Indices of a obj file format

Question

If I have the obj file like below, what would be my indices. How do I find the indices? What would be the order of the indices also.(the triangles)

v 1.000000 0.000000 -1.000000

v -1.000000 0.000000 -1.000000

v 1.000000 0.000000 1.000000

v -1.000000 0.000000 1.000000


vt 0.9562 0.0001

vt 0.0001 0.9999

vt 0.0001 0.0001

vt 0.9562 0.9999


vn 0.0000 1.0000 0.0000

f 2/1/1 3/2/1 1/3/1

f 2/1/1 4/4/1 3/2/1

Answer 1

Wavefront files don't contain the indices in a traditional sense. The lines starting with "f" are the closest one you get. Each one of those marks a triangle with 3 vertices. These vertices are represented in the v/vt/n format. The first value is the id of the vertex, the second is the uv coordinate and the third is the normal.

Neither D3D, nor OpenGL can deal with a format like this, so you'll have to manually convert it to the preferred format.

This makes the already pretty large file a bit smaller, because for example a flat shaded cube would require 24 different vertex declarations if you use the traditional way, but only 8 this way.

Answer 2

//================needed classes or structs =================
   public struct Vertex 
    {
        private Vector3 position;
        private Vector3 normal;
        private Vector2 textureCoords;
        public Vertex(Vector3 position, Vector3 normal, Vector2 textureCoords )
        {
            this.normal = normal;
            this.position = position;
            this.textureCoords = textureCoords;

        }

        public static int Size => (3 + 3 + 2) * 4;  

    }


 public class OBJLoader
    {

   public static RawModel LoadObjModel(String name)
        {
            string path = @"Models\" + name + ".obj";
            List<Vertex> Vertices = new List<Vertex>();
            List<string> VertexString = new List<string>();
            List<int> Indices = new List<int>();
            List<Vector3> Positions = new List<Vector3>();
            List<Vector2> TextureCoordinates = new List<Vector2>();
            List<Vector3> Normals = new List<Vector3>();
            List<string> Lines = new List<string>();
            string Line;
            System.IO.StreamReader file = new System.IO.StreamReader(path);
            while ((Line = file.ReadLine()) != null)
            {
                Lines.Add(Line);
            }
            file.Close();
            string FacePattern = "^f .*$";
            string PositionPattern = "^v .*$";
            string TextureCorrdinatesPattern = "^vt .*$";
            string NormalPattern = "^vn .*$";
            string FloatingPointPattern = @"[-+]?[0-9]*\.?[0-9]+";
            string VertexFacePattern = @"[0-9]*/[0-9]*/[0-9]*";
            string NumberPatten = @"\d+";
            MatchCollection FloatingPoints = null;
            MatchCollection FaceVertices = null;
            Lines.Where(L => Regex.IsMatch(L, PositionPattern)).ToList().ForEach(L =>
            {
                FloatingPoints = Regex.Matches(L, FloatingPointPattern);
                Positions.Add(new Vector3(float.Parse(FloatingPoints[0].ToString()), float.Parse(FloatingPoints[1].ToString()), float.Parse(FloatingPoints[2].ToString())));
            });
            Lines.Where(L => Regex.IsMatch(L, NormalPattern)).ToList().ForEach(L =>
            {
                FloatingPoints = Regex.Matches(L, FloatingPointPattern);
                Normals.Add(new Vector3(float.Parse(FloatingPoints[0].ToString()), float.Parse(FloatingPoints[1].ToString()), float.Parse(FloatingPoints[2].ToString())));
            });
            Lines.Where(L => Regex.IsMatch(L, TextureCorrdinatesPattern)).ToList().ForEach(L =>
            {
                FloatingPoints = Regex.Matches(L, FloatingPointPattern);
                TextureCoordinates.Add(new Vector2(float.Parse(FloatingPoints[0].ToString()), float.Parse(FloatingPoints[1].ToString())));
            });
            Lines.Where(L => Regex.IsMatch(L, FacePattern)).ToList().ForEach(L =>
            {
                FaceVertices = Regex.Matches(L, VertexFacePattern);
                VertexString.Add(FaceVertices[0].ToString());
                VertexString.Add(FaceVertices[1].ToString());
                VertexString.Add(FaceVertices[2].ToString());
            });
            List<string> AllVerticesStrings = new List<string>();
            AllVerticesStrings = VertexString;
            VertexString = VertexString.Distinct().ToList();
            MatchCollection indices = null;
            VertexString.ForEach(Vertex =>
            {
                indices = Regex.Matches(Vertex, NumberPatten);
                Vertices.Add(new VertexData.Vertex(Positions[int.Parse(indices[0].ToString()) - 1], Normals[int.Parse(indices[2].ToString()) - 1], TextureCoordinates[int.Parse(indices[1].ToString()) - 1]));
            });
            Dictionary<string, int> IndexKeyDictionary = new Dictionary<string, int>();
            for (int i = 0; i < VertexString.Count; i++)
            {
                IndexKeyDictionary.Add(VertexString[i], i);
            }
            AllVerticesStrings.ToList().ForEach(Key => Indices.Add(IndexKeyDictionary[Key]));
            return new RawModel(Vertices.ToArray(), Indices.ToArray());
        }

}