Thanks, I will work on testing some of this later when I am able.
When you turn an attachment into a mesh, where is this mesh vertex data stored?
If I were to export a set of lower resolution sprites into an atlas using the spine texture packer, then manually load the low res atlas in unity and assign it to the skeleton data (that was exported with the high res atlas), would this break any animations that rely on having mesh attachments?
JDBB When you turn an attachment into a mesh, where is this mesh vertex data stored?
What do you mean by "when you turn an attachment into a mesh", do you mean in the Spine Editor?
If you want to know whether mesh attachments directly store atlas texture coordinates: no, they store coords relative to the atlas region boundaries, and the atlas regions are defined by the atlas. Thus the atlas layout can be different and mesh attachments still work fine.
JDBB If I were to export a set of lower resolution sprites into an atlas using the spine texture packer, then manually load the low res atlas in unity and assign it to the skeleton data (that was exported with the high res atlas), would this break any animations that rely on having mesh attachments?
It does not break anything.
Hey, that answers my question, thanks.
I thought that I had it working by simply setting the 'atlas assets array' to be empty in the 'skeleton data asset'. This allowed the 'skeleton animation' object in my scene to load up fine with no atlases. Then I could load the desired atlas using addressables and assign it as you explained before.
This worked fine in the editor but failed in a build with the error: "Error reading skeleton JSON file for SkeletonData asset: skeleton_SkeletonData"
In the inspector for the 'skeleton data asset' it says "AtlasAsset array is empty. Skeleton's attachments will load without being mapped to images.". Which sounds like the exact behavior that I want and also sounds like having an empty atlas array is supported.
Does this suggest that the error that I'm getting in my build is actually a bug?
I have also seen references to 'RegionlessAttachmentLoader' in the spine source which also may suggest that having no atlas is supposed to work.
I tried to look into making a custom attachment loader but I couldn't see how to make the 'skeleton animation' object in my scene use my custom loader.
Sorry for so many questions, hopefully nearly there. Thanks again for all the help.
I have done some more work on this and everything seems to be working perfectly in both the build and in the editor, apart from the error that gets thrown when running the build (mentioned above).
As soon as I have a skeleton data asset with an empty atlas list then I get the error in the build.
I could just ignore the error but I would much rather fix it.
Glad to hear you've got a working solution already! Sorry for the troubles with the error message, we will check whether we can remove it safely. I'll get back to you here on the forum once we have a fix ready.
@JDBB Unfortunately we could not reproduce the issue, we received no error message when entering play mode or building with a scene which uses a skeleton with the SkeletonDataAsset's Atlas Assets size set to 0.
Which exact version of the spine-unity runtime (name of the unitypackage, also listed in Assets/Spine/version.txt or in the Package Manager) are you using?
The version of the Unity runtime is: 4.2.67
The C Sharp runtime is: 4.2.23
I am using Unity version: 2023.2.20
I did have the addressables and on demand loading packages installed at one point but have since removed them from the project.
I have just made a fresh project to test on and I am still seeing the error.
I can send you the project if you would like.
Steps that I took:
Hopefully I didn't miss anything.
In case it helps the full error is:
Error reading skeleton JSON file for SkeletonData asset: skeleton_SkeletonData
Error reading attachment: God_Ray_0, skin: default
at Spine.SkeletonJson.ReadSkeletonData (System.IO.TextReader reader) [0x013d8] in .\Library\PackageCache\com.esotericsoftware.spine.spine-csharp@568e5ef049\SkeletonJson.cs:369
at Spine.Unity.SkeletonDataAsset.ReadSkeletonData (System.String text, Spine.AttachmentLoader attachmentLoader, System.Single scale) [0x00017] in .\Library\PackageCache\com.esotericsoftware.spine.spine-unity@568e5ef049\Runtime\spine-unity\Asset Types\SkeletonDataAsset.cs:295
at Spine.Unity.SkeletonDataAsset.GetSkeletonData (System.Boolean quiet) [0x000c4] in .\Library\PackageCache\com.esotericsoftware.spine.spine-unity@568e5ef049\Runtime\spine-unity\Asset Types\SkeletonDataAsset.cs:194
UnityEngine.Debug:ExtractStackTraceNoAlloc (byte*,int,string)
UnityEngine.StackTraceUtility:ExtractStackTrace () (at C:/build/output/unity/unity/Runtime/Export/Scripting/StackTrace.cs:37)
UnityEngine.DebugLogHandler:Internal_Log (UnityEngine.LogType,UnityEngine.LogOption,string,UnityEngine.Object)
UnityEngine.DebugLogHandler:LogFormat (UnityEngine.LogType,UnityEngine.Object,string,object[])
UnityEngine.Logger:Log (UnityEngine.LogType,object,UnityEngine.Object)
UnityEngine.Debug:LogError (object,UnityEngine.Object)
Spine.Unity.SkeletonDataAsset:GetSkeletonData (bool) (at ./Library/PackageCache/com.esotericsoftware.spine.spine-unity@568e5ef049/Runtime/spine-unity/Asset Types/SkeletonDataAsset.cs:197)
Spine.Unity.SkeletonRenderer:Initialize (bool,bool) (at ./Library/PackageCache/com.esotericsoftware.spine.spine-unity@568e5ef049/Runtime/spine-unity/Components/SkeletonRenderer.cs:476)
Spine.Unity.SkeletonAnimation:Initialize (bool,bool) (at ./Library/PackageCache/com.esotericsoftware.spine.spine-unity@568e5ef049/Runtime/spine-unity/Components/SkeletonAnimation.cs:189)
Spine.Unity.SkeletonRenderer:Awake () (at ./Library/PackageCache/com.esotericsoftware.spine.spine-unity@568e5ef049/Runtime/spine-unity/Components/SkeletonRenderer.cs:408)
(Filename: ./Library/PackageCache/com.esotericsoftware.spine.spine-unity@568e5ef049/Runtime/spine-unity/Asset Types/SkeletonDataAsset.cs Line: 197)
I just realised that there may be a misunderstanding.
The error shows when I run the build and the first scene loads, not when entering play mode in the editor or when I build the build.
JDBB I just realised that there may be a misunderstanding.
The error shows when I run the build and the first scene loads, not when entering play mode in the editor or when I build the build.
Thanks for the clarification, I have indeed overlooked this important part!
(Your reproduction steps list above is also missing "7. Run the built executable and observe the error message in the console" 
).
We've just improved the error message that is given in this case:
Shader "Spine/Special/HiddenPass" not found while loading SkeletonDataAsset" with 0 Atlas Assets. Please add this shader to Project Settings - Graphics - Always Included Shaders, or make sure your SkeletonDataAssets all have an AtlasAsset assigned.
So the issue was just the missing shader which was not found in the built executable which is found in the Unity Editor in play mode.
A new spine-unity 4.2 unitypackage is available for download:
https://esotericsoftware.com/spine-unity-download
Issue ticket URL for later reference:
EsotericSoftware/spine-runtimes2544
Yeah, my bad for sure. I should have been more clear. I did try editing my post to add the 7th step after I realised my mistake but it said that I don't have permissions.
Thanks, I will try adding the shader as suggested.
Looks like it is working great. Thanks so much for the help 
. It's very easy to forget to mention steps, especially those one considers trivial or "clear anyway".