code: package org.sunflow.core.shader;
  
  import org.sunflow.SunflowAPI;
  import org.sunflow.core.ParameterList;
  import org.sunflow.core.Ray;
  import org.sunflow.core.Shader;
  import org.sunflow.core.ShadingState;
  import org.sunflow.image.Color;
  import org.sunflow.math.OrthoNormalBasis;
  import org.sunflow.math.Vector3;
  
  public class QuickGrayShader implements Shader {
      public QuickGrayShader() {
      }
  
      public boolean update(ParameterList pl, SunflowAPI api) {
          return true;
      }
  
      public Color getRadiance(ShadingState state) {
          if (state.getNormal() == null) {
              // if this shader has been applied to an infinite instance because
              // of shader overrides
              // run the default shader, otherwise, just shade black
              return state.getShader() != this ? state.getShader().getRadiance(state) : Color.BLACK;
          }
          // make sure we are on the right side of the material
          state.faceforward();
          // setup lighting
          state.initLightSamples();
          state.initCausticSamples();
          return state.diffuse(Color.GRAY);
      }
  
      public void scatterPhoton(ShadingState state, Color power) {
          Color diffuse;
          // make sure we are on the right side of the material
          if (Vector3.dot(state.getNormal(), state.getRay().getDirection()) > 0.0) {
              state.getNormal().negate();
              state.getGeoNormal().negate();
          }
          diffuse = Color.GRAY;
          state.storePhoton(state.getRay().getDirection(), power, diffuse);
          float avg = diffuse.getAverage();
          double rnd = state.getRandom(0, 0, 1);
          if (rnd < avg) {
              // photon is scattered
              power.mul(diffuse).mul(1.0f / avg);
              OrthoNormalBasis onb = state.getBasis();
              double u = 2 * Math.PI * rnd / avg;
              double v = state.getRandom(0, 1, 1);
              float s = (float) Math.sqrt(v);
              float s1 = (float) Math.sqrt(1.0 - v);
              Vector3 w = new Vector3((float) Math.cos(u) * s, (float) Math.sin(u) * s, s1);
              w = onb.transform(w, new Vector3());
              state.traceDiffusePhoton(new Ray(state.getPoint(), w), power);
          }
      }
  }