To achieve the "see-through" effect with Spine characters in Unity, you need to handle transparency and depth sorting carefully. Here are some steps and considerations based on the provided contexts:

1. Render to a RenderTexture: One effective method is to render your Spine character to a RenderTexture at full opacity and then use this texture in your scene with the desired transparency. This approach avoids the common issue where back parts of the skeleton show through when transparency is applied directly.

2. Use SkeletonRenderTexture: Spine provides a component called SkeletonRenderTexture which can be used to render the skeleton to a texture. You can then control the opacity of this texture. This method is demonstrated in the example scene RenderTexture FadeOut Transparency.

3. Shader Adjustments: Ensure you are using the correct shaders for URP. Spine provides URP-compatible shaders which you can find in the Spine URP Shaders UPM package. Make sure not to mix 2D and 3D URP shaders.

4. Depth and Transparency Settings: When dealing with transparency, you might need to adjust the depth settings. For instance, enabling ZWrite and setting the Render Queue to AlphaTest instead of Transparent can help with depth sorting issues.

5. Custom Shader: If the built-in solutions do not meet your needs, you might need to create a custom shader. This shader should handle transparency and depth correctly. You can start by modifying existing Spine shaders to suit your requirements.

Here is a summary of the steps you can follow:

1. Render to RenderTexture:

   • Use SkeletonRenderTexture to render your Spine character to a texture.
   • Apply this texture to a quad or another object in your scene.

2. Shader Adjustments:

   • Ensure you are using Spine's URP shaders.
   • Adjust the Render Queue to AlphaTest if necessary.

3. Example Scene:

   • Check the example scene RenderTexture FadeOut Transparency for a practical implementation.

For more detailed guidance, you can refer to the following resources:

• Spine Unity Rendering
• [Spine URP Shaders UPM Package](https://esotericsoftware.com/spine-unity#URP-Shaders

It is visible when the depth test is set to "Less Equal"

Then which depth test did you set to Less Equal? If it's behind, it should not render with Less Equal, so something in your setup or the description is wrong.

pixelmeat As it says there, the error appears when a PMA texture is used in the material but your Unity project's color space is set to Linear. If it is your intention to use the PMA texture, set the Color Space to Gamma in the Player - Other Settings in Project Settings:

If exporting the textures as PMA itself is an error, uncheck Premultiply alpha in the Texture Packer settings in the Spine Editor and check Bleed instead. You will then be able to export straight alpha textures.

pixelmeat Ah, sorry my explanation was not complete. If the texture and material settings are not set appropriately for the Straight alpha texture, the error will not be resolved. See this section of the documentation for the correct settings:
Correct Texture Packer export and Texture and Material import settings

pixelmeat

I haven't touched the PMA option and it was checked (Is this the default?)

Yes, the PMA option is enabled by default. This is because there are some advantages to using PMA textures.

• Using PMA textures can reduce draw calls when using the Additive blend mode for skeleton slots.
• Better results when creating mip maps.
  To understand this, watch the following video from 1:45-4:50:
  

@pixelmeat And note that the Universal Render Pipeline/Spine/Skeleton shader is for use with URP without a 2D Renderer, while Universal Render Pipeline/2D/Spine/Skeleton is for URP when using a 2D Renderer.

See the documentation here:
https://esotericsoftware.com/spine-unity-rendering#URP-Shaders-Extension-Package

pixelmeat Alright! Now I have configured the color space as Gamma and enabled PMA back. The warnings are gone and the character renders if I use the Spine/Skeleton shader.

Glad to hear you've decided to switch to Gamma color space! This will make many things behave more intuitively.

@pixelmeat Very glad to hear you've found the issue!

pixelmeat Just one more question for now. Are there versions of the outline shaders for the URP/2D category?

It's a bit more complicated currently, unfortunately there is no single-shader version available yet. This is planned in this issue ticket:
EsotericSoftware/spine-runtimes1824
However in the meantime you can use e.g. the RenderExistingMesh component and the Universal Render Pipeline/Spine/Outline/Skeleton-OutlineOnly shader as described here in the documentation.

You can find an outline URP example scene Outline Shaders URP.unity in the Samples of the Spine URP Shaders UPM package (installed via Package Manager under the Samples tab), there you can see this setup in action.

pixelmeat How can I make the outline object be mask-interactive?

By mask-interactive, do you mean the masks in the way of Inside-Mask Outside-Mask options of the Mask Interaction parameter of SkeletonAnimation? If so, that's the Stencil Comparison parameter (_StencilComp in the shader and in the Material properties under debug mode). Values are as follows (as names and parameter int-values):

• None - Inside and outside (Default): Always, 8
• Inside: LessEqual, 4
• Outside: Greater, 5

The integer values for each comparison operation can also be found under UnityEngine.Rendering.CompareFunction.