/**
  * Updates the object model matrix and applies scaling.
  *
  * @param modelMatrix A 4x4 model-to-world transformation matrix, stored in column-major order.
  * @param scaleFactor A separate scaling factor to apply before the {@code modelMatrix}.
  * @see android.opengl.Matrix
  */
 public void updateModelMatrix(float[] modelMatrix, float scaleFactor)
 {
     float[] scaleMatrix = new float[16];
     Matrix.SetIdentityM(scaleMatrix, 0);
     scaleMatrix[0]  = scaleFactor;
     scaleMatrix[5]  = scaleFactor;
     scaleMatrix[10] = scaleFactor;
     Matrix.MultiplyMM(mModelMatrix, 0, modelMatrix, 0, scaleMatrix, 0);
 }
        /**
         * Creates and initializes OpenGL resources needed for rendering the model.
         *
         * @param context Context for loading the shader and below-named model and texture assets.
         * @param objAssetName  Name of the OBJ file containing the model geometry.
         * @param diffuseTextureAssetName  Name of the PNG file containing the diffuse texture map.
         */
        public void CreateOnGlThread(Context context, string objAssetName, string diffuseTextureAssetName)
        {
            // Read the texture.
            var textureBitmap = BitmapFactory.DecodeStream(context.Assets.Open(diffuseTextureAssetName));

            GLES20.GlActiveTexture(GLES20.GlTexture0);
            GLES20.GlGenTextures(mTextures.Length, mTextures, 0);
            GLES20.GlBindTexture(GLES20.GlTexture2d, mTextures[0]);

            GLES20.GlTexParameteri(GLES20.GlTexture2d,
                                   GLES20.GlTextureMinFilter, GLES20.GlLinearMipmapLinear);
            GLES20.GlTexParameteri(GLES20.GlTexture2d,
                                   GLES20.GlTextureMagFilter, GLES20.GlLinear);
            GLUtils.TexImage2D(GLES20.GlTexture2d, 0, textureBitmap, 0);
            GLES20.GlGenerateMipmap(GLES20.GlTexture2d);
            GLES20.GlBindTexture(GLES20.GlTexture2d, 0);

            textureBitmap.Recycle();

            ShaderUtil.CheckGLError(TAG, "Texture loading");

            // Read the obj file.
            var objInputStream = context.Assets.Open(objAssetName);
            var obj            = ObjReader.Read(objInputStream);

            // Prepare the Obj so that its structure is suitable for
            // rendering with OpenGL:
            // 1. Triangulate it
            // 2. Make sure that texture coordinates are not ambiguous
            // 3. Make sure that normals are not ambiguous
            // 4. Convert it to single-indexed data
            obj = ObjUtils.ConvertToRenderable(obj);

            // OpenGL does not use Java arrays. ByteBuffers are used instead to provide data in a format
            // that OpenGL understands.

            // Obtain the data from the OBJ, as direct buffers:
            IntBuffer   wideIndices = ObjData.GetFaceVertexIndices(obj, 3);
            FloatBuffer vertices    = ObjData.GetVertices(obj);
            FloatBuffer texCoords   = ObjData.GetTexCoords(obj, 2);
            FloatBuffer normals     = ObjData.GetNormals(obj);

            // Convert int indices to shorts for GL ES 2.0 compatibility
            ShortBuffer indices = ByteBuffer.AllocateDirect(2 * wideIndices.Limit())
                                  .Order(ByteOrder.NativeOrder()).AsShortBuffer();

            while (wideIndices.HasRemaining)
            {
                indices.Put((short)wideIndices.Get());
            }
            indices.Rewind();

            var buffers = new int[2];

            GLES20.GlGenBuffers(2, buffers, 0);
            mVertexBufferId = buffers[0];
            mIndexBufferId  = buffers[1];

            // Load vertex buffer
            mVerticesBaseAddress  = 0;
            mTexCoordsBaseAddress = mVerticesBaseAddress + 4 * vertices.Limit();
            mNormalsBaseAddress   = mTexCoordsBaseAddress + 4 * texCoords.Limit();
            int totalBytes = mNormalsBaseAddress + 4 * normals.Limit();

            GLES20.GlBindBuffer(GLES20.GlArrayBuffer, mVertexBufferId);
            GLES20.GlBufferData(GLES20.GlArrayBuffer, totalBytes, null, GLES20.GlStaticDraw);
            GLES20.GlBufferSubData(
                GLES20.GlArrayBuffer, mVerticesBaseAddress, 4 * vertices.Limit(), vertices);
            GLES20.GlBufferSubData(
                GLES20.GlArrayBuffer, mTexCoordsBaseAddress, 4 * texCoords.Limit(), texCoords);
            GLES20.GlBufferSubData(
                GLES20.GlArrayBuffer, mNormalsBaseAddress, 4 * normals.Limit(), normals);
            GLES20.GlBindBuffer(GLES20.GlArrayBuffer, 0);

            // Load index buffer
            GLES20.GlBindBuffer(GLES20.GlElementArrayBuffer, mIndexBufferId);
            mIndexCount = indices.Limit();
            GLES20.GlBufferData(
                GLES20.GlElementArrayBuffer, 2 * mIndexCount, indices, GLES20.GlStaticDraw);
            GLES20.GlBindBuffer(GLES20.GlElementArrayBuffer, 0);

            ShaderUtil.CheckGLError(TAG, "OBJ buffer load");

            int vertexShader = ShaderUtil.LoadGLShader(TAG, context,
                                                       GLES20.GlVertexShader, Resource.Raw.object_vertex);
            int fragmentShader = ShaderUtil.LoadGLShader(TAG, context,
                                                         GLES20.GlFragmentShader, Resource.Raw.object_fragment);

            mProgram = GLES20.GlCreateProgram();
            GLES20.GlAttachShader(mProgram, vertexShader);
            GLES20.GlAttachShader(mProgram, fragmentShader);
            GLES20.GlLinkProgram(mProgram);
            GLES20.GlUseProgram(mProgram);

            ShaderUtil.CheckGLError(TAG, "Program creation");

            mModelViewUniform           = GLES20.GlGetUniformLocation(mProgram, "u_ModelView");
            mModelViewProjectionUniform =
                GLES20.GlGetUniformLocation(mProgram, "u_ModelViewProjection");

            mPositionAttribute = GLES20.GlGetAttribLocation(mProgram, "a_Position");
            mNormalAttribute   = GLES20.GlGetAttribLocation(mProgram, "a_Normal");
            mTexCoordAttribute = GLES20.GlGetAttribLocation(mProgram, "a_TexCoord");

            mTextureUniform = GLES20.GlGetUniformLocation(mProgram, "u_Texture");

            mLightingParametersUniform = GLES20.GlGetUniformLocation(mProgram, "u_LightingParameters");
            mMaterialParametersUniform = GLES20.GlGetUniformLocation(mProgram, "u_MaterialParameters");

            ShaderUtil.CheckGLError(TAG, "Program parameters");

            Matrix.SetIdentityM(mModelMatrix, 0);
        }