diff --git a/CMakeLists.txt b/CMakeLists.txt
index a1ee659f..1a1d4dc5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -689,6 +689,25 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
                                 PROPERTIES COMPILE_FLAGS ${TUSDZ_COMPILE_FLAGS})
   endif()
 
+# Increase warning level for gcc.
+elseif(CMAKE_CXX_COMPILER_ID MATCHES "GNU")
+
+  set(TUSDZ_COMPILE_FLAGS
+      "-Wall "
+  )
+
+  option(TINYUSDZ_NO_WERROR "Don't set -Werror when building the tinyusdz library" ${TINYUSDZ_DEFAULT_NO_WERROR})
+  if (NOT TINYUSDZ_NO_WERROR)
+    set(TUSDZ_COMPILE_FLAGS "${TUSDZ_COMPILE_FLAGS} -Werror")
+  endif()
+
+  set_source_files_properties(${TINYUSDZ_SOURCES}
+                              PROPERTIES COMPILE_FLAGS ${TUSDZ_COMPILE_FLAGS})
+
+  if (TINYUSDZ_WITH_C_API)
+    set_source_files_properties(${TINYUSDZ_C_API_SOURCES}
+                                PROPERTIES COMPILE_FLAGS ${TUSDZ_COMPILE_FLAGS})
+  endif()
 endif()
 
 if(TINYUSDZ_CUSTOM_COMPILE_FLAGS)
diff --git a/README.md b/README.md
index 8006dfd1..cc05e5a8 100644
--- a/README.md
+++ b/README.md
@@ -10,6 +10,7 @@
     * [examples/openglviewer](examples/openglviewer) OpenGL viewer
     * [examples/sdlviewer](examples/sdlviewer) Software raytracing viewer 
   * For Vulkan and Android Vulkan example, please refer https://github.com/syoyo/Vulkan-glTF-USDZ-PBR for a while
+  * For OpenGL + MaterialX example, please refer ASF MaterialXViewer fork to load USD model through TinyUSDZ https://github.com/lighttransport/materialx
  
 ## Mid-term todo
 
@@ -154,13 +155,6 @@ USD itself is a generic container of 3D scene data.
 Tydra is an interface to Renderers/Viewers and other DCCs.
 Tydra may be something like Tiny version of pxrUSD Hydra, but its API is completely different. See [src/tydra/README.md](src/tydra/README.md) for the background.
 
-* Image color space
-  * sRGB
-  * Linear
-  * Rec.709
-  * [ ] Partial support of OCIO(OpenColor IO) through TinyColorIO https://github.com/syoyo/tinycolorio . Currently SPI3DLut only.
-* More details are T.B.W.
-
 ## Notice
 
 TinyUSDZ does not support Reality Composer file format(`.reality`) since it uses proprietary file format and we cannot understand it(so no conversion support from/to Reality also).
diff --git a/src/crate-reader.cc b/src/crate-reader.cc
index 6ab41a70..f6b26808 100644
--- a/src/crate-reader.cc
+++ b/src/crate-reader.cc
@@ -2795,7 +2795,7 @@ bool CrateReader::UnpackValueRep(const crate::ValueRep &rep,
 
         CHECK_MEMORY_USAGE(sizeof(double));
 
-        double v;
+        double v{0.0};
         if (!_sr->read_double(&v)) {
           PUSH_ERROR("Failed to read Double value.");
           return false;
diff --git a/src/external/tiny_dng_loader.h b/src/external/tiny_dng_loader.h
index 777d0353..2e652f27 100644
--- a/src/external/tiny_dng_loader.h
+++ b/src/external/tiny_dng_loader.h
@@ -34,6 +34,7 @@ THE SOFTWARE.
 
 #include <string>
 #include <vector>
+#include <array>
 
 namespace tinydng {
 
@@ -251,6 +252,9 @@ struct DNGImage {
   std::vector<unsigned char>
       data;  // Decoded pixel data(len = spp * width * height * bps / 8)
 
+  std::array<int32_t, 2> shutter_speed{0,0}; // numerator, denominator
+  std::array<int32_t, 2> aperture_value{0,0}; // numerator, denominator
+
   // Custom fields
   std::vector<FieldData> custom_fields;
 };
@@ -346,7 +350,7 @@ bool IsDNGFromMemory(const char* mem, unsigned int size, std::string* msg);
 #pragma clang diagnostic ignored "-Weverything"
 #endif
 
-#define TINY_DNG_LOADER_DEBUG
+//#define TINY_DNG_LOADER_DEBUG
 #ifdef TINY_DNG_LOADER_DEBUG
 #define TINY_DNG_DPRINTF(...) printf(__VA_ARGS__)
 #else
@@ -1928,6 +1932,8 @@ int lj92_encode(uint16_t* image, int width, int height, int bitdepth,
 #endif
 #endif
 
+// NOTE: - https://exiftool.org/TagNames/EXIF.html
+//       - https://helpx.adobe.com/photoshop/kb/dng-specification-tags.html
 typedef enum {
   TAG_NEW_SUBFILE_TYPE = 254,
   TAG_SUBFILE_TYPE = 255,
@@ -1941,6 +1947,11 @@ typedef enum {
   TAG_ROWS_PER_STRIP = 278,
   TAG_STRIP_BYTE_COUNTS = 279,
   TAG_PLANAR_CONFIGURATION = 284,
+  TAG_TRANSFER_FUNCTION = 301, // int16u[768]
+  TAG_SOFTWARE = 305, // string
+  TAG_MODIFY_DATA = 306, // string
+  TAG_ARTIST = 315, // string
+  //TAG_HOST_COMPUTER = 316,
   TAG_PREDICTOR = 317,
   TAG_SUB_IFDS = 330,
   TAG_TILE_WIDTH = 322,
@@ -1979,6 +1990,28 @@ typedef enum {
   TAG_CR2_SLICES = 50752,
   TAG_CR2_META2 = 50885,
 
+  TAG_EXPOSURE_TIME = 0x829a, // rational64u
+  TAG_F_NUMBER = 0x829d, // rational64u
+  TAG_EXPOSURE_PROGRAM = 0x8822, // int16u
+  TAG_GPS_INFO = 0x8825,
+  TAG_ISO = 0x8827, // int16u[n]
+
+  TAG_SHUTTER_SPEED_VALUE = 0x9201, // rational64s
+  TAG_APERTURE_VALUE = 0x9202, // rational64u
+  TAG_BRIGHTNESS_VALUE = 0x9203, // rational64s
+  TAG_EXPOSURE_COMPENSATION = 0x9204, // rational64s
+  TAG_MAX_APERTURE_VALUE = 0x9205, // rational64u
+  TAG_SUBJECT_DISTANCE = 0x9206, // rational64u
+  TAG_METERING_MODE = 0x9207, // int16u
+  TAG_LIGHT_SOURCE = 0x9208, // int16u
+  TAG_FLASH = 0x9209, // int16u
+  TAG_FOCAL_LENGTH = 0x920a, // rational64u
+  
+  TAG_LENS_INFO = 0xa432, // rational64u[4]
+  TAG_LENS_MAKE = 0xa433, // string
+  TAG_LENS_MODEL = 0xa434, // string
+  TAG_LENS_SERIAL_NUMBER = 0xa435, // string
+
   //
   // OpCodeList
   //
@@ -1986,10 +2019,11 @@ typedef enum {
   TAG_OPCODE_LIST2 = 0xc741,
   TAG_OPCODE_LIST3 = 0xc742,
 
-  TAG_NOISE_PROFILE = 51041,
+
+  TAG_NOISE_PROFILE = 0xc761,
 
   // DNG 1.6(Apple ProRAW)
-  // ahttps://helpx.adobe.com/photoshop/kb/dng-specification-tags.html
+  // https://helpx.adobe.com/photoshop/kb/dng-specification-tags.html
   TAG_SEMANTIC_NAME = 52526,  // Type: ASCII, Count: String length including
                               // null, Value: null-terminated string
 
@@ -2447,6 +2481,56 @@ class StreamReader {
     // never come here.
   }
 
+  bool read_rational(int type, uint32_t* ret0, uint32_t *ret1) const {
+    // @todo { Support more types. }
+    
+    if (!ret0 || !ret1) {
+      return false;
+    }
+
+    if (type == TYPE_RATIONAL) {
+      unsigned int num;
+      if (!read4(&num)) {
+        return false;
+      }
+      unsigned int denom;
+      if (!read4(&denom)) {
+        return false;
+      }
+
+      (*ret0) = num;
+      (*ret1) = denom;
+      return true;
+    }
+
+    return false;
+  }
+
+  bool read_srational(int type, int32_t* ret0, int32_t *ret1) const {
+    // @todo { Support more types. }
+    
+    if (!ret0 || !ret1) {
+      return false;
+    }
+
+    if (type == TYPE_SRATIONAL) {
+      int num;
+      if (!read4(&num)) {
+        return false;
+      }
+      int denom;
+      if (!read4(&denom)) {
+        return false;
+      }
+
+      (*ret0) = num;
+      (*ret1) = denom;
+      return true;
+    }
+
+    return false;
+  }
+
   //
   // Returns a memory address. The begining of address is computed in
   // relative(based on current seek pos). This function is useful when you just
@@ -4039,7 +4123,7 @@ static bool ParseTIFFIFD(const StreamReader& sr,
 
           return false;
         }
-        size_t readLen = len * sizeof(float);
+        //size_t readLen = len * sizeof(float);
 
         const size_t kMaxSamples = 1024 * 1024;
 
@@ -4112,7 +4196,7 @@ static bool ParseTIFFIFD(const StreamReader& sr,
             }
           }
 
-          if (len != (image.samples_per_pixel * 2)) {
+          if (size_t(len) != (size_t(image.samples_per_pixel) * 2)) {
             if (err) {
               (*err) += "Counts in NoisProfile must be 2 * SamplesPerPixel.\n";
             }
@@ -4501,7 +4585,7 @@ static bool ParseTIFFIFD(const StreamReader& sr,
         unsigned int strip_offset;
         if (!sr.read4(&strip_offset)) {
           if (err) {
-            (*err) += "Failed to read StripOffset value.";
+            (*err) += "Failed to read StripOffset value.\n";
           }
           return false;
         }
@@ -5847,7 +5931,7 @@ bool LoadDNGFromMemory(const char* mem, unsigned int size,
           }
           return false;
         }
-        TINY_DNG_DPRINTF("image.data.size = %lld\n", len);
+        TINY_DNG_DPRINTF("image.data.size = %u\n", uint32_t(len));
 
         image->data.resize(len);
         TINY_DNG_DPRINTF("image.data.size = %d\n", int(len));
diff --git a/src/image-loader.cc b/src/image-loader.cc
index 81b1e62b..dc298094 100644
--- a/src/image-loader.cc
+++ b/src/image-loader.cc
@@ -47,6 +47,13 @@
 #pragma clang diagnostic ignored "-Weverything"
 #endif
 
+#if defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wunused-variable"
+#pragma GCC diagnostic ignored "-Wunused-function"
+#endif
+
+
 #if defined(TINYUSDZ_USE_WUFFS_IMAGE_LOADER)
 
 #include "external/wuffs-unsupported-snapshot.c"
@@ -92,6 +99,10 @@
 #pragma clang diagnostic pop
 #endif
 
+#if defined(__GNUC__)
+#pragma GCC diagnostic pop
+#endif
+
 #include "image-loader.hh"
 #include "io-util.hh"
 
diff --git a/src/image-util.cc b/src/image-util.cc
index 3fd336cb..72912155 100644
--- a/src/image-util.cc
+++ b/src/image-util.cc
@@ -12,6 +12,13 @@
 #pragma clang diagnostic ignored "-Weverything"
 #endif
 
+#if defined(__GNUC__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wunused-variable"
+#pragma GCC diagnostic ignored "-Wunused-function"
+#pragma GCC diagnostic ignored "-Warray-bounds"
+#endif
+
 #if !defined(TINYUSDZ_NO_STB_IMAGE_RESIZE_IMPLEMENTATION)
 #define STB_IMAGE_RESIZE_IMPLEMENTATION
 #endif
@@ -23,6 +30,10 @@
 #pragma clang diagnostic pop
 #endif
 
+#ifdef __GNUC__
+#pragma GCC diagnostic pop
+#endif
+
 #include "image-util.hh"
 #include "value-types.hh"
 #include "common-macros.inc"
diff --git a/src/linear-algebra.cc b/src/linear-algebra.cc
index 67f661e7..e361d89b 100644
--- a/src/linear-algebra.cc
+++ b/src/linear-algebra.cc
@@ -46,13 +46,14 @@ value::quatf slerp(const value::quatf &a, const value::quatf &b, const float t)
   linalg::vec<float, 4> qb;    
   linalg::vec<float, 4> qret;    
 
-  memcpy(&qa, &a, sizeof(float) * 4);
-  memcpy(&qb, &b, sizeof(float) * 4);
+  memcpy(reinterpret_cast<value::quatf *>(&qa), &a, sizeof(float) * 4);
+  memcpy(reinterpret_cast<value::quatf *>(&qb), &b, sizeof(float) * 4);
 
   qret = linalg::slerp(qa, qb, t);
   
-  
-  return *(reinterpret_cast<value::quatf *>(&qret));
+  value::quatf ret;
+  memcpy(&ret, reinterpret_cast<value::quatf *>(&qret), sizeof(float) * 4);
+  return ret;
 }
 
 value::quatd slerp(const value::quatd &a, const value::quatd &b, const double t) {
@@ -61,13 +62,15 @@ value::quatd slerp(const value::quatd &a, const value::quatd &b, const double t)
   linalg::vec<double, 4> qb;    
   linalg::vec<double, 4> qret;    
 
-  memcpy(&qa, &a, sizeof(double) * 4);
-  memcpy(&qb, &b, sizeof(double) * 4);
+  memcpy(reinterpret_cast<value::quatd *>(&qa), &a, sizeof(double) * 4);
+  memcpy(reinterpret_cast<value::quatd *>(&qb), &b, sizeof(double) * 4);
 
   qret = linalg::slerp(qa, qb, t);
-  
-  return *(reinterpret_cast<value::quatd *>(&qret));
 
+  value::quatd ret;
+  memcpy(&ret, reinterpret_cast<value::quatd *>(&qret), sizeof(double) * 4);
+  return ret;
+  
 }
 
 float vlength(const value::float3 &a) {
diff --git a/src/stream-reader.hh b/src/stream-reader.hh
index 578366f8..3a5ce470 100644
--- a/src/stream-reader.hh
+++ b/src/stream-reader.hh
@@ -293,7 +293,7 @@ class StreamReader {
       return false;
     }
 
-    float value;
+    float value{};
     if (!read4(reinterpret_cast<int *>(&value))) {
       return false;
     }
@@ -308,7 +308,7 @@ class StreamReader {
       return false;
     }
 
-    double value;
+    double value{};
     if (!read8(reinterpret_cast<uint64_t *>(&value))) {
       return false;
     }
diff --git a/src/tydra/render-data.hh b/src/tydra/render-data.hh
index f1fddd02..7f582134 100644
--- a/src/tydra/render-data.hh
+++ b/src/tydra/render-data.hh
@@ -1060,10 +1060,18 @@ struct UDIMTexture {
   std::unordered_map<uint32_t, int32_t> imageTileIds;
 };
 
+// workaround for GCC
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
+#endif
+
 // T or TextureId
 template <typename T>
-struct ShaderParam {
-  ShaderParam(const T &t) { value = t; }
+class ShaderParam {
+ public:
+  ShaderParam() = default;
+  ShaderParam(const T &t) : value(t) { }
 
   bool is_texture() const { return texture_id >= 0; }
 
@@ -1078,12 +1086,14 @@ struct ShaderParam {
     memcpy(&value, &val, sizeof(T));
   }
 
-  T value;
+ //private:
+  T value{};
   int32_t texture_id{-1};  // negative = invalid
 };
 
 // UsdPreviewSurface
-struct PreviewSurfaceShader {
+class PreviewSurfaceShader {
+ public:
   bool useSpecularWorkflow{false};
 
   ShaderParam<vec3> diffuseColor{{0.18f, 0.18f, 0.18f}};
@@ -1103,6 +1113,10 @@ struct PreviewSurfaceShader {
   uint64_t handle{0};  // Handle ID for Graphics API. 0 = invalid
 };
 
+#if defined(__GNUC__) && !defined(__clang__)
+#pragma GCC diagnostic pop
+#endif
+
 // Material + Shader
 struct RenderMaterial {
   std::string name;  // elementName in USD (e.g. "pbrMat")
diff --git a/src/usdMtlx.cc b/src/usdMtlx.cc
index c2b45c30..4a2ae451 100644
--- a/src/usdMtlx.cc
+++ b/src/usdMtlx.cc
@@ -377,7 +377,7 @@ bool ParseMaterialXValue(const std::string &str, T *value, std::string *err) {
                             str.size(), /* swap endian */ false);
   tinyusdz::ascii::AsciiParser parser(&sr);
 
-  T val;
+  T val{};
 
   if (!ParseValue(parser, val, err)) {
     return false;
@@ -522,14 +522,14 @@ static bool ConvertPlace2d(const pugi::xml_node &node, PrimSpec &ps,
   }
 
   if (pugi::xml_attribute pivot_attr = node.attribute("pivot")) {
-    value::float2 value;
+    value::float2 value{};
     if (!ParseMaterialXValue(pivot_attr.as_string(), &value, err)) {
       ps.props()["inputs:pivot"] = Property(Attribute::Uniform(value));
     }
   }
 
   if (pugi::xml_attribute scale_attr = node.attribute("scale")) {
-    value::float2 value;
+    value::float2 value{};
     if (!ParseMaterialXValue(scale_attr.as_string(), &value, err)) {
       PUSH_ERROR_AND_RETURN(
           "Failed to parse `rotate` attribute of `place2d`.\n");
@@ -538,7 +538,7 @@ static bool ConvertPlace2d(const pugi::xml_node &node, PrimSpec &ps,
   }
 
   if (pugi::xml_attribute rotate_attr = node.attribute("rotate")) {
-    float value;
+    float value{};
     if (!ParseMaterialXValue(rotate_attr.as_string(), &value, err)) {
       PUSH_ERROR_AND_RETURN(
           "Failed to parse `rotate` attribute of `place2d`.\n");
@@ -548,7 +548,7 @@ static bool ConvertPlace2d(const pugi::xml_node &node, PrimSpec &ps,
 
   pugi::xml_attribute offset_attr = node.attribute("offset");
   if (offset_attr) {
-    value::float2 value;
+    value::float2 value{};
     if (!ParseMaterialXValue(offset_attr.as_string(), &value, err)) {
       PUSH_ERROR_AND_RETURN(
           "Failed to parse `offset` attribute of `place2d`.\n");
@@ -923,6 +923,7 @@ bool WriteMaterialXToString(const MtlxModel &mtlx, std::string &xml_str,
   if (auto usdps = mtlx.shader.as<MtlxUsdPreviewSurface>()) {
     return detail::WriteMaterialXToString(*usdps, xml_str, warn, err);
   } else if (auto adskss = mtlx.shader.as<MtlxAutodeskStandardSurface>()) {
+    (void)adskss;
     // TODO
     PUSH_ERROR_AND_RETURN("TODO: AutodeskStandardSurface");
   } else {
diff --git a/src/usdc-reader.cc b/src/usdc-reader.cc
index edd9ad84..6ca5a416 100644
--- a/src/usdc-reader.cc
+++ b/src/usdc-reader.cc
@@ -751,7 +751,7 @@ USDCReader::Impl::DecodeListOp(const ListOp<T> &arg) {
     }
   }
 
-  return std::move(dst);
+  return dst;
 }
 
 bool USDCReader::Impl::BuildPropertyMap(const std::vector<size_t> &pathIndices,
@@ -2039,6 +2039,7 @@ bool USDCReader::Impl::ParsePrimSpec(const crate::FieldValuePairVector &fvs,
       }
     } else if (fv.first == "inherits") {  // `inherits` composition
       if (auto pvb = fv.second.as<value::ValueBlock>()) {
+        (void)pvb;
         // make empty array
         primMeta.inherits =
             std::make_pair(ListEditQual::ResetToExplicit, std::vector<Path>());
@@ -2067,6 +2068,7 @@ bool USDCReader::Impl::ParsePrimSpec(const crate::FieldValuePairVector &fvs,
 
     } else if (fv.first == "references") {  // `references` composition
       if (auto pvb = fv.second.as<value::ValueBlock>()) {
+        (void)pvb;
         // make empty array
         primMeta.references = std::make_pair(ListEditQual::ResetToExplicit,
                                              std::vector<Reference>());
@@ -2096,6 +2098,7 @@ bool USDCReader::Impl::ParsePrimSpec(const crate::FieldValuePairVector &fvs,
       }
     } else if (fv.first == "payload") {  // `payload` composition
       if (auto pvb = fv.second.as<value::ValueBlock>()) {
+        (void)pvb;
         // make empty array
         primMeta.payload = std::make_pair(ListEditQual::ResetToExplicit,
                                              std::vector<Payload>());
diff --git a/src/usdc-writer.cc b/src/usdc-writer.cc
index 69ba5126..d9bc89aa 100644
--- a/src/usdc-writer.cc
+++ b/src/usdc-writer.cc
@@ -208,6 +208,7 @@ class Packer {
                    // by Index()(= ~0)
 };
 
+#if 0 // not used atm.
 crate::TokenIndex Packer::AddToken(const Token &token) {
   if (token_to_index_map.count(token)) {
     return token_to_index_map[token];
@@ -271,6 +272,7 @@ crate::FieldSetIndex Packer::AddFieldSet(
 
   return fieldset_to_index_map[fieldset];
 }
+#endif
 
 class Writer {
  public:
diff --git a/src/value-types.cc b/src/value-types.cc
index 1febf030..df6a37cb 100644
--- a/src/value-types.cc
+++ b/src/value-types.cc
@@ -896,7 +896,7 @@ bool RoleTypeCast(const uint32_t roleTyId, value::Value &inout) {
         if (auto pv = inout.get_value<__srcBaseTy>()) {                        \
           __srcBaseTy val = pv.value();                                        \
           __roleTy newval;                                                     \
-          memcpy(&newval, &val, sizeof(__srcBaseTy));                          \
+          memcpy(reinterpret_cast<__srcBaseTy *>(&newval), &val, sizeof(__srcBaseTy));                          \
           inout = newval;                                                      \
           return true;                                                         \
         }                                                                      \
@@ -909,7 +909,7 @@ bool RoleTypeCast(const uint32_t roleTyId, value::Value &inout) {
           std::vector<__srcBaseTy> val = pv.value();                           \
           std::vector<__roleTy> newval;                                        \
           newval.resize(val.size());                                           \
-          memcpy(newval.data(), val.data(), sizeof(__srcBaseTy) * val.size()); \
+          memcpy(reinterpret_cast<__srcBaseTy *>(newval.data()), val.data(), sizeof(__srcBaseTy) * val.size()); \
           inout = newval;                                                      \
           return true;                                                         \
         }                                                                      \