Draw wrong faces in obj model with OpenGL VBO

Question

I am trying to draw a 3D model from an .obj file, and have met big trouble. I think my model draws with wrong faces, but I can't understand why.

Original model:

This is my code for reading the model:

        using (StreamReader streamReader = new StreamReader("Content/Models/" + path))
        {
            while (!streamReader.EndOfStream)
            {
                List<string> words = new List<string>(streamReader.ReadLine().ToLower().Split(' '));
                words.RemoveAll(s => s == string.Empty);

                if (words.Count == 0)
                    continue;

                string type = words[0];
                words.RemoveAt(0);

                switch (type)
                {
                    // vertex
                    case "v":
                        vertices.Add(new Vector4(float.Parse(words[0], new CultureInfo("en-US").NumberFormat),
                            float.Parse(words[1], new CultureInfo("en-US").NumberFormat),
                            float.Parse(words[2], new CultureInfo("en-US").NumberFormat),
                            words.Count < 4 ? 1 : float.Parse(words[3], new CultureInfo("en-US").NumberFormat)));
                        break;

                    case "vt":
                        textureVertices.Add(new Vector3(float.Parse(words[0], new CultureInfo("en-US").NumberFormat),
                            float.Parse(words[1], new CultureInfo("en-US").NumberFormat),
                            words.Count < 3 ? 0 : float.Parse(words[2], new CultureInfo("en-US").NumberFormat)));
                        break;

                    case "vn":
                        normals.Add(new Vector3(float.Parse(words[0], new CultureInfo("en-US").NumberFormat),
                            float.Parse(words[1], new CultureInfo("en-US").NumberFormat),
                            float.Parse(words[2], new CultureInfo("en-US").NumberFormat)));
                        break;

                    // face
                    case "f":
                        foreach (string w in words)
                        {
                            if (w.Length == 0)
                                continue;

                            string[] comps = w.Split('/');

                            // subtract 1: indices start from 1, not 0
                            vertexIndices.Add(uint.Parse(comps[0]) - 1);

                            if (comps.Length > 1 && comps[1].Length != 0)
                                textureIndices.Add(uint.Parse(comps[1]) - 1);

                            if (comps.Length > 2)
                                normalIndices.Add(uint.Parse(comps[2]) - 1);
                        }
                        break;

                    default:
                        break;
                }
            }
        }

This is code is also available on GitHub - github.com/Freaky-Brainstorm/Deep.

Answer 1

There are better ways than what I show here, but if you're working on a small game (aka.: you're not working on an A++ game) then you'll be fine with this:

The part where you extract the vertex, texture and the normal information from the file is good and should work, but you need to rearrange them so the information for the same vertex should have the same index.

So, I assume you have three lists, vertices for the vertices (obviously) the textureVertices for the UV coords, and the normals for the normals.

The output lists are going to be verts, texs, norms and indices for the vertex positions, UV coords, normals and indices respectively.

vertexIndex := 0
// Processing stuff for the vertices, UV coords and normals
// the "row" string contains the current row
case "f":

   data := split(row, " ")
   // data: ["f", "1/2/3", "4/5/6", "7/8/9"]
   for (i = 1; i <= 3) do
       info := split(data[i], "/")
       // info: ["1", "2", "3"]
       verts.add(vertices[parse(info[0] - 1)])       // Rearrange vertex data
       texs.add(textureVertices[parse(info[1] - 1)])
       norms.add(normals[parse(info[2] - 1)])
       indices.add(vertexIndex)                      // Add the index for the vertex (if you use drawArrays, you can leave out this part)
       vertexIndex = vertexIndex + 1
   end

   break