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Some times it's easier to add a quick sphere and move it so it intersects a part of an existing model rather than trying to connect the existing model to the sphere.

When are these kinds of shortcuts acceptable (or not) for good topology?

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This technique has lots of advantages, the biggest of which is of course speed of modeling. There are also some drawbacks you might want to consider:

  • Stacking is troublesome if you need bevelling. The intersections will stay sharp, no matter what.

bevelling

  • Freestyle won't produce the result you're after. It uses face boundaries to find edges and there are none.

Freestyle edges

  • In the past, Blender sometimes had trouble to precisely calculate the intersections, resulting in a jagged appearance. I haven't noticed that lately so it might be fine.

  • If you deform the surface, the object might move relative to one another, resulting in a shift of the edge. Or in Z-fighting.

  • Transparent materials will show that you tried to cheat. Nothing against cheating in 3D as long as you can get away with it. The left image is how it should look, the right is how it looks if it's stacked. This effect gets worse when you rotate around the object.

Transparency

  • If you don't use it for joints or movable parts but just for speeding up the modeling, and you don't trim the parts inside the model, it always results in unnecessary geometry. Depends on the case what's more important. But even with trimming, it's still faster than manually connecting the detail or cleaning up boolean artefacts.

  • Adding small details has the advantage of not distorting the big geometry which can be quite time consuming to repair or plan in advance.

  • Tools like the Loop Cut will ignore the detail when you work on the main mesh.

  • It affects your UV mapping workflow.

  • One of the biggest factors still is screen size. Extreme closeups will reveal possible problems. If it's just a small detail, you can employ a lot of simplification techniques, saving time.

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  • $\begingroup$ Thanks for your answer :) What about any pros/cons for adding medium/small details in hard-surface modeling? $\endgroup$ – gandalf3 Oct 12 '13 at 1:44
  • $\begingroup$ Re: In the past, Blender sometimes had trouble to precisely calculate the intersections, Some graphics cards have bad depth precision, Or you may have clip-near/far set to large, irrespective - this is a general precision issues and not really Blender specific. $\endgroup$ – ideasman42 Oct 12 '13 at 3:12
  • $\begingroup$ In the viewport, yes. But it also happened in rendering, if I'm not totally mistaken, I think when the camera settings for clipping were off. And this seems to have been fixed. $\endgroup$ – Haunt_House Oct 12 '13 at 3:33
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Some reasons, off the top of my head:

  • Style: Maybe you are going for an early Pixar from the 90's look, with some primitives slapped together, I'm not gonna judge.
  • Full rotational potential: If you are building, say, a robot with ball and socket joints, it would make sense not to connect the meshes. There is a chance any part of the sphere is visible, so you do not want to go and connect the two at any point.
  • Interchangeability: Dota 2, the game, has characters that are made up of many different meshes, so the game can swap out parts at will. Similar things may need to happen if you are making a game or something that another program needs to modify. Guns and Characters will not be connected, as a more broad example.
  • Speed: Sometimes it is just plain faster for you to model and rig and animate if the meshes don't need to connect at any point. Hair, spikes, that sort of thing are legitimate reasons.
  • Modifier dodging: Sometimes you need parts of a mesh that will not be affected by an object-wide mesh modifier. Splitting the item away and parenting it in place is a quick and easy solution (I realize that there are vertex weights, but that is imperfect and on a modifier-by-modifier basis).
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  • $\begingroup$ Thanks for your answer :) What about any pros/cons for adding medium/small details in hard-surface modeling? $\endgroup$ – gandalf3 Oct 12 '13 at 1:43
  • $\begingroup$ From Experience, it can gets very unwieldy after a while. There is overhead for each stand-alone object, and after about a hundred or so, my computer starts to get really slow, but YMMV. Just keep in the back of your mind that you will need to keep it all organized. $\endgroup$ – Kirbinator Oct 13 '13 at 18:02
  • $\begingroup$ If you make each part into a separate object, then yes. But e.g. making a car with separate parts for the different panels (hood, doors, roof, etc.) you may want to keep them in the same object in some cases. (BTW, sweet model :) ) $\endgroup$ – gandalf3 Nov 4 '13 at 19:53

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