While the Mesh Extract tool is a great tool in Zbrush, you are not limited to setting up the extraction in Zbrush alone.
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What you can do is take a copy of your original mesh in your original 3d program, subdivide it a few times then use booleans and other tools to get a more precise shape of what you want to extract.
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It doesn’t matter if you get polgons with more than 4 points. Just triple these.
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Now, just import these extracted parts into ZB3 and use the mesh extraction tool without masking or hiding.
The reason why I am explaining this option to you is that sometimes, when you are masking your mesh in Zbrush, you are restricted to the polygon flow of your object. Even if you subdivided your object many times, you can still get irregular bumps around the edges of your extracted mesh. For example, if you have a human figure model and you want to extract a wrist band shape, you may get irregular bumps because of the edgeflow of the muscles. By actually making a crude, manually cut mesh in your native 3d program for ZB to extract, you define the exact edges of the extraction.
However, mesh extraction can’t handle sharp corners so whatever you plan to
extract, make sure it has decently curved corners (IE not too sharp).
If you want to do a test, I suggest you import a Poser model into ZB3 and try to extract a writband or something.
Oh, and an extra little tip. You don’t need to use the mask tool for mesh extraction directly. You can import a black and white texture, convert it into a mask then extract the shape from that.
I hope this has been insightful.
Cheers,
Revanto
PS: If you want some images to get a better idea of what I mean, please let me know and I will make some and post them up.