Question

I am trying to render a model in openGL 4 using the glDrawElements/Arrays function. I am currently reading in files with vertex data and indices for polygons. I cannot get anyting to display on screen though except the axes im using for reference. I have used this general approach a bunch of times with success. I dont know what I could be doing wrong and have been stuck for almost a day.

This is where I buffer my data

// get position of "in vec4 vPosition;" from shader program
// displays axes properly
vPosition = glGetAttribLocation( program, "vPosition" );

// create buffers
glGenVertexArrays(2, vao);
glGenBuffers( 3, vbo );

// here I bind vao[0] and vbo[0] then buffer
// a set of XYZ axes which display correctly

// buffer model
glBindVertexArray( vao[1] );
glBindBuffer( GL_ARRAY_BUFFER, vbo[1] );
glBufferData( GL_ARRAY_BUFFER, sizeof(board->verts),
                board->verts, GL_STATIC_DRAW );

glEnableVertexAttribArray( vPosition );
glVertexAttribPointer( vPosition, 4, GL_FLOAT, GL_FALSE, 0,
                        BUFFER_OFFSET(0));

glBindBuffer( GL_ELEMENT_ARRAY_BUFFER , vbo[2] );
glBufferData(
        GL_ELEMENT_ARRAY_BUFFER,
        sizeof(board->indices),
        board->indices.data(),
        GL_STATIC_DRAW);

fyi board is a model of a surfboard in which the vertices and indices for glDrawElements are read in. These vertices and indices are printed out correctly if the following code is included just above the call to glBufferData

for( int i = 0; i < board->numVerts; i++ ){
    std::cerr << i << ":  "<<board->verts[i].x << " "<<board->verts[i].y<<" "<<
            board->verts[i].z << " " << board->verts[i].w << std::endl;
}

the same goes for the indices if a similar print loop is put in. Here is where I attempt to draw the model:

void draw_model(){

    glUniform4fv( color_loc, 1, glm::value_ptr(blue));
    glBindVertexArray( vao[1] );

    glDrawArrays( GL_LINE_STRIP, 0, 600 );

    //glDrawElements(
    //  GL_LINES,
    //  board->indices.size(),
    //  GL_UNSIGNED_INT,
    //  NULL
    //);

}

the call to DrawElements (commented out) does not display anything. the call to DrawArrays was a troubleshooting effort to see if I was just using DrawElements incorrectly (i have only used drawArrays in the past). board is defined as model3d *board. here is its class

class model3d{
public:
    glm::vec4 *verts;
    std::vector<int> indices;
    int numVerts;
    int numPolys;
    model3d( const char*, const char* );
private:
    void load_coords( const char* );
    void load_polys( const char* );
};

why does my data not seem to buffer?

Answer 1

Don’t use sizeof(board->verts) as it’ll only return the size of the pointer.