Hello!
Creating an inner glow effect on a Spine character is definitely possible, though it requires some shader work since there's no built-in feature for this specific effect. Let me explain the approaches you can take:
Understanding the Challenge
You're correct that the main issue is that Spine's alpha channel is typically binary (opaque or transparent), which doesn't provide the gradient information needed for an inner glow falloff. The included Spine Outline shaders work by sampling surrounding pixels to detect edges, but they're designed more like a stroke effect rather than a smooth inner glow with gradient falloff.
Recommended Approaches
1. Distance Field Texture Approach (Most Efficient)
The most performance-friendly solution is to create an additional distance field texture that encodes the distance from each pixel to the nearest edge:
- Generate this texture in an image editor (like Photoshop using an inner glow/bevel effect) or programmatically in Unity
- You could encode this information in an unused channel (like the alpha channel of a separate texture, or even in your main texture if you have room)
- In your shader, sample this distance field to drive your glow intensity
This approach is mentioned in the context as a cost-effective alternative to runtime calculations.
2. Multi-Pass Sampling in Shader (More Expensive)
You could modify your shader to sample surrounding pixels in multiple iterations with increasing distance:
// Pseudo-code concept
float GetDistanceToEdge(float2 uv) {
float maxDistance = _GlowWidth; // in pixels
int samples = 8; // or more for smoother results
for (int i = 1; i <= samples; i++) {
// Sample surrounding pixels at distance i
// If you hit transparency, return distance
}
return maxDistance;
}
Warning: This approach requires multiple texture samples per pixel and can be costly, especially with many characters on screen. The relevant shader code for single-pass outline detection can be found at Spine-Outline-Common.cginc, which you could extend for multiple iterations.
3. Normal Maps with Lighting Trick
If you want to create a rim-light style inner glow, you could:
- Enable
Advanced - Add Normals and Advanced - Solve Tangents at your component
- Use normal maps where normals face outward at edges
- In your shader, detect edge pixels by checking if the normal's outward component exceeds a threshold
- Apply your glow color based on this detection
This is similar to the gradient rim lighting approach mentioned in the context.
Practical Implementation Tips
For URP 2D setup:
- Start with a copy of your
Universal Render Pipeline/2D/Spine/Skeleton Lit shader
- Add a property for glow color and intensity
- Implement one of the approaches above
- Remember these Spine shader requirements:
- Keep
Cull Off (no backface culling)
- Consider whether you need PMA vertex colors enabled
- Test with your texture's alpha export settings (PMA vs Straight)
Quick prototype:
For a simple hit flash effect, you might not need a true distance-based inner glow. Consider using:
- Tint/additive color overlay on the entire character (simpler and very common in games)
- Scale pulse combined with color change
- Vertex color animation for a flash effect
These are often sufficient for hit feedback and much more performance-friendly than complex shader effects.
Additional Resources
You can find shader examples and discussions in the forum about custom effects. The Spine/Sprite/Vertex Lit shader (found in Spine Examples/Other Examples/Sprite Shaders) shows sophisticated lighting techniques you might be able to adapt.
If you need further assistance with shader code or want to discuss your specific implementation, feel free to ask!
