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diff --git a/hw6/src/compoundShader.cpp b/hw6/src/compoundShader.cpp
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+/******************************************************************/
+/* This file is part of the homework assignments for CSCI-427/527 */
+/* at The College of William & Mary and authored by Pieter Peers. */
+/* No part of this file, whether altered or in original form, can */
+/* be distributed or used outside the context of CSCI-427/527     */
+/* without consent of either the College of William & Mary or     */
+/* Pieter Peers.                                                  */
+/******************************************************************/
+
+#include <algorithm>
+#include "compoundShader.h"
+#include "random_number.h"
+#include <iostream> // DEBUG
+
+//////////////////
+// Constructors //
+//////////////////
+compoundShader::compoundShader(const std::vector<std::shared_ptr<const shader_base>>& shader_list)
+  : shader_base()
+{
+  _compound = shader_list;
+}
+
+
+/////////////
+// Methods //
+/////////////
+color compoundShader::shade(const intersectionPoint& ip, const vec3d& light_dir) const
+{
+  color result(0.0f, 0.0f, 0.0f);
+  for_each(_compound.begin(), _compound.end(), [&](const std::shared_ptr<const shader_base>& shader)
+  {
+    result += shader->shade(ip, light_dir);
+  });
+
+  // Done.
+  return result;
+}
+
+
+color compoundShader::reflectance(const intersectionPoint& ip, const vec3d& light_dir) const
+{
+  color result(0.0f, 0.0f, 0.0f);
+  for_each(_compound.begin(), _compound.end(), [&](const std::shared_ptr<const shader_base>& shader)
+  {
+    result += shader->reflectance(ip, light_dir);
+  });
+
+  // Done.
+  return result;
+}
+
+
+brdfSample compoundShader::sample(const intersectionPoint& ip, float r1, float r2) const
+{
+  // santiy check
+  if(_compound.empty()) return brdfSample();
+
+  // initialize
+  auto currentCompound = _compound.begin();
+  float pdf = 1.0f;
+
+  // HW6: implement importance sampling of BRDF, where you select
+  //      a BRDF propertional to their relative reflectivity.
+  //      For example, if the compound shader has two BRDFs with
+  //      an reflectivity of 20% and 40%, then the likelihood that
+  //      the second BRDF is sampled is twice that of the first BRDF.
+  //      Practically, you need to point the 'currentCompound' to the BRDF
+  //      you want sample, and set 'pdf' to the PDF of the selection.
+  // Modifies: nothing
+  // Returns: brdfSample
+
+  float cdf = 0.0f;
+  float choice = random_float(1.0f) * reflectivity(ip);
+
+  // compute cdf until it exceeds choice probability and select that compound
+  while (currentCompound < _compound.end() && choice >= cdf) {
+    pdf = (**currentCompound).reflectivity(ip);
+    cdf += pdf;
+    currentCompound++;
+  }
+
+  // move currentCompound back to selection
+  currentCompound--;
+
+  // generate sample (do not remove -- this line will produce the correct
+  //  result if currentCompound and pdf are set correctly).
+  return (*currentCompound)->sample(ip, r1, r2) * pdf;
+}
+
+
+float compoundShader::reflectivity(const intersectionPoint& ip) const
+{
+  // return sum of reflectivity
+  float total_reflectivity = 0.0f;
+  for_each(_compound.begin(), _compound.end(), [&](const std::shared_ptr<const shader_base>& shader)
+  {
+    total_reflectivity += shader->reflectivity(ip);
+  });
+
+  // Done.
+  return total_reflectivity;
+}
+
+
+color compoundShader::emittance(const intersectionPoint& ip) const
+{
+  // return total sum of emittance
+  color total_emittance;
+  for_each(_compound.begin(), _compound.end(), [&](const std::shared_ptr<const shader_base>& shader)
+  {
+    total_emittance += shader->emittance(ip);
+  });
+
+  // Done.
+  return total_emittance;
+}
+
+
+shaderProperties compoundShader::properties(const intersectionPoint& ip) const
+{
+  // if any component has a property set, then set it for the compound
+  shaderProperties result(false, false);
+  for(unsigned int i=0; i < _compound.size(); i++)
+    result |= _compound[i]->properties(ip);
+
+  // Done.
+  return result;
+}
+
+
+void compoundShader::_print(std::ostream& s) const
+{
+  s << "Compound Shader (" << _compound.size() << " components): {";
+  for(unsigned int i=0; i < _compound.size(); i++)
+  {
+    s << *_compound[i];
+    if(i != _compound.size()-1) s << ", ";
+  }
+  s << "}";
+}