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Like the question says, can I manipulate normals explicitly?

The reason I'm asking is because I'm making a game and I want to make some objects that fit together with other objects as they are placed by the engine. For instance, I want to model the dug-up sand besides a hole and want it to transition naturally into the (flat) ground around it, or model a skirt for the character model and want it to fit together with that character's torso.

However, when doing that the "naïve" way, the normals are (of course) calculated as if the object existed in isolation, which means that they point in a different direction from the surface they're supposed to fit together with in the end, creating a sharp edge in the lighting of the compound object, which is a bit disturbing.

Is there any way I can just "set" the normals in a particular direction? Like, straight upwards for the normals on the outer vertices of the mentioned sand, or straight outwards for vertices along the upper edge of the mentioned skirt.

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    $\begingroup$ There is a custom split normals branch being worked on by Bastien Montagne(Mont29). The version I tried used a modifier which I gather is supposed to alter the normals of one object using another but I could not get any results at all so I did not bother to take it any further since I tend to use inset and control loops to alter normals so I can bake them. Anyways, if you feel like digging around some more, that Blender branch may be a solution for you. $\endgroup$ – MarcClintDion Oct 11 '14 at 10:18
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As @MarcClintDion said, this is not possible... Yet. There is a custom Split Normals branch being worked on by Bastien Montagne(Mont29) to try to incorporate this into Blender; However, it's not yet finished - Loop Normal Edition is the current sub-project under development.

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I am just solving the exact same problem also for my game and the best solution (yet not perfect) I could figure out is to manually select the vertices along the edge and then hit Alt + N and select Rotate..., then I manually rotated them a little bit so they face the desired direction and that made my game pieces smooth together perfectly.

It is not perfect though because it also rotates little bit the normals on the edge, as you will see. Not a huge deal though.

I yet haven't found a way how to affect only one normal and not all the normals coming from the vertex... Some add-on for 2.8 would be very welcome :)

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