assimp/code/glTF2Asset.h

/*
Open Asset Import Library (assimp)
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All rights reserved.

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/** @file glTFAsset.h
 * Declares a glTF class to handle gltf/glb files
 *
 * glTF Extensions Support:
 *   KHR_materials_pbrSpecularGlossiness full
 *   KHR_materials_unlit full
 */
#ifndef GLTF2ASSET_H_INC
#define GLTF2ASSET_H_INC

#ifndef ASSIMP_BUILD_NO_GLTF_IMPORTER

#include <map>
#include <string>
#include <list>
#include <vector>
#include <algorithm>
#include <stdexcept>

#define RAPIDJSON_HAS_STDSTRING 1
#include <rapidjson/rapidjson.h>
#include <rapidjson/document.h>
#include <rapidjson/error/en.h>

#ifdef ASSIMP_API
#   include <memory>
#   include <assimp/DefaultIOSystem.h>
#   include <assimp/ByteSwapper.h>
#else
#   include <memory>
#   define AI_SWAP4(p)
#   define ai_assert
#endif


#if _MSC_VER > 1500 || (defined __GNUC___)
#       define ASSIMP_GLTF_USE_UNORDERED_MULTIMAP
#   else
#       define gltf_unordered_map map
#endif

#ifdef ASSIMP_GLTF_USE_UNORDERED_MULTIMAP
#   include <unordered_map>
#   if _MSC_VER > 1600
#       define gltf_unordered_map unordered_map
#   else
#       define gltf_unordered_map tr1::unordered_map
#   endif
#endif

#include <assimp/StringUtils.h>

namespace glTF2
{
#ifdef ASSIMP_API
    using Assimp::IOStream;
    using Assimp::IOSystem;
    using std::shared_ptr;
#else
    using std::shared_ptr;

    typedef std::runtime_error DeadlyImportError;
    typedef std::runtime_error DeadlyExportError;

    enum aiOrigin { aiOrigin_SET = 0, aiOrigin_CUR = 1, aiOrigin_END = 2 };
    class IOSystem;
    class IOStream
    {
        FILE* f;
    public:
        IOStream(FILE* file) : f(file) {}
        ~IOStream() { fclose(f); f = 0; }

        size_t Read(void* b, size_t sz, size_t n) { return fread(b, sz, n, f); }
        size_t Write(const void* b, size_t sz, size_t n) { return fwrite(b, sz, n, f); }
        int    Seek(size_t off, aiOrigin orig) { return fseek(f, off, int(orig)); }
        size_t Tell() const { return ftell(f); }

        size_t FileSize() {
            long p = Tell(), len = (Seek(0, aiOrigin_END), Tell());
            return size_t((Seek(p, aiOrigin_SET), len));
        }
    };
#endif

    using rapidjson::Value;
    using rapidjson::Document;

    class Asset;
    class AssetWriter;

    struct BufferView; // here due to cross-reference
    struct Texture;
    struct Skin;

    // Vec/matrix types, as raw float arrays
    typedef float (vec3)[3];
    typedef float (vec4)[4];
    typedef float (mat4)[16];

    namespace Util
    {
        void EncodeBase64(const uint8_t* in, size_t inLength, std::string& out);

        size_t DecodeBase64(const char* in, size_t inLength, uint8_t*& out);

        inline size_t DecodeBase64(const char* in, uint8_t*& out)
        {
            return DecodeBase64(in, strlen(in), out);
        }

        struct DataURI
        {
            const char* mediaType;
            const char* charset;
            bool base64;
            const char* data;
            size_t dataLength;
        };

        //! Check if a uri is a data URI
        inline bool ParseDataURI(const char* uri, size_t uriLen, DataURI& out);
    }


    //! Magic number for GLB files
	#define AI_GLB_MAGIC_NUMBER "glTF"
	#include <assimp/pbrmaterial.h>

    #ifdef ASSIMP_API
        #include "./../include/assimp/Compiler/pushpack1.h"
    #endif

    //! For binary .glb files
    //! 12-byte header (+ the JSON + a "body" data section)
    struct GLB_Header
    {
        uint8_t magic[4];     //!< Magic number: "glTF"
        uint32_t version;     //!< Version number (always 2 as of the last update)
        uint32_t length;      //!< Total length of the Binary glTF, including header, scene, and body, in bytes
    } PACK_STRUCT;

    struct GLB_Chunk
    {
        uint32_t chunkLength;
        uint32_t chunkType;
    } PACK_STRUCT;

    #ifdef ASSIMP_API
        #include "./../include/assimp/Compiler/poppack1.h"
    #endif


    //! Values for the GLB_Chunk::chunkType field
    enum ChunkType
    {
        ChunkType_JSON = 0x4E4F534A,
        ChunkType_BIN  = 0x004E4942
    };

    //! Values for the mesh primitive modes
    enum PrimitiveMode
    {
        PrimitiveMode_POINTS = 0,
        PrimitiveMode_LINES = 1,
        PrimitiveMode_LINE_LOOP = 2,
        PrimitiveMode_LINE_STRIP = 3,
        PrimitiveMode_TRIANGLES = 4,
        PrimitiveMode_TRIANGLE_STRIP = 5,
        PrimitiveMode_TRIANGLE_FAN = 6
    };

    //! Values for the Accessor::componentType field
    enum ComponentType
    {
        ComponentType_BYTE = 5120,
        ComponentType_UNSIGNED_BYTE = 5121,
        ComponentType_SHORT = 5122,
        ComponentType_UNSIGNED_SHORT = 5123,
        ComponentType_UNSIGNED_INT = 5125,
        ComponentType_FLOAT = 5126
    };

    inline unsigned int ComponentTypeSize(ComponentType t)
    {
        switch (t) {
            case ComponentType_SHORT:
            case ComponentType_UNSIGNED_SHORT:
                return 2;

            case ComponentType_UNSIGNED_INT:
            case ComponentType_FLOAT:
                return 4;

            case ComponentType_BYTE:
            case ComponentType_UNSIGNED_BYTE:
                return 1;
            default:
                throw DeadlyImportError("GLTF: Unsupported Component Type " + to_string(t));
        }
    }

    //! Values for the BufferView::target field
    enum BufferViewTarget
    {
        BufferViewTarget_ARRAY_BUFFER = 34962,
        BufferViewTarget_ELEMENT_ARRAY_BUFFER = 34963
    };

    //! Values for the Sampler::magFilter field
    enum class SamplerMagFilter: unsigned int
    {
        UNSET = 0,
        SamplerMagFilter_Nearest = 9728,
        SamplerMagFilter_Linear = 9729
    };

    //! Values for the Sampler::minFilter field
    enum class SamplerMinFilter: unsigned int
    {
        UNSET = 0,
        SamplerMinFilter_Nearest = 9728,
        SamplerMinFilter_Linear = 9729,
        SamplerMinFilter_Nearest_Mipmap_Nearest = 9984,
        SamplerMinFilter_Linear_Mipmap_Nearest = 9985,
        SamplerMinFilter_Nearest_Mipmap_Linear = 9986,
        SamplerMinFilter_Linear_Mipmap_Linear = 9987
    };

    //! Values for the Sampler::wrapS and Sampler::wrapT field
    enum class SamplerWrap: unsigned int
    {
        UNSET = 0,
        Clamp_To_Edge = 33071,
        Mirrored_Repeat = 33648,
        Repeat = 10497
    };

    //! Values for the Texture::format and Texture::internalFormat fields
    enum TextureFormat
    {
        TextureFormat_ALPHA = 6406,
        TextureFormat_RGB = 6407,
        TextureFormat_RGBA = 6408,
        TextureFormat_LUMINANCE = 6409,
        TextureFormat_LUMINANCE_ALPHA = 6410
    };

    //! Values for the Texture::target field
    enum TextureTarget
    {
        TextureTarget_TEXTURE_2D = 3553
    };

    //! Values for the Texture::type field
    enum TextureType
    {
        TextureType_UNSIGNED_BYTE = 5121,
        TextureType_UNSIGNED_SHORT_5_6_5 = 33635,
        TextureType_UNSIGNED_SHORT_4_4_4_4 = 32819,
        TextureType_UNSIGNED_SHORT_5_5_5_1 = 32820
    };


    //! Values for the Accessor::type field (helper class)
    class AttribType
    {
    public:
        enum Value
            { SCALAR, VEC2, VEC3, VEC4, MAT2, MAT3, MAT4 };

    private:
        static const size_t NUM_VALUES = static_cast<size_t>(MAT4)+1;

        struct Info
            { const char* name; unsigned int numComponents; };

        template<int N> struct data
            { static const Info infos[NUM_VALUES]; };

    public:
        inline static Value FromString(const char* str)
        {
            for (size_t i = 0; i < NUM_VALUES; ++i) {
                if (strcmp(data<0>::infos[i].name, str) == 0) {
                    return static_cast<Value>(i);
                }
            }
            return SCALAR;
        }

        inline static const char* ToString(Value type)
        {
            return data<0>::infos[static_cast<size_t>(type)].name;
        }

        inline static unsigned int GetNumComponents(Value type)
        {
            return data<0>::infos[static_cast<size_t>(type)].numComponents;
        }
    };

    // must match the order of the AttribTypeTraits::Value enum!
    template<int N> const AttribType::Info
    AttribType::data<N>::infos[AttribType::NUM_VALUES] = {
        { "SCALAR", 1 }, { "VEC2", 2 }, { "VEC3", 3 }, { "VEC4", 4 }, { "MAT2", 4 }, { "MAT3", 9 }, { "MAT4", 16 }
    };



    //! A reference to one top-level object, which is valid
    //! until the Asset instance is destroyed
    template<class T>
    class Ref
    {
        std::vector<T*>* vector;
        unsigned int index;

    public:
        Ref() : vector(0), index(0) {}
        Ref(std::vector<T*>& vec, unsigned int idx) : vector(&vec), index(idx) {}

        inline unsigned int GetIndex() const
            { return index; }

        operator bool() const
            { return vector != 0; }

        T* operator->()
            { return (*vector)[index]; }

        T& operator*()
            { return *((*vector)[index]); }
    };

    //! Helper struct to represent values that might not be present
    template<class T>
    struct Nullable
    {
        T value;
        bool isPresent;

        Nullable() : isPresent(false) {}
        Nullable(T& val) : value(val), isPresent(true) {}
    };


    //! Base class for all glTF top-level objects
    struct Object
    {
        int index;        //!< The index of this object within its property container
        int oIndex;       //!< The original index of this object defined in the JSON
        std::string id;   //!< The globally unique ID used to reference this object
        std::string name; //!< The user-defined name of this object

        //! Objects marked as special are not exported (used to emulate the binary body buffer)
        virtual bool IsSpecial() const
            { return false; }

        virtual ~Object() {}

        //! Maps special IDs to another ID, where needed. Subclasses may override it (statically)
        static const char* TranslateId(Asset& /*r*/, const char* id)
            { return id; }
    };

    //
    // Classes for each glTF top-level object type
    //

    //! A typed view into a BufferView. A BufferView contains raw binary data.
    //! An accessor provides a typed view into a BufferView or a subset of a BufferView
    //! similar to how WebGL's vertexAttribPointer() defines an attribute in a buffer.
    struct Accessor : public Object
    {
        Ref<BufferView> bufferView;  //!< The ID of the bufferView. (required)
        unsigned int byteOffset;     //!< The offset relative to the start of the bufferView in bytes. (required)
        ComponentType componentType; //!< The datatype of components in the attribute. (required)
        unsigned int count;          //!< The number of attributes referenced by this accessor. (required)
        AttribType::Value type;      //!< Specifies if the attribute is a scalar, vector, or matrix. (required)
        std::vector<float> max;      //!< Maximum value of each component in this attribute.
        std::vector<float> min;      //!< Minimum value of each component in this attribute.

        unsigned int GetNumComponents();
        unsigned int GetBytesPerComponent();
        unsigned int GetElementSize();

        inline uint8_t* GetPointer();

        template<class T>
        bool ExtractData(T*& outData);

        void WriteData(size_t count, const void* src_buffer, size_t src_stride);

        //! Helper class to iterate the data
        class Indexer
        {
            friend struct Accessor;

            Accessor& accessor;
            uint8_t* data;
            size_t elemSize, stride;

            Indexer(Accessor& acc);

        public:

            //! Accesses the i-th value as defined by the accessor
            template<class T>
            T GetValue(int i);

            //! Accesses the i-th value as defined by the accessor
            inline unsigned int GetUInt(int i)
            {
                return GetValue<unsigned int>(i);
            }

            inline bool IsValid() const
            {
                return data != 0;
            }
        };

        inline Indexer GetIndexer()
        {
            return Indexer(*this);
        }

        Accessor() {}
        void Read(Value& obj, Asset& r);
    };

    //! A buffer points to binary geometry, animation, or skins.
    struct Buffer : public Object
	{
		/********************* Types *********************/
	public:

		enum Type
		{
			Type_arraybuffer,
			Type_text
		};

		/// \struct SEncodedRegion
		/// Descriptor of encoded region in "bufferView".
		struct SEncodedRegion
		{
			const size_t Offset;///< Offset from begin of "bufferView" to encoded region, in bytes.
			const size_t EncodedData_Length;///< Size of encoded region, in bytes.
			uint8_t* const DecodedData;///< Cached encoded data.
			const size_t DecodedData_Length;///< Size of decoded region, in bytes.
			const std::string ID;///< ID of the region.

			/// \fn SEncodedRegion(const size_t pOffset, const size_t pEncodedData_Length, uint8_t* pDecodedData, const size_t pDecodedData_Length, const std::string pID)
			/// Constructor.
			/// \param [in] pOffset - offset from begin of "bufferView" to encoded region, in bytes.
			/// \param [in] pEncodedData_Length - size of encoded region, in bytes.
			/// \param [in] pDecodedData - pointer to decoded data array.
			/// \param [in] pDecodedData_Length - size of encoded region, in bytes.
			/// \param [in] pID - ID of the region.
			SEncodedRegion(const size_t pOffset, const size_t pEncodedData_Length, uint8_t* pDecodedData, const size_t pDecodedData_Length, const std::string pID)
				: Offset(pOffset), EncodedData_Length(pEncodedData_Length), DecodedData(pDecodedData), DecodedData_Length(pDecodedData_Length), ID(pID)
			{}

			/// \fn ~SEncodedRegion()
			/// Destructor.
			~SEncodedRegion() { delete[] DecodedData; }
		};

		/******************* Variables *******************/

		//std::string uri; //!< The uri of the buffer. Can be a filepath, a data uri, etc. (required)
		size_t byteLength; //!< The length of the buffer in bytes. (default: 0)
		//std::string type; //!< XMLHttpRequest responseType (default: "arraybuffer")

		Type type;

		/// \var EncodedRegion_Current
		/// Pointer to currently active encoded region.
		/// Why not decoding all regions at once and not to set one buffer with decoded data?
		/// Yes, why not? Even "accessor" point to decoded data. I mean that fields "byteOffset", "byteStride" and "count" has values which describes decoded
		/// data array. But only in range of mesh while is active parameters from "compressedData". For another mesh accessors point to decoded data too. But
		/// offset is counted for another regions is encoded.
		/// Example. You have two meshes. For every of it you have 4 bytes of data. That data compressed to 2 bytes. So, you have buffer with encoded data:
		/// M1_E0, M1_E1, M2_E0, M2_E1.
		/// After decoding you'll get:
		/// M1_D0, M1_D1, M1_D2, M1_D3, M2_D0, M2_D1, M2_D2, M2_D3.
		/// "accessors" must to use values that point to decoded data - obviously. So, you'll expect "accessors" like
		/// "accessor_0" : { byteOffset: 0, byteLength: 4}, "accessor_1" : { byteOffset: 4, byteLength: 4}
		/// but in real life you'll get:
		/// "accessor_0" : { byteOffset: 0, byteLength: 4}, "accessor_1" : { byteOffset: 2, byteLength: 4}
		/// Yes, accessor of next mesh has offset and length which mean: current mesh data is decoded, all other data is encoded.
		/// And when before you start to read data of current mesh (with encoded data ofcourse) you must decode region of "bufferView", after read finished
		/// delete encoding mark. And after that you can repeat process: decode data of mesh, read, delete decoded data.
		///
		/// Remark. Encoding all data at once is good in world with computers which do not has RAM limitation. So, you must use step by step encoding in
		/// exporter and importer. And, thanks to such way, there is no need to load whole file into memory.
		SEncodedRegion* EncodedRegion_Current;

	private:

		shared_ptr<uint8_t> mData; //!< Pointer to the data
		bool mIsSpecial; //!< Set to true for special cases (e.g. the body buffer)

		/// \var EncodedRegion_List
		/// List of encoded regions.
		std::list<SEncodedRegion*> EncodedRegion_List;

		/******************* Functions *******************/

	public:

		Buffer();
		~Buffer();

		void Read(Value& obj, Asset& r);

        bool LoadFromStream(IOStream& stream, size_t length = 0, size_t baseOffset = 0);

		/// \fn void EncodedRegion_Mark(const size_t pOffset, const size_t pEncodedData_Length, uint8_t* pDecodedData, const size_t pDecodedData_Length, const std::string& pID)
		/// Mark region of "bufferView" as encoded. When data is request from such region then "bufferView" use decoded data.
		/// \param [in] pOffset - offset from begin of "bufferView" to encoded region, in bytes.
		/// \param [in] pEncodedData_Length - size of encoded region, in bytes.
		/// \param [in] pDecodedData - pointer to decoded data array.
		/// \param [in] pDecodedData_Length - size of encoded region, in bytes.
		/// \param [in] pID - ID of the region.
		void EncodedRegion_Mark(const size_t pOffset, const size_t pEncodedData_Length, uint8_t* pDecodedData, const size_t pDecodedData_Length, const std::string& pID);

		/// \fn void EncodedRegion_SetCurrent(const std::string& pID)
		/// Select current encoded region by ID. \sa EncodedRegion_Current.
		/// \param [in] pID - ID of the region.
		void EncodedRegion_SetCurrent(const std::string& pID);

		/// \fn bool ReplaceData(const size_t pBufferData_Offset, const size_t pBufferData_Count, const uint8_t* pReplace_Data, const size_t pReplace_Count)
		/// Replace part of buffer data. Pay attention that function work with original array of data (\ref mData) not with encoded regions.
		/// \param [in] pBufferData_Offset - index of first element in buffer from which new data will be placed.
		/// \param [in] pBufferData_Count - count of bytes in buffer which will be replaced.
		/// \param [in] pReplace_Data - pointer to array with new data for buffer.
		/// \param [in] pReplace_Count - count of bytes in new data.
		/// \return true - if successfully replaced, false if input arguments is out of range.
		bool ReplaceData(const size_t pBufferData_Offset, const size_t pBufferData_Count, const uint8_t* pReplace_Data, const size_t pReplace_Count);
		bool ReplaceData_joint(const size_t pBufferData_Offset, const size_t pBufferData_Count, const uint8_t* pReplace_Data, const size_t pReplace_Count);

        size_t AppendData(uint8_t* data, size_t length);
        void Grow(size_t amount);

        uint8_t* GetPointer()
            { return mData.get(); }

        void MarkAsSpecial()
            { mIsSpecial = true; }

        bool IsSpecial() const
            { return mIsSpecial; }

        std::string GetURI()
            { return std::string(this->id) + ".bin"; }

        static const char* TranslateId(Asset& r, const char* id);
    };

    //! A view into a buffer generally representing a subset of the buffer.
    struct BufferView : public Object
    {
        Ref<Buffer> buffer; //! The ID of the buffer. (required)
        size_t byteOffset; //! The offset into the buffer in bytes. (required)
        size_t byteLength; //! The length of the bufferView in bytes. (default: 0)
        unsigned int byteStride; //!< The stride, in bytes, between attributes referenced by this accessor. (default: 0)

        BufferViewTarget target; //! The target that the WebGL buffer should be bound to.

        void Read(Value& obj, Asset& r);
    };

    struct Camera : public Object
    {
        enum Type
        {
            Perspective,
            Orthographic
        };

        Type type;

        union
        {
            struct {
                float aspectRatio; //!<The floating - point aspect ratio of the field of view. (0 = undefined = use the canvas one)
                float yfov;  //!<The floating - point vertical field of view in radians. (required)
                float zfar;  //!<The floating - point distance to the far clipping plane. (required)
                float znear; //!< The floating - point distance to the near clipping plane. (required)
            } perspective;

            struct {
                float xmag;  //! The floating-point horizontal magnification of the view. (required)
                float ymag;  //! The floating-point vertical magnification of the view. (required)
                float zfar;  //! The floating-point distance to the far clipping plane. (required)
                float znear; //! The floating-point distance to the near clipping plane. (required)
            } ortographic;
        } cameraProperties;

        Camera() {}
        void Read(Value& obj, Asset& r);
    };


    //! Image data used to create a texture.
    struct Image : public Object
    {
        std::string uri; //! The uri of the image, that can be a file path, a data URI, etc.. (required)

        Ref<BufferView> bufferView;

        std::string mimeType;

        int width, height;

    private:
        uint8_t* mData;
        size_t mDataLength;

    public:

        Image();
        void Read(Value& obj, Asset& r);

        inline bool HasData() const
            { return mDataLength > 0; }

        inline size_t GetDataLength() const
            { return mDataLength; }

        inline const uint8_t* GetData() const
            { return mData; }

        inline uint8_t* StealData();

        inline void SetData(uint8_t* data, size_t length, Asset& r);
	};

	const vec4 defaultBaseColor = {1, 1, 1, 1};
	const vec3 defaultEmissiveFactor = {0, 0, 0};
	const vec4 defaultDiffuseFactor = {1, 1, 1, 1};
	const vec3 defaultSpecularFactor = {1, 1, 1};

    struct TextureInfo
    {
        Ref<Texture> texture;
        unsigned int index;
        unsigned int texCoord = 0;
    };

    struct NormalTextureInfo : TextureInfo
    {
        float scale = 1;
    };

    struct OcclusionTextureInfo : TextureInfo
    {
        float strength = 1;
    };

    struct PbrMetallicRoughness
    {
        vec4 baseColorFactor;
        TextureInfo baseColorTexture;
        TextureInfo metallicRoughnessTexture;
        float metallicFactor;
        float roughnessFactor;
    };

    struct PbrSpecularGlossiness
    {
        vec4 diffuseFactor;
        vec3 specularFactor;
        float glossinessFactor;
        TextureInfo diffuseTexture;
		TextureInfo specularGlossinessTexture;

		PbrSpecularGlossiness() { SetDefaults(); }
		void SetDefaults();
    };

    //! The material appearance of a primitive.
    struct Material : public Object
    {
        //PBR metallic roughness properties
        PbrMetallicRoughness pbrMetallicRoughness;

        //other basic material properties
        NormalTextureInfo normalTexture;
        OcclusionTextureInfo occlusionTexture;
        TextureInfo emissiveTexture;
        vec3 emissiveFactor;
        std::string alphaMode;
        float alphaCutoff;
        bool doubleSided;

        //extension: KHR_materials_pbrSpecularGlossiness
        Nullable<PbrSpecularGlossiness> pbrSpecularGlossiness;

        //extension: KHR_materials_unlit 
        bool unlit;

        Material() { SetDefaults(); }
        void Read(Value& obj, Asset& r);
        void SetDefaults();
    };

    //! A set of primitives to be rendered. A node can contain one or more meshes. A node's transform places the mesh in the scene.
    struct Mesh : public Object
    {
        typedef std::vector< Ref<Accessor> > AccessorList;

        struct Primitive
        {
            PrimitiveMode mode;

            struct Attributes {
                AccessorList position, normal, tangent, texcoord, color, joint, jointmatrix, weight;
            } attributes;

            Ref<Accessor> indices;

            Ref<Material> material;

            struct Target {
                AccessorList position, normal, tangent;
            };
            std::vector<Target> targets;
        };

        std::vector<Primitive> primitives;

        std::vector<float> weights;

        Mesh() {}

		/// \fn void Read(Value& pJSON_Object, Asset& pAsset_Root)
		/// Get mesh data from JSON-object and place them to root asset.
		/// \param [in] pJSON_Object - reference to pJSON-object from which data are read.
		/// \param [out] pAsset_Root - reference to root assed where data will be stored.
		void Read(Value& pJSON_Object, Asset& pAsset_Root);
    };

    struct Node : public Object
    {
        std::vector< Ref<Node> > children;
        std::vector< Ref<Mesh> > meshes;

        Nullable<mat4> matrix;
        Nullable<vec3> translation;
        Nullable<vec4> rotation;
        Nullable<vec3> scale;

        Ref<Camera> camera;

        std::vector< Ref<Node> > skeletons;       //!< The ID of skeleton nodes. Each of which is the root of a node hierarchy.
        Ref<Skin> skin;                           //!< The ID of the skin referenced by this node.
        std::string jointName;                    //!< Name used when this node is a joint in a skin.

        Ref<Node> parent;                         //!< This is not part of the glTF specification. Used as a helper.

        Node() {}
        void Read(Value& obj, Asset& r);
    };

    struct Program : public Object
    {
        Program() {}
        void Read(Value& obj, Asset& r);
    };


    struct Sampler : public Object
    {
        SamplerMagFilter magFilter; //!< The texture magnification filter.
        SamplerMinFilter minFilter; //!< The texture minification filter.
        SamplerWrap wrapS;          //!< The texture wrapping in the S direction.
        SamplerWrap wrapT;          //!< The texture wrapping in the T direction.

        Sampler() { SetDefaults(); }
        void Read(Value& obj, Asset& r);
        void SetDefaults();
    };

    struct Scene : public Object
    {
        std::vector< Ref<Node> > nodes;

        Scene() {}
        void Read(Value& obj, Asset& r);
    };

    struct Shader : public Object
    {
        Shader() {}
        void Read(Value& obj, Asset& r);
    };

    struct Skin : public Object
    {
        Nullable<mat4> bindShapeMatrix;       //!< Floating-point 4x4 transformation matrix stored in column-major order.
        Ref<Accessor> inverseBindMatrices;    //!< The ID of the accessor containing the floating-point 4x4 inverse-bind matrices.
        std::vector<Ref<Node>> jointNames;    //!< Joint names of the joints (nodes with a jointName property) in this skin.
        std::string name;                     //!< The user-defined name of this object.

        Skin() {}
        void Read(Value& obj, Asset& r);
    };

    //! A texture and its sampler.
    struct Texture : public Object
    {
        Ref<Sampler> sampler; //!< The ID of the sampler used by this texture. (required)
        Ref<Image> source;    //!< The ID of the image used by this texture. (required)

        //TextureFormat format; //!< The texture's format. (default: TextureFormat_RGBA)
        //TextureFormat internalFormat; //!< The texture's internal format. (default: TextureFormat_RGBA)

        //TextureTarget target; //!< The target that the WebGL texture should be bound to. (default: TextureTarget_TEXTURE_2D)
        //TextureType type; //!< Texel datatype. (default: TextureType_UNSIGNED_BYTE)

        Texture() {}
        void Read(Value& obj, Asset& r);
    };

    struct Animation : public Object
    {
        struct AnimSampler {
            std::string id;               //!< The ID of this sampler.
            std::string input;            //!< The ID of a parameter in this animation to use as key-frame input.
            std::string interpolation;    //!< Type of interpolation algorithm to use between key-frames.
            std::string output;           //!< The ID of a parameter in this animation to use as key-frame output.
        };

        struct AnimChannel {
            int sampler;                 //!< The index of a sampler in the containing animation's samplers property.

            struct AnimTarget {
                Ref<Node> node;          //!< The node to animate.
                std::string path;        //!< The name of property of the node to animate ("translation", "rotation", or "scale").
            } target;
        };

        struct AnimParameters {
            Ref<Accessor> TIME;           //!< Accessor reference to a buffer storing a array of floating point scalar values.
            Ref<Accessor> rotation;       //!< Accessor reference to a buffer storing a array of four-component floating-point vectors.
            Ref<Accessor> scale;          //!< Accessor reference to a buffer storing a array of three-component floating-point vectors.
            Ref<Accessor> translation;    //!< Accessor reference to a buffer storing a array of three-component floating-point vectors.
        };

        // AnimChannel Channels[3];            //!< Connect the output values of the key-frame animation to a specific node in the hierarchy.
        // AnimParameters Parameters;          //!< The samplers that interpolate between the key-frames.
        // AnimSampler Samplers[3];            //!< The parameterized inputs representing the key-frame data.

        std::vector<AnimChannel> Channels;            //!< Connect the output values of the key-frame animation to a specific node in the hierarchy.
        AnimParameters Parameters;                    //!< The samplers that interpolate between the key-frames.
        std::vector<AnimSampler> Samplers;         //!< The parameterized inputs representing the key-frame data.

        Animation() {}
        void Read(Value& obj, Asset& r);

        //! Get accessor given an animation parameter name.
        Ref<Accessor> GetAccessor(std::string name) {
            if (name == "TIME") {
                return Parameters.TIME;
            } else if (name == "rotation") {
                return Parameters.rotation;
            } else if (name == "scale") {
                return Parameters.scale;
            } else if (name == "translation") {
                return Parameters.translation;
            }
            return Ref<Accessor>();
        }
    };


    //! Base class for LazyDict that acts as an interface
    class LazyDictBase
    {
    public:
        virtual ~LazyDictBase() {}

        virtual void AttachToDocument(Document& doc) = 0;
        virtual void DetachFromDocument() = 0;

        virtual void WriteObjects(AssetWriter& writer) = 0;
    };


    template<class T>
    class LazyDict;

    //! (Implemented in glTFAssetWriter.h)
    template<class T>
    void WriteLazyDict(LazyDict<T>& d, AssetWriter& w);


    //! Manages lazy loading of the glTF top-level objects, and keeps a reference to them by ID
    //! It is the owner the loaded objects, so when it is destroyed it also deletes them
    template<class T>
    class LazyDict : public LazyDictBase
    {
        friend class Asset;
        friend class AssetWriter;

        typedef typename std::gltf_unordered_map< unsigned int, unsigned int > Dict;
        typedef typename std::gltf_unordered_map< std::string, unsigned int > IdDict;

        std::vector<T*>     mObjs;         //! The read objects
        Dict                mObjsByOIndex; //! The read objects accessible by original index
        IdDict              mObjsById;     //! The read objects accessible by id
        const char*         mDictId;       //! ID of the dictionary object
        const char*         mExtId;        //! ID of the extension defining the dictionary
        Value*              mDict;         //! JSON dictionary object
        Asset&              mAsset;        //! The asset instance

        void AttachToDocument(Document& doc);
        void DetachFromDocument();

        void WriteObjects(AssetWriter& writer)
            { WriteLazyDict<T>(*this, writer); }

        Ref<T> Add(T* obj);

    public:
        LazyDict(Asset& asset, const char* dictId, const char* extId = 0);
        ~LazyDict();

        Ref<T> Retrieve(unsigned int i);

        Ref<T> Get(unsigned int i);
        Ref<T> Get(const char* id);

        Ref<T> Create(const char* id);
        Ref<T> Create(const std::string& id)
            { return Create(id.c_str()); }

        unsigned int Remove(const char* id);

        inline unsigned int Size() const
            { return unsigned(mObjs.size()); }

        inline T& operator[](size_t i)
            { return *mObjs[i]; }

    };


    struct AssetMetadata
    {
        std::string copyright; //!< A copyright message suitable for display to credit the content creator.
        std::string generator; //!< Tool that generated this glTF model.Useful for debugging.

        struct {
            std::string api;     //!< Specifies the target rendering API (default: "WebGL")
            std::string version; //!< Specifies the target rendering API (default: "1.0.3")
        } profile; //!< Specifies the target rendering API and version, e.g., WebGL 1.0.3. (default: {})

        std::string version; //!< The glTF format version

        void Read(Document& doc);

        AssetMetadata() : version("") {}
    };

    //
    // glTF Asset class
    //

    //! Root object for a glTF asset
    class Asset
    {
        typedef std::gltf_unordered_map<std::string, int> IdMap;

        template<class T>
        friend class LazyDict;

        friend struct Buffer; // To access OpenFile

        friend class AssetWriter;

    private:
        IOSystem* mIOSystem;

        std::string mCurrentAssetDir;

        size_t mSceneLength;
        size_t mBodyOffset, mBodyLength;

        std::vector<LazyDictBase*> mDicts;

        IdMap mUsedIds;

        Ref<Buffer> mBodyBuffer;

        Asset(Asset&);
        Asset& operator=(const Asset&);

    public:

        //! Keeps info about the enabled extensions
        struct Extensions
        {
            bool KHR_materials_pbrSpecularGlossiness;
            bool KHR_materials_unlit;

        } extensionsUsed;

        AssetMetadata asset;


        // Dictionaries for each type of object

        LazyDict<Accessor>    accessors;
        LazyDict<Animation>   animations;
        LazyDict<Buffer>      buffers;
        LazyDict<BufferView>  bufferViews;
        LazyDict<Camera>      cameras;
        LazyDict<Image>       images;
        LazyDict<Material>    materials;
        LazyDict<Mesh>        meshes;
        LazyDict<Node>        nodes;
        LazyDict<Sampler>     samplers;
        LazyDict<Scene>       scenes;
        LazyDict<Skin>        skins;
        LazyDict<Texture>     textures;

        Ref<Scene> scene;

    public:
        Asset(IOSystem* io = 0)
            : mIOSystem(io)
            , asset()
            , accessors     (*this, "accessors")
            , animations    (*this, "animations")
            , buffers       (*this, "buffers")
            , bufferViews   (*this, "bufferViews")
            , cameras       (*this, "cameras")
            , images        (*this, "images")
            , materials     (*this, "materials")
            , meshes        (*this, "meshes")
            , nodes         (*this, "nodes")
            , samplers      (*this, "samplers")
            , scenes        (*this, "scenes")
            , skins         (*this, "skins")
            , textures      (*this, "textures")
        {
            memset(&extensionsUsed, 0, sizeof(extensionsUsed));
        }

        //! Main function
        void Load(const std::string& file, bool isBinary = false);

        //! Enables binary encoding on the asset
        void SetAsBinary();

        //! Search for an available name, starting from the given strings
        std::string FindUniqueID(const std::string& str, const char* suffix);

        Ref<Buffer> GetBodyBuffer()
            { return mBodyBuffer; }

    private:
        void ReadBinaryHeader(IOStream& stream, std::vector<char>& sceneData);

        void ReadExtensionsUsed(Document& doc);

        IOStream* OpenFile(std::string path, const char* mode, bool absolute = false);
    };

}

// Include the implementation of the methods
#include "glTF2Asset.inl"

#endif // ASSIMP_BUILD_NO_GLTF_IMPORTER

#endif // GLTF2ASSET_H_INC