Shadow Rendering with ShaderMaterial and Alpha Maps

When using a ShaderMaterial with alpha maps, you might encounter issues with shadows not respecting the alpha transparency. This is a common challenge in Three.js.

The Issue

Your ShaderMaterial with an alpha map is casting rectangular shadows instead of shadows that match the visible shape defined by the alpha map. This happens because the shadow mapping system doesn't automatically use the alpha information from your shader.

Solution

To make shadows respect your alpha map in a ShaderMaterial, you need to:

1. Enable alpha testing in your shader by adding a custom alphaTest value
2. Make sure your shadow map is properly configured

Implementing Alpha Testing in ShaderMaterial

In your fragment shader, you need to implement alpha testing manually:

GLSLCopied
// In your fragment shader
uniform sampler2D alphaMap;
uniform float alphaTest;

void main() {
  // Your existing shader code
  vec4 diffuseColor = /* your color calculation */;
  
  // Apply alpha map
  float alpha = texture2D(alphaMap, vUv).g; // Using green channel as per Three.js convention
  diffuseColor.a *= alpha;
  
  // Alpha test (discard fragments below threshold)
  if (diffuseColor.a < alphaTest) discard;
  
  gl_FragColor = diffuseColor;
}

Setting Up Your Renderer

Make sure your renderer has shadow mapping enabled and configured:

JAVASCRIPTCopied
// Configure renderer
renderer.shadowMap.enabled = true;
renderer.shadowMap.type = THREE.PCFSoftShadowMap; // Or another type based on your needs

Material Configuration

When creating your ShaderMaterial, pass the alphaMap and set appropriate uniforms:

JAVASCRIPTCopied
const material = new THREE.ShaderMaterial({
  uniforms: {
    alphaMap: { value: alphaMapTexture },
    alphaTest: { value: 0.5 }, // Adjust threshold as needed
    // Your other uniforms
  },
  vertexShader: vertexShaderCode,
  fragmentShader: fragmentShaderCode,
  transparent: true
});

This approach should help your shadows respect the alpha map's shape rather than casting a rectangular shadow.

If you're still having issues, you might need to adjust your shadow map resolution or consider using a different shadow map type for better quality.