2d code only draws black screen - noob question...

OpenGL OpenGL: Basic Coding
1

Hi

Using these lines:


  glBegin(GL_LINES);
  glVertex3f(10, 10, 0);
  glVertex3f(500, 500, 0);
  glEnd();

I want to try to plot some 2D code (make a straight line in this example)… However, all I get is a black screen - I’m trying to modify the below code:


//    g++ openGL.cpp  -lGL -lglut -lGLU -lm && ./a.out

#ifdef __APPLE_CC__
#include <GLUT/glut.h>
#else
#include <GL/glut.h>
#endif

static float x = 0, y = 0;

// Handles the keyboard event: the left and right arrows bend the elbow, the
// up and down keys bend the shoulder.
void special(int key, int, int)
{
  switch (key) {
    case GLUT_KEY_LEFT: (x -= 1); break;
    case GLUT_KEY_RIGHT: (x += 1); break;
    case GLUT_KEY_UP: (y += 1); break;
    case GLUT_KEY_DOWN: (y -= 1); break;
    default: return;
  }
  glutPostRedisplay();
}


void display()
{
  glClear(GL_COLOR_BUFFER_BIT);
  glMatrixMode(GL_MODELVIEW);
  //==================
  glPushMatrix();
  glTranslatef(x, y, 0.0);

  glBegin(GL_LINES);
  glVertex3f(10, 10, 0);
  glVertex3f(500, 500, 0);
  glEnd();

  glPopMatrix();
  //==================
  glFlush();
}

// Handles the reshape event by setting the viewport so that it takes up the
// whole visible region, then sets the projection matrix to something reason-
// able that maintains proper aspect ratio.
void reshape(GLint w, GLint h)
{
  glViewport(0, 0, w, h);
  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  /*  gluPerspective(65.0, // fovy: field of angle view (degrees), in y direction
				 GLfloat(w)/GLfloat(h), // ratio of x (width) to y (height)
				 1.0, // distance from the viewer to the near clipping plane
				 20.0); // distance from the viewer to the far clipping plane
  */
}


// Initializes GLUT, the display mode, and main window; registers callbacks;
// does application initialization; enters the main event loop.
int main(int argc, char** argv)
{
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
  glutInitWindowSize(800, 600);
  glutCreateWindow("Testing...");
  glutDisplayFunc(display);
  glutReshapeFunc(reshape);
  glutSpecialFunc(special);

  //-----------------
  glShadeModel(GL_FLAT);
  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();
  /*  gluLookAt(
			1,2,8, // eyeX, eyeY, eyeZ
			0,0,0, // centerX, centerY, centerZ
			0,1,0); // upX, upY, upZ
  */
  //-----------------

  glutMainLoop();
}

Please advice, thank you very much!

2

Aargh… sorry… Inserting:

gluOrtho2D(0.0, (GLdouble) w, 0.0, (GLdouble) h);

in reshape(…) seems to fix the problem… Merry christmas…

3

Hmm, why do you have gluLookAt and gluPerspective commented out?

Try removing the /* */ and let me know what happens!

4

Hi LearningOGL,

The problem was solved with:

gluOrtho2D(0.0, (GLdouble) w, 0.0, (GLdouble) h);

in reshape(…)… Answer to your question: gluLookAt and gluPerspective is commented out because the original code was 3d, I then changed it into 2D-code…

I only have one tiny/simple question:


//    g++ openGL.cpp  -lGL -lglut -lGLU -lm && ./a.out

#ifdef __APPLE_CC__
#include <GLUT/glut.h>
#else
#include <GL/glut.h>
#endif

static float x = 0, y = 0;

// Handles the keyboard event: the left and right arrows bend the elbow, the
// up and down keys bend the shoulder.
void special(int key, int, int)
{
  switch (key) {
    case GLUT_KEY_LEFT: (x -= 5); break;
    case GLUT_KEY_RIGHT: (x += 5); break;
    case GLUT_KEY_UP: (y += 5); break;
    case GLUT_KEY_DOWN: (y -= 5); break;
    default: return;
  }
  glutPostRedisplay();
}


void display()
{
  glClear(GL_COLOR_BUFFER_BIT);
  glMatrixMode(GL_MODELVIEW);
  //==================
  glPushMatrix();
  glTranslatef(x, y, 0.0);

  glColor3f(1.0, 1.0, 1.0);
  glBegin(GL_LINES);
  glVertex2f(-50, -50);
  glVertex2f(50, 50);
  glVertex2f(-50, 50);
  glVertex2f(50, -50);
  glEnd();

  glPopMatrix();
  //==================
  glutSwapBuffers(); // also does glFlush();
}

// Handles the reshape event by setting the viewport so that it takes up the
// whole visible region, then sets the projection matrix to something reason-
// able that maintains proper aspect ratio.
void reshape(GLint w, GLint h)
{
  GLdouble myLgt = 100;
  bool aspect = false;
  //  bool aspect = true;

  if (!aspect)
	{
	  glViewport(0, 0, // x, y
				 (GLsizei) w, (GLsizei) h); // width, height
	  gluOrtho2D(-myLgt, myLgt, // left, right
				 -myLgt, myLgt); // bottom, top
	}

  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();

  if (aspect)
	{
	  glViewport(0, 0, // x, y
				 (GLsizei) w, (GLsizei) h); // width, height

	  if (w <= h) // tall
		gluOrtho2D( -myLgt, myLgt,
				   -myLgt*(GLfloat)h/(GLfloat)w, myLgt*(GLfloat)h/(GLfloat)w );
	  else // wide
		gluOrtho2D( -myLgt*(GLfloat)w/(GLfloat)h, myLgt*(GLfloat)w/(GLfloat)h,
				   -myLgt, myLgt);
	}
  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();
}


void keyboard(unsigned char key, int x, int y)
{
   switch (key) {
      case 27:
         exit(0);
         break;
   }
}


// Initializes GLUT, the display mode, and main window; registers callbacks;
// does application initialization; enters the main event loop.
int main(int argc, char** argv)
{
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
  glutInitWindowSize(800, 600);
  glutCreateWindow("Testing...");
  //-----------------
  glShadeModel(GL_FLAT);
  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();
  //-----------------
  glutDisplayFunc(display);
  glutReshapeFunc(reshape);
  glutSpecialFunc(special);
  glutKeyboardFunc(keyboard);

  glutMainLoop();
  return 0;
}

When I run this, why does the cross move completely out of the picture when I press any arrow key (note the difference, when “bool aspect” value is negated => this is the behaviour that I would expect in any case) ???

I don’t understand that/why the current cross moves out of the screen by a single arrow-key-press…

5

hmm… No answers…

Please allow me to rephrase: Copy/paste the code into a file and type (or press F5 with MS Visual studio):

g++ openGL.cpp -lGL -lglut -lGLU -lm && ./a.out

Then single-type any of the 4 arrow keys. Notice/please explain to me the difference between using:

bool aspect = false;
bool aspect = true;

I cannot understand, why the image moves “out-of-the-screen” by a single key-press (I don’t like this)…???

Any explanation for this behaviour is greatly appreciated!

6

In your resize function:
move


  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();

before the first “if”.

7

Aah, that was stupid of me -

THANK you very much, uruk!!!

I’m very grateful for your time (even though I should have seen it myselt) :slight_smile: