Shader adding a mask over another texture, modifed Kagamma example

model1.x3dv

#X3D V4.0 utf8

PROFILE Interchange

Shape {
  appearance Appearance {
    material UnlitMaterial {
    }
    texture ImageTexture {
      url "texture.png"
      textureProperties TextureProperties {
        magnificationFilter "NEAREST_PIXEL"
        minificationFilter "NEAREST_PIXEL"
      }
    }
    effects Effect {
      language "GLSL"
      initializeOnly SFNode mask_texture ImageTexture {
        url "mask.png"
        textureProperties TextureProperties {
          magnificationFilter "NEAREST_PIXEL"
          minificationFilter "NEAREST_PIXEL"
        }
      }
      parts EffectPart {
        type "FRAGMENT"
        url "data:text/plain,
        uniform sampler2D mask_texture;

        void PLUG_texture_color(inout vec4 texture_color, const in sampler2D texture_in, const in vec4 tex_coord) {
          float alpha = texture2D(mask_texture, tex_coord.st).r; // r=g=b so pick one...
          texture_color = vec4(texture_color.xyz, alpha);
        }"
      }
    }
  }
  geometry IndexedFaceSet {
    solid FALSE
    coord Coordinate { point [ -0.5 -0.5 0, 0.5 -0.5 0, 0.5 0.5 0, -0.5 0.5 0 ] }
    texCoord TextureCoordinate { point [ 0 0, 1 0, 1 1, 0 1 ] }
    coordIndex [ 0 1 2 3 ]
  }
}