mentioned backface culling. This is a culling method supported by graphics APIs and hardware. Enable this.

A good rule of thumb is to let your API do as much as possible. IE, rather than managing all your transforms yourself, use the API''s matrix stacks. Sometimes, you will get hardware acceleration, which is good. Other times, the API will implement it in its software layer, but it will probably do a better job than you can. (They have spent a lot of time on this, they can detect CPU type and use SSE extensions, etc).

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Yeah, I understand - I made the "clipping" in the topic title before I knew that "culling" was what I was talking about. I''m letting OGL clip everything, but I was looking for a good way to cull my objects (good method to go about it).

I kinda found a good tutorial for it, but it does all this fancy math to find the frustum planes and stuff to compensate for huge 3D worlds, and I think what I''m looking for is alot more simple than that.

I have object x,y coords, and camera x,y coords, along with a camera z coord to simulate a zooming effect. As of right now, my game is 2D, so I just have a bunch of tiles that face the camera with a texture on them. I think with the coordinate & zoom information, coupled with the projection matrix settings I should be able to come up with a simple EQ for testing, ya know?

SO yeah, I think I''m going to find out what PROJECTION matrix angle I''m using and see if I can just use a simple trig EQ to test if my simple polygons are on the screen or not. Unless someone knows a better idea THanks for all the help

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