Fixing OpenGL App: Black Background & White Triangle

[cscott]

I'm having trouble getting the basic OpenGL app running properly. When it opens the window is entirely white when it should have a black background and white triangle.

I will explain the code in bold bellow

#include <OpenGL/gl.h>
#include <OpenGL/glu.h>
#include <GLUT/glut.h>

int initGL(GLvoid);
GLvoid drawScene(GLvoid);
GLvoid resizeScene(int width, int height);

int
main(int argc, char *argv[])
{
	const int WINDOW_HEIGHT = 300;
	const int WINDOW_WIDTH = 400;
	
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
	glutInitWindowSize(WINDOW_WIDTH, WINDOW_HEIGHT);
	glutInitWindowPosition(100, 100);
	glutCreateWindow(argv[0]);
	
	initGL();
	
	glutDisplayFunc(drawScene);
	glutReshapeFunc(resizeScene);
	[b]
   glClearColor(0.0, 0.0, 0.0, 0.0);
   glClear(GL_COLOR_BUFFER_BIT);
   glColor3f(0.0, 1.0, 1.0);
   glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0); 
   glBegin(GL_POLYGON);
      glVertex2f(-0.5, -0.5);
      glVertex2f(-0.5, 0.5);
      glVertex2f(0.5, 0.5);
      glVertex2f(0.5, -0.5);
   glEnd();
   glFlush();
	[/b]
	glutMainLoop();
	
	return 0;
}

int 
initGL(GLvoid)
{
	glShadeModel(GL_SMOOTH);
	glClearColor(1.0f, 0.0f, 0.0f, 0.0f);
	
	glClearDepth(1.0f);
	glEnable(GL_DEPTH_TEST);
	glDepthFunc(GL_LEQUAL);
	
	glHint(GL_PERSPECTIVE_CORRECTION_HINT, GL_NICEST);
	
	return 0;
}

GLvoid 
resizeScene(int width, int height)
{
	if (height == 0)
		height = 1;
		
	glViewport(0, 0, width, height);
	
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	gluPerspective(45.0f, (GLfloat)width / (GLfloat)height, 0.1f, 100.0f);
	
	glMatrixMode(GL_MODELVIEW);
	glLoadIdentity();
	
	return;
}

GLvoid 
drawScene(GLvoid)
{
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	glLoadIdentity();
	
	glTranslatef(-1.5f, 0.0f, -6.0f);
	glColor3f(1.0,0.0,0.0);
	glBegin(GL_TRIANGLES);
		glVertex3f(0.0f, 1.0f, 0.0f);
		glVertex3f(-1.0f, -1.0f, 0.0f);
		glVertex3f(1.0f, -1.0f, 0.0f);
	glEnd();
	
	return;
}

The code in bold, when added to the rest, produces a black background and white square like it's suppose to. I don't know if that tells you anything about the rest of my code or not...

 

[cscott]

Argh! It seems that a silly glFlush(); fixed my problems. The code in bold above was from a book and gave me the hint

 

[ravenprp]

Thank you for providing the code and describing the issue you are having. It seems like the main issue is with the colors and background not being displayed correctly. Let's take a look at the code and see where the problem might be.

First, in the initGL() function, you are setting the clear color to red (1.0, 0.0, 0.0). This means that the background will be red instead of black. To fix this, change the values to (0.0, 0.0, 0.0) to set the background to black.

Next, in the resizeScene() function, you are setting the viewport to the width and height of the window, but you are not setting the projection matrix properly. Instead of using gluPerspective(), you should use glOrtho() to set up an orthographic projection. This will allow you to see the triangle in the center of the screen. You can use the same values that are currently being used in glOrtho() in the main() function.

Finally, in the drawScene() function, you are setting the color to red again (1.0, 0.0, 0.0) before drawing the triangle. This will make the triangle red instead of white. Change the color to (1.0, 1.0, 1.0) to make the triangle white.

With these changes, the background should be black and the triangle should be white. I hope this helps you get your OpenGL app running properly. If you continue to have issues, feel free to reach out for further assistance.