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Separating geometry or keep it as one?

polycounter lvl 11
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Tobbo polycounter lvl 11
How do you know when to keep geometry separate and when to connect it to each other, even when you know it's going to have to be done by adding additional triangles?

I'm working on a medieval lantern. I have been separating certain parts based on how it would be in real life. E.g. the candle and the chain holding the lantern up is completely separate from the lantern.

But then I got to thinking, why don't artists do it for other parts as well. E.g. Making the top of the lantern be completely separate geometry from the bottom part and so on? Why do they insist on keeping it as one?

In order for it all to be connected to each other I would have to do so by adding additional triangles.

If there's really no disadvantage for not having all the pieces be a single whole, why do other artists go through the trouble of making it all connected by adding additional triangles?

Thoughts?

Replies

  • Psyk0
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    Psyk0 polycounter lvl 18
    For a high poly i prefer to build into separate pieces it's faster and it mimics how it's built in real life, no one cares how it's put together as long as it looks good.

    For low poly meant for normal maps, it's more convenient to keep it as one because it's easier to unwrap, doesn't waste space on hidden parts and most importantly, it keeps the baking cage conformed to your model. The only time i want to have separate geo is when moving parts are involved.
  • Mark Dygert
    It's really situational question. There isn't one extreme rule governing everything, "always join everything". But just some situtaions where it makes sense.

    If the mesh is going to be deforming then having two things clip into each other doesn't offer any real way to keep one piece anchored on the surface of the other.
    SpikesRigging.jpg
    But if they are joined then you can control the points of contact and move them together.

    If one mesh is going to float over another and they are going to deform then they need to share similar topology.
    StrapTopo.gif
    If they share similar topology then you might as well join the two pieces together and delete the faces that aren't seen. This will also prevent any gaps or slips between the two pieces.

    For example if you have a cylinder going through a box and they aren't joined, you have a lot of polygons that are hidden inside the box, making large sections of your unwrap wasted space. The point where they interest won't capture correctly, but if you join them together the cage will be smooth and capture correctly.

    The dark band in the image below is wasted space on the cylinder. In order to maintain a consistent pixel density on the pieces of the cylinder that are visible you need to waste a bunch of space. You could add edges and remove that dark spot but then you might as well cut the box and join them together, making it a much better cage.

    NonContiguous02.jpg
    This will also capture very very poorly if there was a normal map, which is HUGE if you need a smooth transition there, or if you had weld marks and didn't want a seam.

    Often contiguous pieces are easier to unwrap and easier to pack.
    NonContiguous01.jpg
  • Tobbo
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    Tobbo polycounter lvl 11
    Thanks for the responses Psyk0 and Mark.

    @Mark Thanks for taking the time to try and clarify it further. It helped.
  • EarthQuake
    Great post Mark.

    One more point: With contiguous/"water tight" meshes you'll get less aliasing than you will with intersecting meshes. I try to use contiguous meshes as much as possible for this + all the reasons Mark pointed out.

    Basically:
    A. If needs to animate(ie: the bolt on a rifle) I use a separate mesh chunk
    B. If it needs to be removed (ie: swappable parts) I use a separate mesh chunk
    C. If its complex overlapping geometry* that will cause issues when baking I use a separate mesh chunk (and explode bake it)

    But for pretty much everything else I try to keep it contiguous.

    * You can often reduce overlapping bake issues a great deal by merging your meshes together, where it is logical to do so at least.
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