Hello,

I am trying to learn how to make some CAS parts and I followed a few tutorials here and there to do so.
If anything I have understood sounds wrong, please don't hesitate to correct me.

In particular I'd like to do multi-part meshes, because as far as I know it's the only way I can apply multiple shaders to the same CAS part, like transparent or shiny stuff (actual reflective material, not baked textures with heavy static patches of white), along with a normal SimSkin stuff part.
I seem to understand it's kind of a hack to begin with, because EA apparently just uses multipart to split meshes with too many bones, leading to part _1 just getting a few fingers or whatever.

I have Blender 2.93 with GEOM tools 2.1.4, CTU, S3PE, Mesh toolkit (for now only used to convert existing morphs since GEOM tools needs them converted to mesh).

I am at the point that I can split an existing mesh in two parts and export them as GEOMs, then inject them through CTU into an already multi-part thing, like the afBodyApron.
I gathered that to do so correctly I needed the two parts to have sequential vertex IDs, so in Blender I made them so by recalculating from 5000 for LOD1_0 and from the last ID + 1 for LOD1_1.
Then in S3PE I changed the shader of one of the parts to what I found on other similar pieces (like 0x1C608236, which S3PE doesn't seem to recognize and I couldn't find referenced there but seems to be code for "Ooo, shiny").

It's looking like it works as far as materials are concerned, but now there's the part where morphs are involved. As far as I know even a multipart CASP only has one set of morphs common to all parts, and I couldn't get that to export in Blender.

Also even if a previous full mesh morph that I could import exists, I assume it would mess it up completely (since it uses vertex IDs to apply the transformation and those have changed).

So now for a few questions :

1. would it be possible to align morph IDs with changed mesh IDs somehow, to keep an existing morph?
2. can GEOM tools export a new morph with a multi-part mesh? Or would I need some way/tool to stitch two separate morphs into one, or something?
3. is there a cleaner/simpler way to "add a part" to an existing package, instead of replacing stuff into one of the few EA CAS parts that already have them?

Thank you for any insight.

This is actually exactly how hat hair works, in order for the hair to use the SimHair shader and the accessory or hat to use SimSkin. It is more of a beast to work with than one group for sure, but in some ways it is more straightforward than some of your conclusions. Which is good news.

Hairs and clothing have some discrepancies, so excuse any I might mention as I relay what I've come to understand after fighting with hat-hair for months.

I use CTU for presets and texture replacement for hat-hairs, but I use S3OC + S3PE for the initial cloning and mesh swap. Many more important parts are included with a full clone rather than the way CTU makes its packages, and it throws less errors about meshes to just right click the GEOMs and 'Replace...' them.
Download Delphy's GEOM editor to modify the shaders on the .geom files directly if you want to skip digging through S3PE for that.

I've never had to have sequential IDs, but again this could be a hair peculiarity specifically. All that I do on the Blender side is make sure I copy data from the same type of piece- LOD1 to LOD1, LOD_1 to LOD_1. (This is a given if you Replace GEOMs in S3PE though, because you need to use all of the clone GEOMs as your Transfer GEOM Data reference for your new pieces to have the correct data.)

1. With some very particular renumbering, taking into account things like vertex IDs starting at 0... it's possible you could find a way to split a morph but it would be a little time and math intensive to lock down a method that works 100%. It's likely more straightforward just to use MeshToolKit to auto-morph both pieces using the old morphs as reference- it should come out more or less identical if you didn't actually change the geometry.
2. EA items usually use GEOM based morphs instead of BGEOs for multiple mesh groups, but you can use either method with MeshToolKit.
   Under Package Tools > Add Morphs to Clothing/Hair.
   These are the settings to add a morph GEOM:
   
   Just untick Add as Morph Meshes if you want to add them as BGEO. You will see that there are inputs for the original and the _1 mesh.
   I don't actually know if there is a benefit to one over the other. Try both
3. The hardest part to keep functional is actually the presets- these have some unique information for CASPs with multiple mesh groups, which is why you need to use a reference that already has these parts in place. It's possible to get an item to recognise more mesh groups on the VPXY level, but I have yet to crack getting those pieces to be textured properly because of complicated references in the presets. Much easier to just find an object already functioning in this way and clone that same one each time.

Doing all of this manually is quite new territory, so if in doubt there is no harm testing theories and digging into stuff until you figure out how it is or isn't working

Thank you very much, there are quite a few missing pieces of the puzzle there, and I am making progress

In particular Mesh Toolkit's function to create a morph from a reference. It did create a functional fat morph, albeit a slightly distorted one in parts.

At this point this may be the result of tinkering too much with the mesh. I'll start experimenting a bit from a cleaner base.

I'm still laughing! You have downloaded my FemmeBots? Or liquid latex outfit, perhaps??? @CardinalSims uses different tools than I, so you get a couple of perspectives here. First, the full shader is 0x1C608236 (476086838) . It is the Plumbot shader, and is not listed in Geom Editor for some reason. At least, I did not see it when I looked.

I generally use TSRW, MTK, Blender 2.80 and s3pe. I have not used CTU in years, so cannot guide you thru that program, and only use S3OC for referencing default replacements. But, the process is relatively simple. For Sim garments, I will use either afBodyDressCocktail, which is 2 groups L1, and 1 group each for L2 and L3. I often do not care so much about detail in L2 and L3, but if you want 2 groups in all LODs, afBodyDressTight_halter or afBodyDressTank_ruffles will work. More work when replacing images in mesh tab.

But, to your issue... MTK does not morph .wsos by vertex ID, it morphs by proximity, which is why custom mesh morphs often need to be "fixed"... which is a whole different tutorial.

If you are re-splitting basic EA meshes, THAT is easy! I work with both .obj and geom. It requires extra steps, but I like the results. I will make my meshes and export as .obj first. Objects export a cleaner mesh. Export each group separately, as groups. Join the meshes, split seams, etc and export as geom. This is your bone reference. Convert objects to geom, add bones using the reference, and fix seams in MTK. Then convert to .wso. In TSRW, export the original .wsos with morphs. In MTK> TSRW tools> FrankenMesh, put the .wsos together. Do not worry if "too many bones" warning, as it is only for morphs. Use the combined .wso as morph reference for both groups, and import them to TSRW.

This part might not be necessary... Replace texture images as necessary, and do not forget the ones in Mesh Tab! Use Browse, and reuse textures already imported in the Textures Tab, so you do not bloat the package. Be sure to replace the normals map.

Export as package. Open in S3PE and replace shader. Close package. NEW TSRW... Import your package that now has your shaders. Save the work file, and create a new package. Now, your work with the new shaders is part of the TSRW work file, if you ever need to go back to make changes.

Somewhere, here on MTS, is a tutorial by BloomBase, on how to actually add a 3rd mesh group. I have used this a couple of times, although I do not use CTU as the tutorial does.

This is a 3 group mesh that I did a couple of years ago. It is a FemmeBot, so is put together as combination of Sim and Plumbot. The mesh uses 2 different shaders, and more than 60 bones split between meshes. Bots do not use morphs, but it would be easy enough to add them.

Actually, err... yeah, I did
And I may have done some exploration on those. For Science, you know.

Thank you for explaining your process too, and the info on the 3rd group (though I am not sure I am ready for stuff that complex yet ). I have not tried TSRW yet, I'll have to see if this makes the whole thing simpler later.
In the meantime I did manage to fix the morphs eventually, starting over from a clean S3OC clone instead of a CTU-made package must have helped. No distortion anymore

There is just one weird bug that is probably due to some irregularity... The first (and only) design I've made works perfectly when I select it (I can CASt it no problem too), but when in Nraas compact CAS mode if I first click on the part in CAS instead it shows up as an erroneous "custom" design with a plain blue-ish colour covering the whole body. I have seen CC from other people do that occasionally, not sure what's causing it, and possibly related to how Nraas hides designs. Not a huge problem for now anyway.

I was very lucky to find that 3rd group thing. I was stuck on how to do that, and yes, it is a bit advanced. Not something I would have tried when first starting out. Since then, I have done 4 groups... only for L1. That tutorial is on MTS, somewhere. I remain amazed at how much, people like BloomBase, Cmar, and so many others figured out so long ago that remain relevent today.

Anyway... Are you referring to thumbnails, or the actual garment? Did you clear caches? If custom thumbnails, you must make one for each preset. If it is the garment, there are a few things that would cause the CAS Sim to be a solid color. Most common for me is old textures, still attached. In TSRW, you must click to use a texture for all presets. This is for all textures, overlay, multiplier, specular, RGB or RGBA mask, etc. Another is reusing a CASP number in a new package, and textures have different ID than they had in the first package. Internal references get messed up, and may search to the basegame folder to use the original textures. @CardinalSims probably knows more about this, using s3oc.

I didn't make or import any custom thumbnail, it's generated, and I did delete the caches.

It's a bit weird, when in normal CAS view, there's only design #1 and it works as expected. It only happens when I toggle "Show in Compact Form: Clothing" in Nraas MasterController (the option that only loads one design per CASP). It also shows as design #1 but when I select it, it's that solid colour, and when I click on CASt it's set on a custom design (big blue * ). This option does weird thing even to EA parts for me, as in I click on a part and it loads a design that is not the first one, is identical to another but is considered a custom CASt design. So yeah, maybe my game's haunted or something.

I suspect you are right that I must have some broken texture link still and this is why it shows up like that though. I need to tinker a bit with it.

Game generated thumbnails are found in the Thumbnails folder. Delete the package for CAS. It will regenerate a new one... eventually, so do not panic when all thumbs are blank when you open CAS. Every thumb for every CC garment and EA CAS, and all presets are being remade... takes time.

But again, it almost sounds like in TSRW, when you do not "copy to all presets" (textures), or do not delete unused presets. And again, @CardinalSims would better know about things made using s3oc.

I actually had a thumbnail issue with the last hat-hair I was working on, but I wasn't sure if it was just something I specifically had done wrong. Thank you for explaining what it was doing in your game!
I think it's related to presets again and MasterController is seeing one that shouldn't be there.

I'll try a few things on my end and let you know if I narrow down what causes that. The TSRW equivalent is a good lead for me to look into how to fix it.

TSRW may definitely be easier in some regards for you to get started making things, especially in terms of the documentation being out there for it. You can get a little boxed in by its limitations, though.
The workflow centered around the GEOM Tools is newer territory, but I suppose you're in luck that it's territory I happen to be charting. Both have their upsides and downsides, so experiment and use the methods you prefer