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View Full Version : Clipping: Good or Bad? (Games)


MayaBug
03-01-2011, 10:41 AM
Hey,

I understand clipping to be where two objects are joined intersecting rather than one being modelled out of the other so that they are a single object. Firstly, is this correct?

If so, it seems like it'd save a lot of time in creating models but can anyone offer a description of when to and when not to use this technique and what are the pros and cons of it etc.?

ezekiel66
03-01-2011, 11:01 AM
The term "clipping" is generally used for a technique to avoid the computation of objects/ polygons that are outside the field of view of the camera. It's used in video games to maximize their possible visual quality/ the possible overall polygon count by only drawing the polygons etc. that are visible to the camera/ the gamer.

A similar technique is "occlusion culling" where also the polygons that are occluded by other polygons will not be drawn by the game engine.

It has nothing to do with modelling. You probably are talking about something different, "clipping" is not the correct term. I'm not sure what you mean though. "Booleans" maybe?

MayaBug
03-01-2011, 12:07 PM
I think we're talking about the same thing.

http://img822.imageshack.us/img822/5558/clippingl.jpg (http://img822.imageshack.us/i/clippingl.jpg/)

On the left you can see that the blocks intersect each other and are a combined mesh (Maya) whereas the model on the right has been painstakingly modelled into shape. The model on the left demonstrates "clipping"? I want to know if this method is ok for modelling in games because in more complex shapes it would undoubtedly be a great time-saver but I don't know if it would cause problems in the game engine or what the do's and don'ts of it are...:curious:

I hope I've been a bit clearer.

earlyworm
03-01-2011, 12:48 PM
The main problem with that technique is that it introduces visual artefacts which can look horrible.

Where two polygons occupy the same space like that - the computer has trouble figuring out which is in front vs which is behind...

http://en.wikipedia.org/wiki/Z-fighting

MayaBug
03-01-2011, 03:28 PM
Ok, thanks for that. Would there be problems if , for instance, the vertical block was smaller and the polygons didn't occupy the same space? Obviously this Z-fighting wouldn't be a problem, but anything else?

The technique itself i.e. sticking one object inside another just appears dodgy to me but I've seen it said that it's ok to do in some video tutorial someplace. Maybe just for inanimate objects in the background (of games)?

Arrimus
03-02-2011, 12:23 PM
Yes, clipping, as you call it, is a valid technique and has been used in virtually all games since games began being 3D. It's primary purpose is to conserve polygons and verticies and make it easier for the game to run. Clipping can be used on all models except on parts of models that will be animated. For example, if you have a character that wears metal armor, don't clip on the limbs and parts that will bend, but you can use clipping for the helmet and armor pieces.

Here is the exact model of the AR2 from Half Life 2. This is what you see in first person view, sans textures. As you can see, there's clipping all over the place and it doesn't detract from the ingame model. Although, you can increase the polygon density in newer games as well as add normal maps. Sometimes you may need to avoid clipping to get the normal map looking good.

http://img196.imageshack.us/img196/1483/halflife2ar2topology.jpg (http://img196.imageshack.us/i/halflife2ar2topology.jpg/)

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