OpenGL draw full of screen

Question

I am trying to draw an 2D football field in the full window. However, it gets black bars at the left and right:

This is my code:

#include <OpenGL/gl.h> // include GLEW and new version of GL on Windows
#include <GLFW/glfw3.h> // GLFW helper library
#include <stdio.h>
#include <math.h>

void Soccer_Field();

int main() {
    // start GL context and O/S window using the GLFW helper library
    if (!glfwInit()) {
        fprintf(stderr, "ERROR: could not start GLFW3\n");
        return 1;
    }

    // uncomment these lines if on Apple OS X
    /*glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
     glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 2);
     glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
     glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);*/

    GLFWwindow* window = glfwCreateWindow(640, 480, "Soccer field", NULL, NULL);
    if (!window) {
        fprintf(stderr, "ERROR: could not open window with GLFW3\n");
        glfwTerminate();
        return 1;
    }
    glfwMakeContextCurrent(window);

    // get version info
    const GLubyte* renderer = glGetString(GL_RENDERER); // get renderer string
    const GLubyte* version = glGetString(GL_VERSION); // version as a string
    printf("Renderer: %s\n", renderer);
    printf("OpenGL version supported %s\n", version);

    /* OTHER STUFF GOES HERE NEXT */
    while(!glfwWindowShouldClose(window)) {
        glMatrixMode(GL_PROJECTION);
        glPushMatrix();
        glLoadIdentity();
        glOrtho(0.0, 1, 0.0, 1, -1.0, 1.0);
        glMatrixMode(GL_MODELVIEW);
        glPushMatrix();

       Soccer_Field();

        // update other events like input handling
        glfwPollEvents();
        // put the stuff we've been drawing onto the display
        glfwSwapBuffers(window);
    }

    // close GL context and any other GLFW resources
    glfwTerminate();
    return 0;
}

void Soccer_Field (void)
{
    float x, y, ang, radius = 0.05;     // Not sure what the radius of the center circle should be?

    static float RAD_DEG = 57.296;

    glBegin (GL_QUADS);
    glColor3f  (0.20, 0.60, 0.20);                           // GreenYard
    glVertex2f (0.10, 0.10); glVertex2f (0.90, 0.10);
    glVertex2f (0.90, 0.90); glVertex2f (0.10, 0.90);
    glColor3f  (1.0, 1.0, 1.0);
    glVertex2f (0.90, 0.35); glVertex2f (0.83, 0.35);        // Inner White Quad - Right
    glVertex2f (0.83, 0.65); glVertex2f (0.90, 0.65);
    glVertex2f (0.10, 0.35); glVertex2f (0.17, 0.35);        // Inner White Quad - Left
    glVertex2f (0.17, 0.65); glVertex2f (0.10, 0.65);
    glEnd ();

    glColor3f (0.0, 0.0, 0.0);                                  // Change color to black

    glBegin (GL_LINES);
    glVertex2f (0.50, 0.10); glVertex2f (0.50, 0.90);        // Mid Line

    // Left side of the Ground
    glVertex2f (0.25, 0.25); glVertex2f (0.25, 0.75);        // Goal keeper front line
    glVertex2f (0.10, 0.75); glVertex2f (0.25, 0.75);        // Goal keeper left line
    glVertex2f (0.10, 0.25); glVertex2f (0.25, 0.25);        // Goal keeper right line

    // Right Side of the Ground
    glVertex2f (0.75, 0.25); glVertex2f (0.75, 0.75);        // Goal keeper front line
    glVertex2f (0.75, 0.25); glVertex2f (0.90, 0.25);        // Goal keeper left  Line
    glVertex2f (0.75, 0.75); glVertex2f (0.90, 0.75);        // Goal keeper right line
    glEnd ();

    glBegin (GL_LINE_LOOP);                                     // Circle at center of field
    for (ang = 0.0; ang < 360.0; ang += 10.0)  {
        x = radius * cos(ang/RAD_DEG) + 1.0;
        y = radius * sin(ang/RAD_DEG) + 0.5;
        glVertex2f (x/2.0, y);
    }
    glEnd ();
}

How can I fix this? I've tried to set the viewport and other things but I can't get it to work.

Hope someone gets me in the straight way.

Thanks

Answer 1

The projection matrix describes the mapping from 3D points of a scene, to 2D points of the viewport. The projection matrix transforms from view space to the clip space, and the coordinates in the clip space are transformed to the normalized device coordinates (NDC) in the range (-1, -1, -1) to (1, 1, 1) by dividing with the w component of the clip coordinates.
At Orthographic Projection the coordinates in the eye space are linearly mapped to normalized device coordinates.

The ortogrpahic projection matrix is definde by glOrtho . With this functions you define the field of view of the scene.

Since all your geometry is in the range from (0.1, 0.1) to (0.9, 0.9), you have to change it to:

GLdouble left   = 0.1;
GLdouble right  = 0.9; 
GLdouble bottom = 0.1;
GLdouble top    = 0.9; 
glOrtho(left, right, bottom, top, -1.0, 1.0);

The second possibility would be to scale the geometry by the model view matrix. You have to scale ( glScale ) the geometry from [(0.1, 0.1), (0.9, 0.9)] to the range which which is set up by the orthographic projection [(0.0, 0.0), (1.0, 1.0)]. Note, in this case you have to "re-center" the geometry ( glTranslate ):

glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

GLdouble scaleX = (1.0 - 0.0) / (0.9 - 0.1);
GLdouble scaleY = (1.0 - 0.0) / (0.9 - 0.1); 
glScaled( scaleX, scaleY, 1.0 );
glTranslated( -0.1, -0.1, 0.0 );