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I have a mesh (a rock wall) that I would like to split into pieces and export each piece for a game engine. The mesh has a smooth, undulating surface and when all the sections are merged into a single mesh, it looks the way I want it to. However, when I split the mesh into pieces, Blender recalculates the normals along the edges causing a very visible change in shading.

I would like Bender to leave the normals alone, or maybe give me some way to edit the normals manually. Can I do this?

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    $\begingroup$ You need to use custom normals. It's the only way to preserve the smooth shading on the edges you split. $\endgroup$ Commented Dec 10, 2015 at 11:50
  • $\begingroup$ How do you use custom normals? I tired adding a normal edit modifier to my mesh. Not only does it automatically recalculate my normals to cause a 'radial' effect, but if I split my mesh it still recalculates again along the new seam. $\endgroup$
    – kitfox
    Commented Dec 11, 2015 at 9:44

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I created a Blender add-on for this exact purpose called TiNA which allows you to easily transfer normals between objects. TiNA basically turns the somewhat overwhelming Data Transfer Modifier into a set of more comprehensible operations.
There's a special branch for those using Blender 2.80 beta.

TiNA|

 Hotkey           | Operation 
------------------|-------------------------------------------------------------------
 Alt+N            | Transfer Normals from selection to active object
 Shift+Alt+N      | Transfer Normals from active object to all other selected objects
 Ctrl+Shift+Alt+N | Clear Custom Normals data for entire selection
 Alt+W            | Wrap Normals

In your case you could transfer the normals from an unsplit duplicate by selecting all objects, and - with the unsplit duplicate as your active object - hitting Shift+Alt+N.

Example results

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    $\begingroup$ maybe you need to detail the feature, usage, effect, or other information...?? $\endgroup$ Commented Feb 27, 2019 at 6:53

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