OBJ files are handy for many reasons:
- they're text files
- well documented
- easy to parse
- and many professional softwares can export them
This is why they are so popular as standard files for 3D graphics.
Their structure is basically a (long) list of vectors definitions (spatial, normals, texture coordinates, and so on) followed by a (long) list of structures defining faces. What are faces? Faces are tuples of indexes, which point specific vertexes, normals and texture coordinate in the previous lists. At least three vertexes are needed for each face, because it forms a triangle, that is the smaller polygon we can create.
Why is this needed? Well, because a single vertex could be shared by many faces (=polygons). That's important in real-time 3D programming, because avoiding redundancy is good for performances!
This is the very same idea behind OpenGL's indexed vertex arrays: rather than sending to the video card a list of vertexes, normals and texture coordinates (the so called immediate mode) we prepare a couple of arrays filled with informations, and another array of indexes which tell OpenGL what vertex use in a certain primitive.
Sounds good, uh? Well, there is a couple of drawbacks actually.
The biggest is that every index points simultaneously at vertex, normals and texture arrays! For instance, if our first index is "1", that face points vertex, normal and texture_coord. That's a real nightmare, because there's no ensurance that our modeling softwares will produce .OBJ files featuring correctly ordered lists! Moreover, there's often a few of vertexes (which are often shared by many faces) and lot of normals!
So what?? Well, OpenGL does not aid us in any way: you gotta solve with your hands. My solution is the following:
- parse the OBJ file, loading vertexes, normals, and faces;
- from faces, create a definitive vertex index and a temporary arrays for normals and textures coordinates;
- for each i in number_of_indexes do
definitive_normal_array[ index_array[i] ] :=
Temporary_normals_array[ TemporaryNormalIndexes[i] ]
The most interesting way for drawing this series of arrays is the use of Vertex Buffer Objects (VBO). They're similar to the indexed vertex arrays, but have interesting features... like dropping geometries in the fastest video card memory and use it! The boost of performances is impressive, and they are easy enough to deal with.
My PLTmesh::Draw() method is this:
Quite simple and clear! :)// PLT_VBOnames are references to the VBOS - see glGenBuffers()
glNormalPointer(GL_FLOAT, 0, NULL);
glVertexPointer(3, GL_FLOAT, 0, (char *) NULL );
glDrawElements(GL_TRIANGLES, this->IndexesArray.size(), GL_UNSIGNED_INT, (GLvoid*)((char*)NULL));