Loading a model and displaying it using OpenGL? - c++

Question

I am tring to load the model and display it using OpenGL. The code runs fine with no errors but nothing is displayed. Can someone help me regarding this issue?
The source code:
int cube;
int number_faces;
int number_vertices;
struct coordinate {
float x, y, z;
coordinate(float a, float b, float c): x(a), y(b), z(c) {};
};
struct face {
int faces[3];
face (int f1, int f2, int f3){
faces[0] = f1;
faces[1] = f2;
faces[2] = f3;
};
};
// function prototype
bool loadObject(const char * file_name);
int main(int argc, char ** argv)
{
//initialize Glut
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowPosition(w_x_position, w_y_position);
glutInitWindowSize(w_width, w_height);
glutCreateWindow("Assignment 1");
//function call
init();
glutDisplayFunc(display);
//loop main
glutMainLoop();
return 0;
}
bool loadObject(const char * file_name){
//initialize stuff
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
//create the vectors that will hold the file coordinates
std::vector<std::string*> points;
std::vector<coordinate*> vertex;
std::vector<face*> faces;
//check if the file opens
if ( !file_name ) {
return false;
}
// open file
FILE *fp = fopen( file_name, "r" );
if ( !fp ) {
return false;
}
// num of vertices and indices
fscanf( fp, "data%d%d", &number_vertices, &number_faces);
//push the content into the vertexs
for(int i=0;i<number_vertices;i++) {
float x_temp, y_temp, z_temp;
fscanf( fp, "%f%f%f", &x_temp, &y_temp,&z_temp );
vertex.push_back(new coordinate(x_temp, y_temp, z_temp));
}
for (int y=0; y<number_faces;y++){
int a,b,c;
fscanf( fp, "%d%d%d", &a, &b, &c);
faces.push_back(new face(a, b, c));
}
//draw the modle
int number;
number = glGenLists(1);
glNewList(number, GL_COMPILE);
for(int i =0; i <faces.size();i++){
glBegin(GL_TRIANGLES);
glVertex3f(vertex[faces[i]->faces[0]-1]->x, vertex[faces[i]->faces[0]-1]->y,vertex[faces[i]->faces[0]-1]->z);
glVertex3f(vertex[faces[i]->faces[1]-1]->x, vertex[faces[i]->faces[1]-1]->y,vertex[faces[i]->faces[1]-1]->z);
glVertex3f(vertex[faces[i]->faces[2]-1]->x, vertex[faces[i]->faces[2]-1]->y,vertex[faces[i]->faces[2]-1]->z);
glEnd();
}
glEndList();
//delete the poiters for memory space
for(int i=0;i<points.size();i++){
delete points[i];
}
for(int i=0;i<faces.size();i++){
delete faces[i];
}
for(int i=0;i<vertex.size();i++){
delete vertex[i];
}
return number;
}

Answer 1

First of all I guess you are mixing something up.
The steps should be:
Start the programm
Create everything that is needed for OpenGL
Load your model. You just need to do this once. For modern openGL you can create a so called VertexBuffer for storing you vertices.
Start the GameLoop which normally should have at least three parts
parse the input from the player like KeyBoard and Mouse
Update the scene corresponding to your game logic and the input of the user
Draw everything to the screen
If the user decided so, close the program and clean everything up.
Using glut you can provide function pointer to glut which calls your input, update and reder method, There is also an idle method and various other you can provide, so you do not need to make your own loop.
The 2nd problem I can see in your code beside the not so good structure is that you are missing to set a Modelview Matrix. You've just set a projection matrix and no modelMatrix. But for a better understanding just let me explain a bit more about the render part of the gameLoop.
The render method will draw everything to the screen/window. For each model you have to provide a modelview matrix which will translate, rotate and scale the mesh/model and a projectionmatrix which will bring the 3D world to a 2D screen (There is a bit more about math and a lot more details but we will keep it short and easy here). In older versions of OpenGL there is already a stack implemented on which you can push or pop the matrices. The matrix on top of the stack will be used to draw the mesh. If you want another projection and/or modelmatrix for a 2nd or 3rd mesh just change the matrix on top of the stack. During the loading of a mesh you do not need to priovide any other information. Just position, uv or color for each vertex.
You drawing will be roughly like this (pseudocode):
ClearEverythingFromLastCall();
glMatrixMode(GL_PROJECTION);
SetProjectionMatrixOnStack(); (pushMatrix)
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
ChangeMatrixForCurrentObject();
drawObject();
glutSwapBuffers(); // This shpuld be the last line of you method
drawObject depends on what technique you are using (fixedFunctionPipeline from older OpenGL versions, or the programmable pipeline from OpenGL 3.0 and newer (which I think is the better one)).
So there are a lot of errors in you code and not the "one" little bug which breaks you code. I highly recommend to start with a good tutorial. For older OpenGL I recommend http://nehe.gamedev.net/ or http://videotutorialsrock.com/ (If you are able to speak german I can give you some tutorials in german also). Both start from the very beginning with the fixed pipeline.