Question

I’m capturing webcam data using OpenCV and display them as the Texture for a GL Window, This works fine.

gluBuild2DMipmaps(GL_TEXTURE_2D, GL_RGB, image->width, image->height, GL_RGB, GL_UNSIGNED_BYTE, image->imageData);

But I want also to overlay some basic shapes, Md2 models on top of this, retaining the texture in the background and in the same window itself.

This is my function that is passed into glutDisplayFunc()

void drawScene() {

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glEnable(GL_TEXTURE_2D);

    // Set Projection Matrix
    glMatrixMode (GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0, VIEWPORT_WIDTH, VIEWPORT_HEIGHT, 0);

    // Setup the view  // Switch to Model View Matrix
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glBegin(GL_QUADS); 
    // Trapezoid
    glVertex3f(-0.7f, -1.5f, -5.0f);
    glVertex3f(0.7f, -1.5f, -5.0f);
    glVertex3f(0.4f, -0.5f, -5.0f);
    glVertex3f(-0.4f, -0.5f, -5.0f);
    glEnd(); //End quadrilateral coordinates

    // start drawing the Background texture
    glBegin(GL_QUADS);
    glTexCoord2f(0.0f, 0.0f); glVertex2f(0.0f, 0.0f);
    glTexCoord2f(1.0f, 0.0f); glVertex2f(VIEWPORT_WIDTH, 0.0f);
    glTexCoord2f(1.0f, 1.0f); glVertex2f(VIEWPORT_WIDTH, VIEWPORT_HEIGHT);
    glTexCoord2f(0.0f, 1.0f); glVertex2f(0.0f, VIEWPORT_HEIGHT);
    glEnd();

    glFlush();
    glutSwapBuffers();
}

EDIT
new order

glBegin(GL_QUADS);
    glTexCoord2f(0.0f, 0.0f); glVertex2f(0.0f, 0.0f);
    glTexCoord2f(1.0f, 0.0f); glVertex2f(VIEWPORT_WIDTH, 0.0f);
    glTexCoord2f(1.0f, 1.0f); glVertex2f(VIEWPORT_WIDTH, VIEWPORT_HEIGHT);
    glTexCoord2f(0.0f, 1.0f); glVertex2f(0.0f, VIEWPORT_HEIGHT);
    glEnd();

    glBegin(GL_QUADS); //Begin quadrilateral coordinates
    // Trapezoid
    glVertex3f(-0.7f, -1.5f, -5.0f);
    glVertex3f(0.7f, -1.5f, -5.0f);
    glVertex3f(0.4f, -0.5f, -5.0f);
    glVertex3f(-0.4f, -0.5f, -5.0f);
    glEnd(); //End quadrilateral coordinates

EDIT:
New order and coordinates:

glDisable(GL_TEXTURE_2D);
glBegin(GL_QUADS);
// Trapezoid to be displayed on top of texture
glVertex3f(0.2f, 0.3f, -0.6f);
glVertex3f(0.5f, -0.5f, -0.5f);
glVertex3f(0.4f, -0.5f, -0.5f);
glVertex3f(-0.4f, -0.5f, -0.5f);
glEnd(); //End quadrilateral coordinates
////
glEnable(GL_TEXTURE_2D); // start drawing the background texture 
glBegin(GL_QUADS);
glTexCoord2f(0.0f, 0.0f); glVertex2f(0.0f, 0.0f);
glTexCoord2f(1.0f, 0.0f); glVertex2f(VIEWPORT_WIDTH, 0.0f);
glTexCoord2f(1.0f, 1.0f); glVertex2f(VIEWPORT_WIDTH, VIEWPORT_HEIGHT);
glTexCoord2f(0.0f, 1.0f); glVertex2f(0.0f, VIEWPORT_HEIGHT);
glEnd();

The result after doing above and Disabling gluOrtho2D()

Answer 1

Check the docs for gluOrtho2D():

gluOrtho2D sets up a two-dimensional orthographic viewing region. This is equivalent to calling glOrtho with near = -1 and far = 1.

You’re placing your trapezoid at Z = -5, way behind your near clipping plane:

glBegin(GL_QUADS);
glVertex3f(-0.7f, -1.5f, -5.0f);
glVertex3f(0.7f, -1.5f, -5.0f);
glVertex3f(0.4f, -0.5f, -5.0f);
glVertex3f(-0.4f, -0.5f, -5.0f);
glEnd(); 

Try -0.5 for your Z coordinates, or at least something between -1 and 0.

EDIT: Example:

#include <GL/glut.h>

int win_w = 0;
int win_h = 0;
GLuint texObj = 0;

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    // set up projection
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    double halfWidth = (win_w / 2.0);;
    double halfHeight = (win_h / 2.0);
    glOrtho( -halfWidth, halfWidth, -halfHeight, halfHeight, -10, 10 );

    // set up modelview
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    // render textured quad
    glEnable(GL_TEXTURE_2D);
    glColor3ub(255,255,255);
    glPushMatrix();
        glScalef(halfWidth * 0.9, halfHeight * 0.9, 1.0);
        glBegin(GL_QUADS);
            glTexCoord2f(0,0);
            glVertex2f(-1,-1);
            glTexCoord2f(1,0);
            glVertex2f(1,-1);
            glTexCoord2f(1,1);
            glVertex2f(1,1);
            glTexCoord2f(0,1);
            glVertex2f(-1,1);
        glEnd();
    glPopMatrix();
    glDisable(GL_TEXTURE_2D);

    // draw quad on top
    glColor3ub(255,0,0);
    glPushMatrix();
        // move quad back a bit so it's above Z=0
        glTranslatef(0,0,-1);
        glScalef(50,50,50);
        glBegin(GL_QUADS);
            glVertex2f(-1,-1);
            glVertex2f(1,-1);
            glVertex2f(1,1);
            glVertex2f(-1,1);
        glEnd();
    glPopMatrix();

    glFlush();
    glutSwapBuffers();
}

void reshape(int w, int h)
{
    win_w = w;
    win_h = h;
    glViewport(0, 0, w, h);
}

GLuint getTexture()
{
    // 2x2 texture
    unsigned char bytes[] =
    {
          0,   0, 255,      255, 255, 255,
        255,   0,   0,      0, 255,   0,
    };

    GLuint texID;
    glGenTextures(1, &texID);
    glBindTexture(GL_TEXTURE_2D, texID);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 2, 2, 0, GL_RGB, GL_UNSIGNED_BYTE, bytes);
    return texID;
}

int main(int argc, char **argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGBA | GLUT_DEPTH | GLUT_DOUBLE);

    glutInitWindowSize(800,600);
    glutCreateWindow("Texture");

    glutDisplayFunc(display);
    glutReshapeFunc(reshape);

    texObj = getTexture();
    glutMainLoop();
    return 0;
}