Hi everyone, is there anybody that can explain me how to do that?
The problem is: and I need to add a texture on the whole nif but the model is split in 3 different parts so the main idea was to merge all these parts in one and then... voilà the job is done, but I don't know how to do that or even if I can do that or if there is another way to apply the texture on the whole .nif without doing that.

ThyLastDay wrote:

I need to add a texture on the whole nif but the model is split in 3 different parts so the main idea was to merge all these parts in one and then... voilà the job is done

Are you sure you need to do it? As I see it every mesh has a uv-map that presets the layout of the texture and thereby tells you which area of the texture-image is to be applied. Those three meshes should still have their original uv-map regardless of whether they're joined or not, meaning: just apply the same texture to each mesh.
If that doesn't work, you can very easily join the meshes in Blender.

I do this a lot on my work. Is this for fo3/nv or fo4?

For fo3/nv
If the mesh uses the exact same texture. open the mesh up in nifskope and make sure all the mesh parts are on the same node. Delete any other nodes so only the three parts are on the file. Open up the bsshader properties and make sure the emmisive colors are all EXCATLY the the same for all three parts or the nif will never combine into one peice.

Import the mesh into blender and click one part then hold down shift and click the other two meshes, then hit ctl +j and accept. Export the file and load it into nifskope and too see if the mesh if one part.
If not, check all the color settings on the textures nodes, then need to all match. Also any nistencil poperties must be removed before joining the meshes together or they will never merge. Delete those those nodes if present. You can always readd it later if needed.

For FO4
Export your 3 meshes as obj files. Then import each one into blender and apply a texture to it. Export each part as ogj and save them as three seperate files. Once the three files have a texture assigned to them, import them all into blender and do the ctrl+j thing once you select them all and join them and export if as an obj.

Open up outfit studio and open the window and drag your obj into it. Export that as a nif. It should be one part. Then open up nifskope and click on the mesh and select copy block. Open up whatever mesh you want your new one to go in and paste it in. The part it wil be replacing should still be in nifskope. Assign that parts bsslightingproperty node to your mesh in the properties. There will also be a property right above that one that says invalid, click on hit and hit delete then enter.

Click on the original mesh part and remove the bsslightproperty from that mesh then remove that nif block. Hit the sanitize button up top and reorder the blocks then save and test until you got it where you need it.

Doing this save a ton of resources the game need to process. Ive seen so many authors recreate parts that all the take the same texture and have items or outfits with like 100 nodes when it could have been 4 or 5. Each node has it`s own texture info the game has to process so if you have an outfit with 20 bullets on it and each bullet is 2098x2098, the game has to load that texture 20x when they all could have been merged into one mesh so it would only have to be loaded once.