split geometry into its own files

src/glTF2Sharp.DOM/README.md

@@ -0,0 +1,51 @@
+# Mesh Building
+
+One of key aspects of building a GPU optimized gltf model is to ensure that all
+meshes in the model share as few vertex and index buffers as possible; in this
+way, the number of state changes required to render a particular model is
+reduced to a minimum.
+
+Given the hierarchical nature of gltf, it is very difficult to populate the meshes
+of a gltf model one by one, since we would need to "grow" the target buffers and
+buffer views, which is far from trivial.
+
+So, in order to solve the problem, we could have an external structure that could
+prepare all the stuff required to fill the data; example:
+
+```c#
+class VertexColumn
+{
+    private string _Attribute;
+    private boolean _Normalized;    
+    private encoding _Encoding;    
+    private dimensions _Dimensions;    
+    private readonly List<Vector4> _Rows = new List<Vector4>();
+}
+
+class Indices
+{
+    private encoding;
+    private readonly List<int> _Rows = new List<int();
+    private primitiveType;
+}
+
+class MeshPrimitive
+{
+    private VertexBufferCreationMode _VbCreationMode; // split columns, interleaved, etc
+    private List<VertexColumn> _Columns = new List<VertexColumn>();
+    private Indices _Indices;
+    private int _MaterialIndex;
+
+    // morphing? how
+}
+
+
+
+```
+
+
+
+
+
+
+ 

src/glTF2Sharp.DOM/Schema2/gltf.Geometry.cs

@@ -1,464 +0,0 @@
-using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Numerics;
-using System.Text;
-
-namespace glTF2Sharp.Schema2
-{
-    using Collections;
-
-    using ROOT = ModelRoot;
-
-    [System.Diagnostics.DebuggerDisplay("MeshPrimitive[{LogicalIndex}] {_mode} {_DebuggerDisplay_TryIdentifyContent()}")]
-    public partial class MeshPrimitive : IChildOf<Mesh>
-    {
-        #region lifecycle
-
-        internal MeshPrimitive()
-        {
-            _attributes = new Dictionary<string, int>();
-            _targets = new List<Dictionary<string, int>>();
-        }        
-
-        #endregion
-
-        #region properties
-
-        public int LogicalIndex => this.LogicalParent.Primitives.IndexOfReference(this);
-
-        public Mesh LogicalParent { get; private set; }
-
-        void IChildOf<Mesh>._SetLogicalParent(Mesh parent) { LogicalParent = parent; }
-
-        public Material Material
-        {
-            get => this._material.HasValue ? LogicalParent.LogicalParent._LogicalMaterials[this._material.Value] : null;
-            set
-            {
-                if (value != null) Guard.MustShareLogicalParent(LogicalParent.LogicalParent, value, nameof(value));
-
-                this._material = value == null ? (int?)null : value.LogicalIndex;
-            }
-        }
-
-        public PrimitiveType DrawPrimitiveType
-        {
-            get => this._mode.AsValue(_modeDefault);
-            set => this._mode = value.AsNullable(_modeDefault);
-        }
-
-        public int MorpthTargets => _targets.Count;
-
-        public BoundingBox3? LocalBounds3 => VertexAccessors["POSITION"]?.LocalBounds3;
-
-        public IReadOnlyDictionary<String, Accessor> VertexAccessors => new ReadOnlyLinqDictionary<String, int, Accessor>(_attributes, alidx => this.LogicalParent.LogicalParent._LogicalAccessors[alidx]);
-
-        public Accessor IndexAccessor
-        {
-            get
-            {
-                if (!this._indices.HasValue) return null;
-
-                return this.LogicalParent.LogicalParent._LogicalAccessors[this._indices.Value];
-            }
-            set
-            {
-                if (value == null) this._indices = null;
-                else
-                {
-                    Guard.MustShareLogicalParent(this.LogicalParent.LogicalParent, value,nameof(value));
-                    this._indices = value.LogicalIndex;
-                }
-            }
-        }
-
-        #endregion
-
-        #region API
-
-        public Accessor GetVertexAccessor(string attributeKey)
-        {
-            Guard.NotNullOrEmpty(attributeKey, nameof(attributeKey));
-
-            if (!_attributes.TryGetValue(attributeKey, out int idx)) return null;
-
-            return this.LogicalParent.LogicalParent._LogicalAccessors[idx];
-        }
-
-        public void SetVertexAccessor(string attributeKey, Accessor accessor)
-        {
-            Guard.NotNullOrEmpty(attributeKey, nameof(attributeKey));
-
-            if (accessor != null)
-            {
-                Guard.MustShareLogicalParent(this.LogicalParent.LogicalParent, accessor, nameof(accessor));
-                _attributes[attributeKey] = accessor.LogicalIndex;                
-            }
-            else
-            {
-                _attributes.Remove(attributeKey);
-            }
-        }        
-
-        public IReadOnlyDictionary<String, Accessor> GetMorphTargetAccessors(int idx)
-        {
-            return new ReadOnlyLinqDictionary<String, int, Accessor>(_targets[idx], alidx => this.LogicalParent.LogicalParent._LogicalAccessors[alidx]);
-        }
-
-        public void SetMorphTargetAccessors(int idx, IReadOnlyDictionary<String, Accessor> accessors)
-        {
-            Guard.NotNull(accessors, nameof(accessors));
-            foreach (var kvp in accessors)
-            {
-                Guard.MustShareLogicalParent(this.LogicalParent, kvp.Value, nameof(accessors));
-            }
-
-            while (_targets.Count <= idx) _targets.Add(new Dictionary<string, int>());
-
-            var target = _targets[idx];
-
-            target.Clear();
-
-            foreach (var kvp in accessors)
-            {
-                target[kvp.Key] = kvp.Value.LogicalIndex;
-            }
-        }
-
-        public IEnumerable<BufferView> GetBufferViews(bool includeIndices, bool includeVertices, bool includeMorphs)
-        {            
-            var accessors = new List<Accessor>();
-
-            var attributes = this._attributes.Keys.ToArray();
-
-            if (includeIndices)
-            {                
-                if (IndexAccessor != null) accessors.Add(IndexAccessor);
-            }
-
-            if (includeVertices)
-            {
-                accessors.AddRange(attributes.Select(k => VertexAccessors[k]));
-            }
-
-            if (includeMorphs)
-            {
-                for (int i = 0; i < MorpthTargets; ++i)
-                {
-                    foreach(var key in attributes)
-                    {
-                        var morpthAccessors = GetMorphTargetAccessors(i);
-                        if (morpthAccessors.TryGetValue(key, out Accessor accessor)) accessors.Add(accessor);
-                    }                    
-                }
-            }
-
-            var indices = accessors
-                .Select(item => item._LogicalBufferViewIndex)
-                .Where(item => item >= 0)
-                .Distinct();
-
-            return indices.Select(idx => this.LogicalParent.LogicalParent.LogicalBufferViews[idx]);            
-        }
-
-        public IReadOnlyList<KeyValuePair<String, Accessor>> GetVertexAccessorsByBuffer(BufferView vb)
-        {
-            Guard.NotNull(vb, nameof(vb));
-            Guard.MustShareLogicalParent(this.LogicalParent, vb, nameof(vb));
-
-            return VertexAccessors
-                .Where(key => key.Value.Buffer == vb)
-                .OrderBy(item => item.Value.ByteOffset)
-                .ToArray();
-        }
-
-        private Accessor _GetAccessor(IReadOnlyDictionary<string, int> attributes, string attribute)
-        {
-            if (!attributes.TryGetValue(attribute, out int idx)) return null;
-
-            return this.LogicalParent.LogicalParent._LogicalAccessors[idx];
-        }
-
-        private String _DebuggerDisplay_TryIdentifyContent()
-        {
-            return String.Join(" ", VertexAccessors.Keys);
-        }
-
-        #endregion
-
-        #region validation
-
-        public override IEnumerable<Exception> Validate()
-        {
-            var exx = base.Validate().ToList();
-
-            // Number of vertices or indices(1) is not compatible with used drawing mode('TRIANGLES').
-
-            var idxAccessor = IndexAccessor;
-
-            if (idxAccessor != null)
-            {
-                switch (DrawPrimitiveType)
-                {
-                    case PrimitiveType.TRIANGLES:
-                        if ((idxAccessor.Count % 3) != 0) exx.Add(new ModelException(this, $"Indices count {idxAccessor.Count} incompatible with Primitive.{DrawPrimitiveType}"));
-                        break;
-
-                }
-            }
-
-            return exx;
-        }
-
-        #endregion
-    }
-
-    [System.Diagnostics.DebuggerDisplay("Mesh[{LogicalIndex}] {Name}")]
-    public partial class Mesh
-    {
-        #region lifecycle
-
-        internal Mesh()
-        {
-            _primitives = new ChildrenCollection<MeshPrimitive, Mesh>(this);
-            _weights = new List<double>();
-        }
-
-        #endregion
-
-        #region properties        
-
-        public int LogicalIndex => this.LogicalParent.LogicalMeshes.IndexOfReference(this);
-
-        public IEnumerable<Node> VisualParents => Node.GetNodesUsingMesh(this);
-
-        public IReadOnlyList<MeshPrimitive> Primitives => _primitives;
-
-        public IReadOnlyList<float> MorphWeights => _weights.Select(item => (float)item).ToArray();
-
-        public MeshPrimitive CreatePrimitive()
-        {
-            var mp = new MeshPrimitive();
-
-            _primitives.Add(mp);
-
-            return mp;
-        }
-
-        internal MeshPrimitive _AddPrimitive(IReadOnlyDictionary<string, Accessor> attributes, Accessor indices, PrimitiveType ptype)
-        {
-            if (attributes == null) throw new ArgumentNullException(nameof(attributes));
-            if (indices == null) throw new ArgumentNullException(nameof(indices));
-            if (!this.SharesLogicalParent(attributes.Values.ToArray())) throw new ArgumentException("Root mismatch",nameof(attributes));
-            if (!this.SharesLogicalParent(indices)) throw new ArgumentException("Root mismatch", nameof(indices));
-
-            // we can also check for Accessor ByteOffset match, padding, etc
-
-            // check parenting
-
-            var mp = new MeshPrimitive();
-
-            _primitives.Add(mp);
-
-            foreach(var kvp in attributes)
-            {
-                mp.SetVertexAccessor(kvp.Key, kvp.Value);
-            }
-
-            mp.IndexAccessor = indices;
-            mp.DrawPrimitiveType = ptype;
-
-            return mp;
-        }
-
-        public BoundingBox3? LocalBounds3 => BoundingBox3.UnionOf(Primitives.Select(item => item.LocalBounds3));
-
-        #endregion
-
-        #region API
-
-        /*
-        internal void _AddPrimitive(MeshPrimitiveBuilder primitive, IReadOnlyDictionary<_DataBuffer, BufferView> sharedBuffers)
-        {
-            Guard.NotNull(primitive, nameof(primitive));
-            Guard.NotNullOrEmpty(sharedBuffers, nameof(sharedBuffers));
-
-            // create vertex accessors
-            var vAccessors = new Dictionary<string, Accessor>();
-            foreach (var vb in primitive._VertexBuffers)
-            {
-                var vbview = sharedBuffers[vb.Buffer];
-
-                foreach (var adesc in vb.Attributes)
-                {
-                    vAccessors[adesc.Name] = LogicalParent._CreateVertexAccessor(vbview, adesc, vb.ByteOffset, vb.Count);
-                }
-            }
-
-            // create index accessor
-            var ib = primitive._IndexBuffer;
-            var ibview = sharedBuffers[ib.Buffer];
-            var iAccessor = LogicalParent._CreateIndexAccessor(ibview, ib.Attributes[0], ib.ByteOffset, ib.Count);
-
-            // create primitive
-            this._AddPrimitive(vAccessors, iAccessor, primitive._PrimitiveType);
-        }*/
-
-        public override IEnumerable<Exception> Validate()
-        {
-            var exx = base.Validate().ToList();
-
-            foreach(var p in this.Primitives)
-            {
-                exx.AddRange(p.Validate());
-            }
-
-            return exx;
-        }
-
-        #endregion
-    }
-
-    [System.Diagnostics.DebuggerDisplay("Skin[{LogicalIndex}] {Name}")]
-    public partial class Skin
-    {
-        // https://github.com/KhronosGroup/glTF/issues/461
-        // https://github.com/KhronosGroup/glTF/issues/100
-        // https://github.com/KhronosGroup/glTF/issues/403
-        // https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Scene/Model.js#L2526
-
-        // max shader joints
-        // https://github.com/KhronosGroup/glTF/issues/283
-
-        #region lifecycle
-
-        internal Skin()
-        {
-            _joints = new List<int>();
-        }
-
-        #endregion
-
-        #region properties
-
-        public int LogicalIndex => this.LogicalParent._LogicalSkins.IndexOfReference(this);
-
-        public IEnumerable<Node> VisualParents => Node.GetNodesUsingSkin(this);
-
-        public int JointsCount => _joints.Count;
-
-        // Skeleton property points to the node that is the root of a joints hierarchy.
-        public Node Skeleton
-        {
-            get => this._skeleton.HasValue ? this.LogicalParent._LogicalNodes[this._skeleton.Value] : null;
-            set
-            {
-                if (value != null) Guard.MustShareLogicalParent(this.LogicalParent, value, nameof(value));
-                this._skeleton = value == null ? (int?)null : value.LogicalIndex;
-            }
-        }
-
-        #endregion
-
-        #region API
-
-        public static IEnumerable<Skin> GetSkinsUsing(Node n)
-        {
-            var idx = n.LogicalIndex;
-
-            return n.LogicalParent._LogicalSkins.Where(s => s._ContainsNode(idx));
-        }
-
-        internal bool _ContainsNode(int nodeIdx) { return _joints.Contains(nodeIdx); }        
-
-        public Accessor GetInverseBindMatricesAccessor()
-        {
-            if (!this._inverseBindMatrices.HasValue) return null;
-
-            return this.LogicalParent._LogicalAccessors[this._inverseBindMatrices.Value];
-        }        
-
-        public KeyValuePair<Node, Matrix4x4> GetJoint(int idx)
-        {
-            var nodeIdx = _joints[idx];
-
-            var node = this.LogicalParent._LogicalNodes[nodeIdx];
-
-            var matrix = (Matrix4x4)GetInverseBindMatricesAccessor().TryGetAttribute<Matrix4x4>()[idx];
-
-            return new KeyValuePair<Node, Matrix4x4>(node, matrix);
-        }
-
-        public override IEnumerable<Exception> Validate()
-        {
-            var exx = base.Validate().ToList();
-
-            // note: this check will fail if the buffers are not set
-
-            /*
-            for(int i=0; i < _joints.Count; ++i)
-            {
-                var j = GetJoint(i);
-
-                var invXform = j.Value;
-
-                if (invXform.M44 != 1) exx.Add(new ModelException(this, $"Joint {i} has invalid inverse matrix"));                
-            }*/
-
-            return exx;
-        }
-
-        public bool IsMatch(Node skeleton, KeyValuePair<Node, Matrix4x4>[] joints)
-        {
-            if (!ReferenceEquals(skeleton, this.Skeleton)) return false;
-
-            if (joints.Length != this._joints.Count) return false;
-
-            for(int i=0; i < this._joints.Count; ++i)
-            {
-                var src = joints[i];
-                var dst = GetJoint(i);
-
-                if (!ReferenceEquals(src.Key, dst.Key)) return false;
-                if (src.Value != dst.Value) return false;
-            }
-
-            return true;
-        }
-
-        /*
-        public void BindJoints(KeyValuePair<Node, Matrix4x4>[] joints)
-        {
-            // inverse bind matrices accessor
-
-            var data = new Byte[joints.Length * 16 * 4];
-
-            var indexer = new Runtime.Encoding.Matrix4x4Indexer(data, 16 * 4, 0, Runtime.Encoding.PackedType.F32);
-
-            for(int i=0; i < joints.Length; ++i) { indexer[i] = joints[i].Value; }            
-
-            var accessor = LogicalParent._CreateDataAccessor(data, Runtime.Encoding.DimensionType.Matrix4x4, joints.Length);
-            this._inverseBindMatrices = accessor.LogicalIndex;
-
-            // joints
-
-            _joints.Clear();
-            _joints.AddRange(joints.Select(item => item.Key.LogicalIndex));
-
-        }*/
-
-        #endregion
-    }
-
-    public partial class ModelRoot
-    {
-        public Mesh CreateMesh()
-        {
-            var dstMesh = new Mesh();
-            this._meshes.Add(dstMesh);
-
-            return dstMesh;
-        }
-    }    
-}

src/glTF2Sharp.DOM/Schema2/gltf.Mesh.cs

@@ -0,0 +1,74 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Numerics;
+using System.Text;
+
+namespace glTF2Sharp.Schema2
+{
+    using Collections;    
+
+    [System.Diagnostics.DebuggerDisplay("Mesh[{LogicalIndex}] {Name}")]
+    public partial class Mesh
+    {
+        #region lifecycle
+
+        internal Mesh()
+        {
+            _primitives = new ChildrenCollection<MeshPrimitive, Mesh>(this);
+            _weights = new List<double>();
+        }
+
+        #endregion
+
+        #region properties        
+
+        public int LogicalIndex => this.LogicalParent.LogicalMeshes.IndexOfReference(this);
+
+        public IEnumerable<Node> VisualParents => Node.GetNodesUsingMesh(this);
+
+        public IReadOnlyList<MeshPrimitive> Primitives => _primitives;
+
+        public IReadOnlyList<float> MorphWeights => _weights.Select(item => (float)item).ToArray();
+
+        public MeshPrimitive CreatePrimitive()
+        {
+            var mp = new MeshPrimitive();
+
+            _primitives.Add(mp);
+
+            return mp;
+        }        
+
+        public BoundingBox3? LocalBounds3 => BoundingBox3.UnionOf(Primitives.Select(item => item.LocalBounds3));
+
+        #endregion
+
+        #region API        
+
+        public override IEnumerable<Exception> Validate()
+        {
+            var exx = base.Validate().ToList();
+
+            foreach (var p in this.Primitives)
+            {
+                exx.AddRange(p.Validate());
+            }
+
+            return exx;
+        }
+
+        #endregion
+    }
+
+    public partial class ModelRoot
+    {
+        public Mesh CreateMesh()
+        {
+            var dstMesh = new Mesh();
+            this._meshes.Add(dstMesh);
+
+            return dstMesh;
+        }
+    }
+}

src/glTF2Sharp.DOM/Schema2/gltf.MeshPrimitive.cs

@@ -0,0 +1,216 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Numerics;
+using System.Text;
+
+namespace glTF2Sharp.Schema2
+{
+    using Collections;
+
+    using ROOT = ModelRoot;
+
+    [System.Diagnostics.DebuggerDisplay("MeshPrimitive[{LogicalIndex}] {_mode} {_DebuggerDisplay_TryIdentifyContent()}")]
+    public partial class MeshPrimitive : IChildOf<Mesh>
+    {
+        #region lifecycle
+
+        internal MeshPrimitive()
+        {
+            _attributes = new Dictionary<string, int>();
+            _targets = new List<Dictionary<string, int>>();
+        }        
+
+        #endregion
+
+        #region properties
+
+        public int LogicalIndex => this.LogicalParent.Primitives.IndexOfReference(this);
+
+        public Mesh LogicalParent { get; private set; }
+
+        void IChildOf<Mesh>._SetLogicalParent(Mesh parent) { LogicalParent = parent; }
+
+        public Material Material
+        {
+            get => this._material.HasValue ? LogicalParent.LogicalParent._LogicalMaterials[this._material.Value] : null;
+            set
+            {
+                if (value != null) Guard.MustShareLogicalParent(LogicalParent.LogicalParent, value, nameof(value));
+
+                this._material = value == null ? (int?)null : value.LogicalIndex;
+            }
+        }
+
+        public PrimitiveType DrawPrimitiveType
+        {
+            get => this._mode.AsValue(_modeDefault);
+            set => this._mode = value.AsNullable(_modeDefault);
+        }
+
+        public int MorpthTargets => _targets.Count;
+
+        public BoundingBox3? LocalBounds3 => VertexAccessors["POSITION"]?.LocalBounds3;
+
+        public IReadOnlyDictionary<String, Accessor> VertexAccessors => new ReadOnlyLinqDictionary<String, int, Accessor>(_attributes, alidx => this.LogicalParent.LogicalParent._LogicalAccessors[alidx]);
+
+        public Accessor IndexAccessor
+        {
+            get
+            {
+                if (!this._indices.HasValue) return null;
+
+                return this.LogicalParent.LogicalParent._LogicalAccessors[this._indices.Value];
+            }
+            set
+            {
+                if (value == null) this._indices = null;
+                else
+                {
+                    Guard.MustShareLogicalParent(this.LogicalParent.LogicalParent, value,nameof(value));
+                    this._indices = value.LogicalIndex;
+                }
+            }
+        }
+
+        #endregion
+
+        #region API
+
+        public Accessor GetVertexAccessor(string attributeKey)
+        {
+            Guard.NotNullOrEmpty(attributeKey, nameof(attributeKey));
+
+            if (!_attributes.TryGetValue(attributeKey, out int idx)) return null;
+
+            return this.LogicalParent.LogicalParent._LogicalAccessors[idx];
+        }
+
+        public void SetVertexAccessor(string attributeKey, Accessor accessor)
+        {
+            Guard.NotNullOrEmpty(attributeKey, nameof(attributeKey));
+
+            if (accessor != null)
+            {
+                Guard.MustShareLogicalParent(this.LogicalParent.LogicalParent, accessor, nameof(accessor));
+                _attributes[attributeKey] = accessor.LogicalIndex;                
+            }
+            else
+            {
+                _attributes.Remove(attributeKey);
+            }
+        }        
+
+        public IReadOnlyDictionary<String, Accessor> GetMorphTargetAccessors(int idx)
+        {
+            return new ReadOnlyLinqDictionary<String, int, Accessor>(_targets[idx], alidx => this.LogicalParent.LogicalParent._LogicalAccessors[alidx]);
+        }
+
+        public void SetMorphTargetAccessors(int idx, IReadOnlyDictionary<String, Accessor> accessors)
+        {
+            Guard.NotNull(accessors, nameof(accessors));
+            foreach (var kvp in accessors)
+            {
+                Guard.MustShareLogicalParent(this.LogicalParent, kvp.Value, nameof(accessors));
+            }
+
+            while (_targets.Count <= idx) _targets.Add(new Dictionary<string, int>());
+
+            var target = _targets[idx];
+
+            target.Clear();
+
+            foreach (var kvp in accessors)
+            {
+                target[kvp.Key] = kvp.Value.LogicalIndex;
+            }
+        }
+
+        public IEnumerable<BufferView> GetBufferViews(bool includeIndices, bool includeVertices, bool includeMorphs)
+        {            
+            var accessors = new List<Accessor>();
+
+            var attributes = this._attributes.Keys.ToArray();
+
+            if (includeIndices)
+            {                
+                if (IndexAccessor != null) accessors.Add(IndexAccessor);
+            }
+
+            if (includeVertices)
+            {
+                accessors.AddRange(attributes.Select(k => VertexAccessors[k]));
+            }
+
+            if (includeMorphs)
+            {
+                for (int i = 0; i < MorpthTargets; ++i)
+                {
+                    foreach(var key in attributes)
+                    {
+                        var morpthAccessors = GetMorphTargetAccessors(i);
+                        if (morpthAccessors.TryGetValue(key, out Accessor accessor)) accessors.Add(accessor);
+                    }                    
+                }
+            }
+
+            var indices = accessors
+                .Select(item => item._LogicalBufferViewIndex)
+                .Where(item => item >= 0)
+                .Distinct();
+
+            return indices.Select(idx => this.LogicalParent.LogicalParent.LogicalBufferViews[idx]);            
+        }
+
+        public IReadOnlyList<KeyValuePair<String, Accessor>> GetVertexAccessorsByBuffer(BufferView vb)
+        {
+            Guard.NotNull(vb, nameof(vb));
+            Guard.MustShareLogicalParent(this.LogicalParent, vb, nameof(vb));
+
+            return VertexAccessors
+                .Where(key => key.Value.Buffer == vb)
+                .OrderBy(item => item.Value.ByteOffset)
+                .ToArray();
+        }
+
+        private Accessor _GetAccessor(IReadOnlyDictionary<string, int> attributes, string attribute)
+        {
+            if (!attributes.TryGetValue(attribute, out int idx)) return null;
+
+            return this.LogicalParent.LogicalParent._LogicalAccessors[idx];
+        }
+
+        private String _DebuggerDisplay_TryIdentifyContent()
+        {
+            return String.Join(" ", VertexAccessors.Keys);
+        }
+
+        #endregion
+
+        #region validation
+
+        public override IEnumerable<Exception> Validate()
+        {
+            var exx = base.Validate().ToList();
+
+            // Number of vertices or indices(1) is not compatible with used drawing mode('TRIANGLES').
+
+            var idxAccessor = IndexAccessor;
+
+            if (idxAccessor != null)
+            {
+                switch (DrawPrimitiveType)
+                {
+                    case PrimitiveType.TRIANGLES:
+                        if ((idxAccessor.Count % 3) != 0) exx.Add(new ModelException(this, $"Indices count {idxAccessor.Count} incompatible with Primitive.{DrawPrimitiveType}"));
+                        break;
+
+                }
+            }
+
+            return exx;
+        }
+
+        #endregion
+    }    
+}

src/glTF2Sharp.DOM/Schema2/gltf.Skin.cs

@@ -0,0 +1,145 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Numerics;
+using System.Text;
+
+namespace glTF2Sharp.Schema2
+{
+    using Collections;
+
+    using ROOT = ModelRoot;
+
+    [System.Diagnostics.DebuggerDisplay("Skin[{LogicalIndex}] {Name}")]
+    public partial class Skin
+    {
+        // https://github.com/KhronosGroup/glTF/issues/461
+        // https://github.com/KhronosGroup/glTF/issues/100
+        // https://github.com/KhronosGroup/glTF/issues/403
+        // https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Scene/Model.js#L2526
+
+        // max shader joints
+        // https://github.com/KhronosGroup/glTF/issues/283
+
+        #region lifecycle
+
+        internal Skin()
+        {
+            _joints = new List<int>();
+        }
+
+        #endregion
+
+        #region properties
+
+        public int LogicalIndex => this.LogicalParent._LogicalSkins.IndexOfReference(this);
+
+        public IEnumerable<Node> VisualParents => Node.GetNodesUsingSkin(this);
+
+        public int JointsCount => _joints.Count;
+
+        // Skeleton property points to the node that is the root of a joints hierarchy.
+        public Node Skeleton
+        {
+            get => this._skeleton.HasValue ? this.LogicalParent._LogicalNodes[this._skeleton.Value] : null;
+            set
+            {
+                if (value != null) Guard.MustShareLogicalParent(this.LogicalParent, value, nameof(value));
+                this._skeleton = value == null ? (int?)null : value.LogicalIndex;
+            }
+        }
+
+        #endregion
+
+        #region API
+
+        public static IEnumerable<Skin> GetSkinsUsing(Node n)
+        {
+            var idx = n.LogicalIndex;
+
+            return n.LogicalParent._LogicalSkins.Where(s => s._ContainsNode(idx));
+        }
+
+        internal bool _ContainsNode(int nodeIdx) { return _joints.Contains(nodeIdx); }
+
+        public Accessor GetInverseBindMatricesAccessor()
+        {
+            if (!this._inverseBindMatrices.HasValue) return null;
+
+            return this.LogicalParent._LogicalAccessors[this._inverseBindMatrices.Value];
+        }
+
+        public KeyValuePair<Node, Matrix4x4> GetJoint(int idx)
+        {
+            var nodeIdx = _joints[idx];
+
+            var node = this.LogicalParent._LogicalNodes[nodeIdx];
+
+            var matrix = (Matrix4x4)GetInverseBindMatricesAccessor().TryGetAttribute<Matrix4x4>()[idx];
+
+            return new KeyValuePair<Node, Matrix4x4>(node, matrix);
+        }
+
+        public override IEnumerable<Exception> Validate()
+        {
+            var exx = base.Validate().ToList();
+
+            // note: this check will fail if the buffers are not set
+
+            /*
+            for(int i=0; i < _joints.Count; ++i)
+            {
+                var j = GetJoint(i);
+
+                var invXform = j.Value;
+
+                if (invXform.M44 != 1) exx.Add(new ModelException(this, $"Joint {i} has invalid inverse matrix"));                
+            }*/
+
+            return exx;
+        }
+
+        public bool IsMatch(Node skeleton, KeyValuePair<Node, Matrix4x4>[] joints)
+        {
+            if (!ReferenceEquals(skeleton, this.Skeleton)) return false;
+
+            if (joints.Length != this._joints.Count) return false;
+
+            for (int i = 0; i < this._joints.Count; ++i)
+            {
+                var src = joints[i];
+                var dst = GetJoint(i);
+
+                if (!ReferenceEquals(src.Key, dst.Key)) return false;
+                if (src.Value != dst.Value) return false;
+            }
+
+            return true;
+        }
+
+        /*
+        public void BindJoints(KeyValuePair<Node, Matrix4x4>[] joints)
+        {
+            // inverse bind matrices accessor
+
+            var data = new Byte[joints.Length * 16 * 4];
+
+            var indexer = new Runtime.Encoding.Matrix4x4Indexer(data, 16 * 4, 0, Runtime.Encoding.PackedType.F32);
+
+            for(int i=0; i < joints.Length; ++i) { indexer[i] = joints[i].Value; }            
+
+            var accessor = LogicalParent._CreateDataAccessor(data, Runtime.Encoding.DimensionType.Matrix4x4, joints.Length);
+            this._inverseBindMatrices = accessor.LogicalIndex;
+
+            // joints
+
+            _joints.Clear();
+            _joints.AddRange(joints.Select(item => item.Key.LogicalIndex));
+
+        }*/
+
+        #endregion
+    }
+
+
+}

src/glTF2Sharp.Tests/Schema2/CreateModelTests.cs

@@ -31,8 +31,7 @@ namespace glTF2Sharp.Schema2
             var mesh = root.CreateMesh();
 
             var primitive = mesh.CreatePrimitive();
-            primitive.DrawPrimitiveType = PrimitiveType.TRIANGLES;
-            // primitive.Material = root.AddLogicalMaterial(typeof(MaterialPBRMetallicRoughness));
+            primitive.DrawPrimitiveType = PrimitiveType.TRIANGLES;            
 
             var positions = root.CreateVector3Buffer
                 (