object loader in opengl

by 
// Get the number of vertices
obj_file >> md->vertices;
// Get the number of faces
obj_file >> md->faces;

Read the spec again. That’s not how OBJs work. You have to parse out the vertexes/texture coordinates/normals/faces as you go along.

Use something like this:

#include <GL/glut.h>
#include <glm/glm.hpp>
#define GLM_ENABLE_EXPERIMENTAL 
#include <glm/gtx/component_wise.hpp>
#include <vector>
#include <fstream>
#include <sstream>

struct Vertex
{
    glm::vec3 position;
    glm::vec2 texcoord;
    glm::vec3 normal;
};

struct VertRef
{
    VertRef( int v, int vt, int vn ) : v(v), vt(vt), vn(vn) { }
    int v, vt, vn;
};

std::vector< Vertex > LoadOBJ( std::istream& in )
{
    std::vector< Vertex > verts;

    std::vector< glm::vec4 > positions( 1, glm::vec4( 0, 0, 0, 0 ) );
    std::vector< glm::vec3 > texcoords( 1, glm::vec3( 0, 0, 0 ) );
    std::vector< glm::vec3 > normals( 1, glm::vec3( 0, 0, 0 ) );
    std::string lineStr;
    while( std::getline( in, lineStr ) )
    {
        std::istringstream lineSS( lineStr );
        std::string lineType;
        lineSS >> lineType;

        // vertex
        if( lineType == "v" )
        {
            float x = 0, y = 0, z = 0, w = 1;
            lineSS >> x >> y >> z >> w;
            positions.push_back( glm::vec4( x, y, z, w ) );
        }

        // texture
        if( lineType == "vt" )
        {
            float u = 0, v = 0, w = 0;
            lineSS >> u >> v >> w;
            texcoords.push_back( glm::vec3( u, v, w ) );
        }

        // normal
        if( lineType == "vn" )
        {
            float i = 0, j = 0, k = 0;
            lineSS >> i >> j >> k;
            normals.push_back( glm::normalize( glm::vec3( i, j, k ) ) );
        }

        // polygon
        if( lineType == "f" )
        {
            std::vector< VertRef > refs;
            std::string refStr;
            while( lineSS >> refStr )
            {
                std::istringstream ref( refStr );
                std::string vStr, vtStr, vnStr;
                std::getline( ref, vStr, "https://stackoverflow.com/" );
                std::getline( ref, vtStr, "https://stackoverflow.com/" );
                std::getline( ref, vnStr, "https://stackoverflow.com/" );
                int v = atoi( vStr.c_str() );
                int vt = atoi( vtStr.c_str() );
                int vn = atoi( vnStr.c_str() );
                v  = (  v >= 0 ?  v : positions.size() +  v );
                vt = ( vt >= 0 ? vt : texcoords.size() + vt );
                vn = ( vn >= 0 ? vn : normals.size()   + vn );
                refs.push_back( VertRef( v, vt, vn ) );
            }

            // triangulate, assuming n>3-gons are convex and coplanar
            for( size_t i = 1; i+1 < refs.size(); ++i )
            {
                const VertRef* p[3] = { &refs[0], &refs[i], &refs[i+1] };

                // http://www.opengl.org/wiki/Calculating_a_Surface_Normal
                glm::vec3 U( positions[ p[1]->v ] - positions[ p[0]->v ] );
                glm::vec3 V( positions[ p[2]->v ] - positions[ p[0]->v ] );
                glm::vec3 faceNormal = glm::normalize( glm::cross( U, V ) );

                for( size_t j = 0; j < 3; ++j )
                {
                    Vertex vert;
                    vert.position = glm::vec3( positions[ p[j]->v ] );
                    vert.texcoord = glm::vec2( texcoords[ p[j]->vt ] );
                    vert.normal = ( p[j]->vn != 0 ? normals[ p[j]->vn ] : faceNormal );
                    verts.push_back( vert );
                }
            }
        }
    }

    return verts;
}

int btn;
glm::ivec2 startMouse;
glm::ivec2 startRot, curRot;
glm::ivec2 startTrans, curTrans;
void mouse(int button, int state, int x, int y )
{
    if( button == GLUT_LEFT_BUTTON && state == GLUT_DOWN )
    {
        btn = button;
        startMouse = glm::ivec2( x, glutGet( GLUT_WINDOW_HEIGHT ) - y );
        startRot = curRot;
    }
    if( button == GLUT_RIGHT_BUTTON && state == GLUT_DOWN )
    {
        btn = button;
        startMouse = glm::ivec2( x, glutGet( GLUT_WINDOW_HEIGHT ) - y );
        startTrans = curTrans;
    }
}

void motion( int x, int y )
{
    glm::ivec2 curMouse( x, glutGet( GLUT_WINDOW_HEIGHT ) - y );
    if( btn == GLUT_LEFT_BUTTON )
    {
        curRot = startRot + ( curMouse - startMouse );
    }
    else if( btn == GLUT_RIGHT_BUTTON )
    {
        curTrans = startTrans + ( curMouse - startMouse );
    }
    glutPostRedisplay();
}

std::vector< Vertex > model;
void display()
{
    glClearColor( 0.2f, 0.2f, 0.2f, 1.0f );
    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

    glMatrixMode( GL_PROJECTION );
    glLoadIdentity();
    double w = glutGet( GLUT_WINDOW_WIDTH );
    double h = glutGet( GLUT_WINDOW_HEIGHT );
    double ar = w / h;
    glTranslatef( curTrans.x / w * 2, curTrans.y / h * 2, 0 );
    gluPerspective( 60, ar, 0.1, 100 );

    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();
    glTranslatef( 0, 0, -10 );

    glPushMatrix();
    {
        glRotatef( curRot.x % 360, 0, 1, 0 );
        glRotatef( -curRot.y % 360, 1, 0, 0 );

        // object
        glColor3ub( 255, 0, 0 );
        glEnableClientState( GL_VERTEX_ARRAY );
        glEnableClientState( GL_TEXTURE_COORD_ARRAY );
        glEnableClientState( GL_NORMAL_ARRAY );
        glVertexPointer( 3, GL_FLOAT, sizeof(Vertex), &model[0].position );
        glTexCoordPointer( 2, GL_FLOAT, sizeof(Vertex), &model[0].texcoord );
        glNormalPointer( GL_FLOAT, sizeof(Vertex), &model[0].normal );
        glDrawArrays( GL_TRIANGLES, 0, model.size() );
        glDisableClientState( GL_VERTEX_ARRAY );
        glDisableClientState( GL_TEXTURE_COORD_ARRAY );
        glDisableClientState( GL_NORMAL_ARRAY );

        // bounding cube
        glDisable( GL_LIGHTING );
        glColor3ub( 255, 255, 255 );
        glutWireCube( 7 );
        glEnable( GL_LIGHTING );
    }
    glPopMatrix();

    glutSwapBuffers();
}

// return the min/max points of pts
template< typename Vec >
std::pair< Vec, Vec > GetExtents( const Vec* pts, size_t stride, size_t count )
{
    unsigned char* base = (unsigned char*)pts;
    Vec pmin( *(Vec*)base );
    Vec pmax( *(Vec*)base );
    for( size_t i = 0; i < count; ++i, base += stride )
    {
        const Vec& pt = *(Vec*)base;
        pmin = glm::min( pmin, pt );
        pmax = glm::max( pmax, pt );
    }

    return std::make_pair( pmin, pmax );
}

// centers geometry around the origin
// and scales it to fit in a size^3 box
template< typename Vec > 
void CenterAndScale( Vec* pts, size_t stride, size_t count, const typename Vec::value_type& size )
{
    typedef typename Vec::value_type Scalar;

    // get min/max extents
    std::pair< Vec, Vec > exts = GetExtents( pts, stride, count );

    // center and scale 
    const Vec center = ( exts.first * Scalar( 0.5 ) ) + ( exts.second * Scalar( 0.5f ) );

    const Scalar factor = size / glm::compMax( exts.second - exts.first );
    unsigned char* base = (unsigned char*)pts;
    for( size_t i = 0; i < count; ++i, base += stride )
    {
        Vec& pt = *(Vec*)base;
        pt = ( ( pt - center ) * factor );
    }    
}

int main( int argc, char **argv )
{
    // https://en.wikipedia.org/wiki/Stanford_bunny
    std::ifstream ifile( "bunny.obj" );
    model = LoadOBJ( ifile );
    CenterAndScale( &model[0].position, sizeof( Vertex ), model.size(), 7 );

    glutInit( &argc, argv );
    glutInitDisplayMode( GLUT_RGBA | GLUT_DEPTH | GLUT_DOUBLE );
    glutInitWindowSize( 640, 480 );
    glutCreateWindow( "OBJ" );
    glutDisplayFunc( display );
    glutMouseFunc( mouse );
    glutMotionFunc( motion );

    glEnable( GL_DEPTH_TEST );

    // set up "headlamp"-like light
    glShadeModel( GL_SMOOTH );
    glEnable( GL_COLOR_MATERIAL );
    glColorMaterial( GL_FRONT_AND_BACK, GL_AMBIENT_AND_DIFFUSE ) ;
    glEnable( GL_LIGHTING );
    glEnable( GL_LIGHT0 );
    glMatrixMode( GL_MODELVIEW );
    glLoadIdentity();
    GLfloat position[] = { 0, 0, 1, 0 };
    glLightfv( GL_LIGHT0, GL_POSITION, position );

    glPolygonMode( GL_FRONT, GL_FILL );
    glPolygonMode( GL_BACK, GL_LINE );
    glutMainLoop();
    return 0;
}

LMB-drag rotates, RMB-drag “pans”.

And use this:

v 0.000000 2.000000 2.000000
v 0.000000 0.000000 2.000000
v 2.000000 0.000000 2.000000
v 2.000000 2.000000 2.000000
f -4 -3 -2 -1

v 2.000000 2.000000 0.000000
v 2.000000 0.000000 0.000000
v 0.000000 0.000000 0.000000
v 0.000000 2.000000 0.000000
f -4 -3 -2 -1

v 2.000000 2.000000 2.000000
v 2.000000 0.000000 2.000000
v 2.000000 0.000000 0.000000
v 2.000000 2.000000 0.000000
f -4 -3 -2 -1

v 0.000000 2.000000 0.000000
v 0.000000 2.000000 2.000000
v 2.000000 2.000000 2.000000
v 2.000000 2.000000 0.000000
f -4 -3 -2 -1

v 0.000000 2.000000 0.000000
v 0.000000 0.000000 0.000000
v 0.000000 0.000000 2.000000
v 0.000000 2.000000 2.000000
f -4 -3 -2 -1

v 0.000000 0.000000 2.000000
v 0.000000 0.000000 0.000000
v 2.000000 0.000000 0.000000
v 2.000000 0.000000 2.000000
f -4 -3 -2 -1

Or this:

v 0.000000 2.000000 0.000000
v 0.000000 0.000000 0.000000
v 2.000000 0.000000 0.000000
v 2.000000 2.000000 0.000000
v 4.000000 0.000000 -1.255298
v 4.000000 2.000000 -1.255298
vn 0.000000 0.000000 1.000000
vn 0.000000 0.000000 1.000000
vn 0.276597 0.000000 0.960986
vn 0.276597 0.000000 0.960986
vn 0.531611 0.000000 0.846988
vn 0.531611 0.000000 0.846988
# 6 vertices

# 6 normals

g all
s 1
f 1//1 2//2 3//3 4//4
f 4//4 3//3 5//5 6//6
# 2 elements

for test.obj.

glm.hpp is from the GLM library.

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