This is a old and known issue, from blender 3.3 they changed the way volumes are rendered, as a result the iray skin gets seams where different skin materials meet. This is because, with the new volume shader, we can have only one volume material per object, that wasn't a limitation before. So to avoid the issue we can use UDIM and make one single skin material. Or the alternative is to use the sss skin in the global settings, which doesn't use volumes, but that is approximated and needs to be fixed by hand most of the times, still it's a viable option.
Thomas did a huge amount of work on UDIM and collateral features in the 5.2 version. The main advantages in 5.2 are listed below as a quick overview. This list is limited to UDIM and doesn't include all the other bug fixes and improvements.
1. Merge geografts. We don't need anymore to specify "last udim" when we merge geografts, it is done automatically when needed. This means easy import works fine both with daz geografts and 3rd party geografts. Also a nasty bug is fixed were a countour seam were visible for geografts if exported at base resolution with sharp edges enabled.
2. Merge materials. Now works with geografts, while in previous versions the geografts and the main figure couldn't merge materials as they were treated as separate objetcs. DIMG groups are also fixed, as previously they were unnecessarily duplicated, also preventing to merge materials.
3. Resize textures. It is much more efficient and requires less memory, that's nice for large scenes or complex figures with lots of textures.
4. Bake LIEs. This new tool allows makeups and tatoos using LIEs to be included in the UDIM textures, that wasn't possible before.
5. Make UDIM and overwrite materials. While previously we had a single tool, the new tool is split in two. This allows to make udim textures for all the materials, if we want to. That's useful for example to paint textures in blender. That wasn't possible with the old tool. Then once we make udims we can overwrite the skin materials as before to make a single skin.
With all these bugs fixed and the new features we can now proceed. As example we take Victoria 9 with makeup and the daz genitalia, plus I added the Strix geografts for nipples and navel as we all know G9 doesn't provide geometry definition for them, apart the HD layer. However the steps are the same for any figure. Test scene provided "makeup-9-gen.duf". I assume the reader is familiar with the addon already so I'll not explain every simgle basic step, but rather focus on the new tools.
Beware renderotica is an adult site, so I'd not post the direct link here. Just search for "renderotica strix nipples" on google.
I don't seem to be able to upload files in the blog, other than images, so you can find the example scene on github. Of course you need the daz content installed for it to work.
1. Import the figure.
As a first step we import the figure with easy import as usual. We enable the BSDF materials and the iray skin in the global settings. Now if we render we see the seams where the skin materials meet, as expected. Depending on the specific skin and light conditions the seams can be more or less visible. Below it's Victoria 9 with the default HDRI, not horrible but still visible.
In the uv editor we see that the daz genitalia is placed in the last udim tile, while strix is merged to the body, also the materials are merged with the geografts. We can also see that all the textures are single images, not udim yet.
2. Save and resize textures then bake LIEs.
In blender UDIM need a special name, not compatible with the daz texture names, so we need to "save local textures" when making udims. Eventually we use "set resolution" in the materials panel, to reduce the texture size. The resolution level is the number of times the textures are halved. So for example with resolution 2 we go from 4K to 1K textures. All meshes will affect all the meshes in the scene, otherwise only the selected ones are affected.
Note that this, of course, will not affect the original textures in the daz library.
If the figure is using daz layered images aka LIEs it is mandatory to bake them, otherwise they can't be included in the UDIM textures. If we are unsure we can just run the tool that will bake if there's any, or just do nothing if there isn't.
After the tool is finished, this can be agood time to cleanup our scene, as we generated new textures and blender keeps the old unused data until we cleanup or close and reopen the scene. This is the usual behavior with blender, so we better get used to cleanup every while and then, especially after complex operations.
file > cleanup > purge unused data
3. Make UDIM and overwrite materials.
We now proceed to make udim textures. As a demonstration we make udim for all the materials, it isn't necessary to make a single skin, but it can be useful for painting in blender, and works fine to make a single skin anyway, so it's a win win.
It is essential to select the head material as active, as it includes the makeup and will be used as template. The "image size" is used to generate images to fill the udim, should any udim tile be missing in the daz textures. Then "combine texture types" will fit udims when a single texture is used with multiple color spaces, otherwise textures with different color spaces are duplicated, it is more a debug option can be always on.
In the image editor we now see the textures are all udims.
Again we can cleanup after making udims to purge memory from the old textures.
file > cleanup > purge unused data
Note. Making udims for all materials is complex, as the daz textures are not always compatible with UDIM, so the addon uses some heuristics to fit them. But the result is usually quite good. Nevertheless, if we encounter oddities in the UDIM textures, we can opt to only turn into UDIM the skin textures we're interested in. This works as the old tool and it is safer.
bug. Personally I get a bug for tiles 1005 1006 in the Victoria normal map, where extra tiles are generated but not filled. This bug Thomas can't reproduce it works fine on his system, so if you encounter the bug please let us know.
To fix this is easy though, we select the 1005 1006 tiles and "fill tile" with the neutral normal map color rgb(0.5,0.5,1.0), this is what the addon should do itself.
Now we can "overwrite materials", that means all the skin materials we select will be overwritten by the active material, again it is important to select the head as active to preserve the makeup. We can eventually exclude the fingernails if we want to keep them as a separate material.
We see the seams are gone as the skin is a single material.
