Add data layout description to README.

README.md

@@ -97,7 +97,7 @@ TinyObjLoader is successfully used in ...
 
 * [ ] Fix obj_sticker example.
 * [ ] More unit test codes.
-* [ ] Texture options
+* [x] Texture options
 * [ ] Normal vector generation
   * [ ] Support smoothing groups
 
@@ -107,10 +107,76 @@ Licensed under MIT license.
 
 ## Usage
 
+### Data format
+
 `attrib_t` contains single and linear array of vertex data(position, normal and texcoord).
+
+```
+attrib_t::vertices => 3 floats per vertex
+
+       v[0]        v[1]        v[2]        v[3]               v[n-1]
+  +-----------+-----------+-----------+-----------+      +-----------+
+  | x | y | z | x | y | z | x | y | z | x | y | z | .... | x | y | z |
+  +-----------+-----------+-----------+-----------+      +-----------+
+
+attrib_t::normals => 3 floats per vertex
+
+       n[0]        n[1]        n[2]        n[3]               n[n-1]
+  +-----------+-----------+-----------+-----------+      +-----------+
+  | x | y | z | x | y | z | x | y | z | x | y | z | .... | x | y | z |
+  +-----------+-----------+-----------+-----------+      +-----------+
+
+attrib_t::texcoords => 2 floats per vertex
+
+       t[0]        t[1]        t[2]        t[3]               t[n-1]
+  +-----------+-----------+-----------+-----------+      +-----------+
+  |  u  |  v  |  u  |  v  |  u  |  v  |  u  |  v  | .... |  u  |  v  |
+  +-----------+-----------+-----------+-----------+      +-----------+
+
+```
+
 Each `shape_t` does not contain vertex data but contains array index to `attrib_t`.
 See `loader_example.cc` for more details.
 
+
+```
+
+mesh_t::indices => array of vertex indices.
+
+  +----+----+----+----+----+----+----+----+----+----+     +--------+
+  | i0 | i1 | i2 | i3 | i4 | i5 | i6 | i7 | i8 | i9 | ... | i(n-1) |
+  +----+----+----+----+----+----+----+----+----+----+     +--------+
+
+Each index has an array index to attrib_t::vertices, attrib_t::normals and attrib_t::texcoords.
+
+mesh_t::num_face_vertices => array of the number of vertices per face(e.g. 3 = triangle, 4 = quad , 5 or more = N-gons).
+
+
+  +---+---+---+        +---+
+  | 3 | 4 | 3 | ...... | 3 |
+  +---+---+---+        +---+
+    |   |   |            |
+    |   |   |            +-----------------------------------------+
+    |   |   |                                                      |
+    |   |   +------------------------------+                       |
+    |   |                                  |                       |
+    |   +------------------+               |                       |
+    |                      |               |                       |
+    |/                     |/              |/                      |/
+
+ mesh_t::indices
+
+  |    face[0]   |       face[1]     |    face[2]   |     |      face[n-1]           |
+  +----+----+----+----+----+----+----+----+----+----+     +--------+--------+--------+
+  | i0 | i1 | i2 | i3 | i4 | i5 | i6 | i7 | i8 | i9 | ... | i(n-3) | i(n-2) | i(n-1) | 
+  +----+----+----+----+----+----+----+----+----+----+     +--------+--------+--------+
+
+```
+
+Note that when `triangulate` flas is true in `tinyobj::LoadObj()` argument, `num_face_vertices` are all filled with 3(triangle).
+
+#### Example code
+
 ```c++
 #define TINYOBJLOADER_IMPLEMENTATION // define this in only *one* .cc
 #include "tiny_obj_loader.h"