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So I am playing around with UE4 to make some concept art for a game. I extracted the model I wanted to use from the game's files and imported it into blender. It's a tank with 4 parts: the hull, the suspension, the turret, and the cannon. The suspension and cannon work as intended, but there is something wrong with the way the hull and turret are rendered. Here I have linked 3 photos of what is happening: http://imgur.com/a/ErpeO

The first screenshot is what the model looks like when rendered in blender (internal and cycles both look like this).

The second screenshot is the model in blender's 3d preview. Looks completely normal.

The third screenshot is what the model looks like in UE4 with textures and normals applied. The outside of the hull and turret meshes are see-through, so only the inside faces on the backside of the mesh are visible.

I have already looked through google and can't find anything to help me. By default the normal direction for the faces (not the vertices) are flipped inside out. Also if I tick "backface culling" blender removes the front faces of the mesh instead of the back faces (should be the other way around). Which means somehow the faces of the 3d mesh are flipped inside out and I need help getting them the way they should be. I have already tried flipping as well as re-calculating the normals (with the "make normals consistent" box ticked) and neither fix the problem. Would really appreciate some help with this. Will upload the model if I have to, but it's not mine so it might be considered copyright infringement.

EDIT: I'm not sure if this matters, but the hull and turret need to be mirrored in blender to match up with the orientation of the cannon/suspension. By defualt they're flipped in the wrong direction. This doesn't seem like a coincidence.

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Double click on the Normal texture inside UE4 and expand the Texture rollout. Below the X-axis and Y-axis Tilting Method drop downs you'll see a check box for "Flip Green Channel". Tick this off and hit save. If this Doesn't fix the problem for you, then the issue isn't with your normal map but with the mesh normals. If this is the case then inside Blender you need to select the normals in question and use the Flip Direction button within the Shading rollout under the heading Normals.

https://blender.stackexchange.com/questions/29132/how-do-i-flip-normals-on-this-model

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