OpenGL black screen/nothing drawn? - c

Question

Why does this piece of code produce a black screen (nothing is drawn...)?
I'm creating a Pong clone but I can't proceed without getting this to work.
#include <GL/glut.h>
struct Rectangle {
int x;
int y;
int width;
int height;
};
struct Ball {
int x;
int y;
int radius;
};
typedef struct Rectangle Rectangle;
typedef struct Ball Ball;
Rectangle rOne, rTwo;
Ball ball;
void display(void);
void reshape(int w, int h);
void drawRectangle(Rectangle *r);
int main(int argc, char* argv[]) {
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_DOUBLE|GLUT_RGB);
glutInitWindowSize(800,600);
glutCreateWindow("Pong");
gluOrtho2D(0,0,800.0,600.0);
rOne.x = 100;
rOne.y = 100;
rOne.width = 100;
rOne.height = 50;
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutMainLoop();
return 0;
}
void display(void) {
glClear(GL_COLOR_BUFFER_BIT);
drawRectangle(&rOne);
glFlush();
glutSwapBuffers();
}
void reshape(int w, int h) {
glViewport(0,0,w,h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0,0,(GLfloat)w,(GLfloat)h);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
void drawRectangle(Rectangle *r) {
glBegin(GL_QUADS);
glVertex2i(r->x,r->y);
glVertex2i(r->x+(r->width-1),r->y);
glVertex2i(r->x+(r->width-1),r->y+(r->height-1));
glVertex2i(r->x,r->y+(r->height-1));
glEnd();
}

Answer 1

You're requesting an ortho projection with zero width:
gluOrtho2D(0,0,(GLfloat)w,(GLfloat)h);
This is probably what you meant:
gluOrtho2D(0,(GLfloat)w,0,(GLfloat)h);
Example:
#include <GL/glut.h>
typedef struct {
int x;
int y;
int width;
int height;
} Rect;
typedef struct {
int x;
int y;
int radius;
} Ball;
Rect rOne, rTwo;
Ball ball;
void display(void);
void reshape(int w, int h);
void drawRectangle(Rect *r);
int main(int argc, char* argv[])
{
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
glutInitWindowSize(800,600);
glutCreateWindow("Pong");
rOne.x = 100;
rOne.y = 100;
rOne.width = 100;
rOne.height = 50;
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutMainLoop();
return 0;
}
void display(void)
{
glClear(GL_COLOR_BUFFER_BIT);
drawRectangle(&rOne);
glFlush();
glutSwapBuffers();
}
void reshape(int w, int h)
{
glViewport(0,0,w,h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(0,(GLfloat)w,0,(GLfloat)h);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
void drawRectangle(Rect *r)
{
glBegin(GL_QUADS);
glVertex2i(r->x,r->y);
glVertex2i(r->x+(r->width-1),r->y);
glVertex2i(r->x+(r->width-1),r->y+(r->height-1));
glVertex2i(r->x,r->y+(r->height-1));
glEnd();
}

Answer 2

Your problem lies here:
gluOrtho2D(0,0,800.0,600.0);
gluOrtho2D is not like glViewport. The parameter of gluOrtho2D are :
gluOrtho2D(left, right, bottom, top);
So you must call it like
gluOrtho(0, w, 0, h);