diff --git a/include/spica.h b/include/spica.h
index ed0de2e..0e12695 100644
--- a/include/spica.h
+++ b/include/spica.h
@@ -9,6 +9,7 @@
 #include "../sources/core/engine.h"
 #include "../sources/core/path.h"
 #include "../sources/core/timer.h"
+#include "../sources/core/memory.h"
 #include "../sources/core/sampling.h"
 #include "../sources/core/spica_dirs.h"
 #include "../sources/core/point2d.h"
@@ -85,7 +86,7 @@
 #include "../sources/texture/spica_texture.h"
 
 // --------------------------------------------
-// Textures
+// Materials
 // --------------------------------------------
 
 #include "../sources/material/spica_material.h"
diff --git a/sources/bxdf/bsdf.cc b/sources/bxdf/bsdf.cc
index b068b07..e730504 100644
--- a/sources/bxdf/bsdf.cc
+++ b/sources/bxdf/bsdf.cc
@@ -7,6 +7,7 @@
 #include "../math/vect_math.h"
 
 #include "bxdf.h"
+#include "fresnel.h"
 
 namespace spica {
 
diff --git a/sources/bxdf/bsdf.h b/sources/bxdf/bsdf.h
index 240caf7..d3c78ff 100644
--- a/sources/bxdf/bsdf.h
+++ b/sources/bxdf/bsdf.h
@@ -16,7 +16,7 @@
 
 namespace spica {
 
-class BSDF {
+class SPICA_EXPORTS BSDF {
 public:
     // Public methods
     BSDF(const SurfaceInteraction& isect, double eta = 1.0);
diff --git a/sources/bxdf/bssrdf.h b/sources/bxdf/bssrdf.h
index 05c8978..38cf4de 100644
--- a/sources/bxdf/bssrdf.h
+++ b/sources/bxdf/bssrdf.h
@@ -54,8 +54,8 @@ class SPICA_EXPORTS SeparableBSSRDF : public BSSRDF {
 
 protected:
     // Protected methods
+    virtual Spectrum Sp(const SurfaceInteraction& pi) const;
     Spectrum Sw(const Vector3d& w) const;
-    Spectrum Sp(const SurfaceInteraction& pi) const;
     Spectrum sampleSp(const Scene& scene, double rand1, const Point2d& rand2,
                       MemoryArena& arena, SurfaceInteraction* pi,
                       double* pdf) const;
diff --git a/sources/bxdf/bxdf.h b/sources/bxdf/bxdf.h
index 8fb2829..7ca8f97 100644
--- a/sources/bxdf/bxdf.h
+++ b/sources/bxdf/bxdf.h
@@ -11,6 +11,7 @@
 #include "../core/forward_decl.h"
 #include "../core/uncopyable.h"
 #include "../core/spectrum.h"
+#include "fresnel.h"
 
 namespace spica {
 
@@ -45,9 +46,6 @@ class SPICA_EXPORTS BxDF : public Uncopyable {
     explicit BxDF(BxDFType type = BxDFType::None);
     virtual ~BxDF();
 
-    BxDF(const BxDF&) = default;
-    BxDF& operator=(const BxDF&) = default;
-
     virtual Spectrum f(const Vector3d& wo, const Vector3d& wi) const = 0;
     virtual Spectrum sample(const Vector3d& wo, Vector3d* wi,
                             const Point2d& rands, double* pdf,
@@ -64,7 +62,7 @@ class SPICA_EXPORTS BxDF : public Uncopyable {
 /**
  * Lambertian refrection.
  */
-class LambertianReflection : public BxDF {
+class SPICA_EXPORTS LambertianReflection : public BxDF {
 public:
     explicit LambertianReflection(const Spectrum& ref);
 
@@ -77,7 +75,7 @@ class LambertianReflection : public BxDF {
 /**
  * Lambertian transmission.
  */
-class LambertianTransmission : public BxDF {
+class SPICA_EXPORTS LambertianTransmission : public BxDF {
 public:
     explicit LambertianTransmission(const Spectrum& tr);
 
@@ -90,7 +88,7 @@ class LambertianTransmission : public BxDF {
 /**
  * Specular reflection (Metal-like effect).
  */
-class SpecularReflection : public BxDF {
+class SPICA_EXPORTS SpecularReflection : public BxDF {
 public:
     // Public methods
     SpecularReflection(const Spectrum& ref, Fresnel* fresnel);
@@ -109,7 +107,7 @@ class SpecularReflection : public BxDF {
 /**
  * Specular transmission.
  */
-class SpecularTransmission : public BxDF {
+class SPICA_EXPORTS SpecularTransmission : public BxDF {
 public:
     // Public methods
     SpecularTransmission(const Spectrum& tr, double etaA, double etaB);
@@ -130,7 +128,7 @@ class SpecularTransmission : public BxDF {
 /**
  * Fresnel specular refraction (Glass-like effect).
  */
-class FresnelSpecular : public BxDF {
+class SPICA_EXPORTS FresnelSpecular : public BxDF {
 public:
     // Public methods
     FresnelSpecular();
@@ -152,7 +150,7 @@ class FresnelSpecular : public BxDF {
 /**
  * Microfacet reflaction.
  */
-class MicrofacetReflection : public BxDF {
+class SPICA_EXPORTS MicrofacetReflection : public BxDF {
 public:
     MicrofacetReflection(const Spectrum& ref,
                          MicrofacetDistribution* distrib, Fresnel* fresnel);
@@ -170,7 +168,7 @@ class MicrofacetReflection : public BxDF {
 /**
  * Microfacet transmission.
  */
-class MicrofacetTransmission : public BxDF {
+class SPICA_EXPORTS MicrofacetTransmission : public BxDF {
 public:
     // Public methods
     MicrofacetTransmission(const Spectrum& tr,
diff --git a/sources/bxdf/fresnel.h b/sources/bxdf/fresnel.h
index 9f986cf..0c19924 100644
--- a/sources/bxdf/fresnel.h
+++ b/sources/bxdf/fresnel.h
@@ -5,6 +5,7 @@
 #ifndef _SPICA_FRESNEL_H_
 #define _SPICA_FRESNEL_H_
 
+#include "../core/common.h"
 #include "../core/spectrum.h"
 
 namespace spica {
@@ -12,7 +13,7 @@ namespace spica {
 /**
  * The inteface for Fresnel reflections.
  */
-class Fresnel {
+class SPICA_EXPORTS Fresnel {
 public:
     virtual ~Fresnel() {}
     virtual Spectrum evaluate(double cosThetaI) const = 0;
@@ -21,7 +22,7 @@ class Fresnel {
 /** 
  * Fresnel conductor class.
  */
-class FresnelConductor : public Fresnel {
+class SPICA_EXPORTS FresnelConductor : public Fresnel {
 public:
     FresnelConductor(const Spectrum& etaI, const Spectrum& etaT,
                      const Spectrum& k);
@@ -34,7 +35,7 @@ class FresnelConductor : public Fresnel {
 /**
  * Fresnel dielectric class.
  */
-class FresnelDielectric : public Fresnel {
+class SPICA_EXPORTS FresnelDielectric : public Fresnel {
 public:
     FresnelDielectric(double etaI, double etaT);
     Spectrum evaluate(double cosThetaI) const override;
@@ -46,7 +47,7 @@ class FresnelDielectric : public Fresnel {
 /**
  * Dammy Fresnel class which returns always one.
  */
-class FresnelNoOp : public Fresnel {
+class SPICA_EXPORTS FresnelNoOp : public Fresnel {
 public:
     FresnelNoOp() {}
     Spectrum evaluate(double cosThetaI) const override {
diff --git a/sources/core/interaction.cc b/sources/core/interaction.cc
index 58bed93..5c82678 100644
--- a/sources/core/interaction.cc
+++ b/sources/core/interaction.cc
@@ -5,6 +5,7 @@
 #include "../core/primitive.h"
 #include "../math/vector3d.h"
 #include "../bxdf/bsdf.h"
+#include "../bxdf/fresnel.h"
 #include "../light/area_light.h"
 
 namespace spica {
diff --git a/sources/integrator/integrator.cc b/sources/integrator/integrator.cc
index 15f0894..5039c56 100644
--- a/sources/integrator/integrator.cc
+++ b/sources/integrator/integrator.cc
@@ -6,6 +6,7 @@
 
 #include "../bxdf/bxdf.h"
 #include "../bxdf/bsdf.h"
+#include "../bxdf/fresnel.h"
 
 #include "../camera/camera.h"
 #include "../image/film.h"
diff --git a/sources/material/subsurface.cc b/sources/material/subsurface.cc
index db566b7..bff8037 100644
--- a/sources/material/subsurface.cc
+++ b/sources/material/subsurface.cc
@@ -43,7 +43,6 @@ void SubsurfaceMaterial::setScatterFuncs(SurfaceInteraction* isect,
     double uRough = uRoughness_ ? uRoughness_->evaluate(*isect) : 0.0;
     double vRough = vRoughness_ ? vRoughness_->evaluate(*isect) : 0.0;
 
-    // TODO: check which method "allocate" is called.
     isect->setBSDF(arena.allocate<BSDF>(*isect, eta_));
     
     if (re.isBlack() && tr.isBlack()) return;
diff --git a/sources/texture/texture.cc b/sources/texture/texture.cc
index 7821f60..5aa3f28 100644
--- a/sources/texture/texture.cc
+++ b/sources/texture/texture.cc
@@ -16,8 +16,8 @@ namespace spica {
 
     Point2d UVMapping2D::map(const SurfaceInteraction& intr, 
                              Vector2d* dstdx, Vector2d* dstdy) const {
-        *dstdx = Vector2d(su_ * intr.dudx(), sv_ * intr.dvdx());
-        *dstdy = Vector2d(su_ * intr.dudy(), sv_ * intr.dvdy());
+        if (dstdx) *dstdx = Vector2d(su_ * intr.dudx(), sv_ * intr.dvdx());
+        if (dstdy) *dstdy = Vector2d(su_ * intr.dudy(), sv_ * intr.dvdy());
         return Point2d(su_ * intr.uv()[0] + du_, sv_ * intr.uv()[1] + dv_);
     }
 
diff --git a/sources/texture/texture.h b/sources/texture/texture.h
index 60983a5..8ca5853 100644
--- a/sources/texture/texture.h
+++ b/sources/texture/texture.h
@@ -16,8 +16,9 @@ namespace spica {
     class SPICA_EXPORTS TextureMapping2D {
     public:
         virtual ~TextureMapping2D() {}
-        virtual Point2d map(const SurfaceInteraction& intr, Vector2d* dstdx,
-                            Vector2d* dstdy) const = 0;
+        virtual Point2d map(const SurfaceInteraction& intr,
+                            Vector2d* dstdx = nullptr,
+                            Vector2d* dstdy = nullptr) const = 0;
     };
 
     class UVMapping2D : public TextureMapping2D {