Why glClear doesn't clear my screen? - c

Question

Here is a simple opengl program by me. I'm trying to clear the screen before I draw a triangle. I've called glClear() in my init() function, however, it seemed that it failed to clear the screen.
#include <stdio.h>
#include <stdlib.h>
#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
void myIdleFunc()
{
glBegin(GL_TRIANGLES);
{
glColor3f(1.0f, 1.0f, 1.0f);
glVertex2f(0.0f, 1.0f);
glVertex2f(-1.0f, -1.0f);
glVertex2f(1.0f, -1.0f);
}
glEnd();
glFlush();
usleep(1000000);
}
void init()
{
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
glFlush();
}
int main(int argc, char **argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE);
glutCreateWindow("Hello, World!");
init();
glutIdleFunc(myIdleFunc);
glutMainLoop();
return 1;
}
Here is a screen-shot, the text is from the gnome terminal in the back ground.

Answer 1

Where's your display callback? You shouldn't use the idle function for drawing.
All drawing needs to take place in the appropriate callbacks, the GL context might not be active until glutMainLoop starts running, and with no active context, your commands simply get ignored (without a context, there might not even be a place to store errors for retrieval with glGetError).
NOTE: Usually you want to clear the buffer at the beginning of every frame. You might get away with clearing just once with single-buffering, but double-buffering is better and requires you to somehow render the entire area between each swap.

Answer 2

Your problem is, that you do clear the screen in your initialization code. But you need to clear it every frame, so right at the start of your display (or in your case idle) function.

Answer 3

I don't use GLUT but a really simple way to display something like this is:
while( !done ) { /* Loop until done. Do drawing. */
glClear( GL_COLOR_BUFFER_BIT);
glLoadIdentity();
glBegin(GL_TRIANGLES);
{
glColor3f(1.0f, 1.0f, 1.0f);
glVertex2f(0.0f, 1.0f);
glVertex2f(-1.0f, -1.0f);
glVertex2f(1.0f, -1.0f);
}
glEnd();
glFlush();
SwapBuffers( );
etc... event handling....
}

Answer 4

Run this code and probably you will get the solution.
#include<GL/gl.h>
#include<GL/glut.h>
#include<stdio.h>
double x_0 = -100;
double y_0 = -25;
double x_1 = 100;
double y_1 = -25;
double x_2 = 100;
double y_2 = 25;
double x_3 = -100;
double y_3 = 25;
void
init(void)
{
/*initialize the x-y co-ordinate*/
glClearColor(0,0,0,0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-320, 319,-240, 239);
glClear(GL_COLOR_BUFFER_BIT);
glFlush();
}
void
drawRectangle()
{
glBegin(GL_LINES);
glVertex2d(x_0, y_0);
glVertex2d(x_1, y_1);
glVertex2d(x_1, y_1);
glVertex2d(x_2, y_2);
glVertex2d(x_2, y_2);
glVertex2d(x_3, y_3);
glVertex2d(x_3, y_3);
glVertex2d(x_0, y_0);
glEnd();
glFlush();
}
int
main(int argc, char *argv[])
{
double x_0, y_0, x_1, y_1;
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(640, 480);
glutInitWindowPosition(400, 400);
glutCreateWindow("Clear Screen");
init();
drawRectangle();
/* clear the screen. You can uncomment following three lines to view the effect of clearing */
// glClearColor(0, 0, 0, 0);
// glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// /* don't forget to flush */
// glFlush();
/***/
glutMainLoop();
}
Compile and run -
gcc file.c -lglut -lGLU -lGL
./a.out