glClear not working on execution - opengl-es-2.0

Question

I am trying to create an interactive openGL program which takes user input and displays a shape. Everything seems to be working once the user inputs something...but on first execution, the glClear in the display function isn't actually clearing the screen...am I missing something somewhere?
Thanks!!
#include <stdlib.h>
#include <stdio.h>
#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
void exit(int);
void myInit(void){
glClearColor(0.0, 0.0, 0.0, 1.0);
glColor3f(1.0f, 0.0f, 0.0f);
glLineWidth(4.0);
}
//SET WINDOW
void setWindow(double left, double right, double bottom, double top){
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(left, right, bottom, top, -1.0, 1.0); //viewing volume at origin, edge length 2
}
//SET VIEWPORT
void setViewport(int x, int y, int width, int height){
glViewport(x, y, width, height);
}
//DISPLAY FUNCTION
void myDisplay(void){
glClear(GL_COLOR_BUFFER_BIT);
}
void myKeyboard(unsigned char key, int mouseX, int mouseY){
switch(key)
{
case 'Q':
case'q':
exit(-1);
case 't':
//displayTriangle();
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_POLYGON);
glVertex2f(-0.5, -0.5);
glVertex2f(0.0, 0.5);
glVertex2f(0.5, -0.5);
glEnd();
glFlush();
glutPostRedisplay();
break;
default:
break;
}
}
int main(int argc, char** argv){
glutInit(&argc, argv); //initialize toolkit
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(640,480);
glutInitWindowPosition(100, 150);
glutCreateWindow("Interactive Shape Display!");
myInit();
glutDisplayFunc(myDisplay);
glutKeyboardFunc(myKeyboard);
glutMainLoop();
return 0;
}

Answer 1

glClear(GL_COLOR_BUFFER_BIT)
will clear the screen to the glClearColor() value. I'll run your code when I get a chance, but the glColor3f(1.0f, 0.0f, 0.0f) will set the color to red for your vertices (glVertex2f(-0.5, -0.5)...) until you change the glColor3f(). After you set the glClearColor() (black), you should be able to clear the screen again, anytime you call glClear(). You call it with myDisplay(). When you enter myKeyboard(), glClear () is called again, but the red glVertex2f()s are called immediately, which will draw over it.
When you say that the screen is not cleared on first execution, are you not seeing black initially? Will run the code and edit this if any results change for me.
Edit:
It's a good idea to move the drawing code to the display function. If you still want keyboard interaction, you could have a glutTimerFunc() in your main method:
glutTimerFunc(1, timerLoopCallback, 0);
The timer's callback would perform any logic based on the keyboard callbacks. So myKeyboard() registers the 't' key and the timer callback determines a triangle must be drawn (maybe setting a variable).
glutPostRedisplay();
should come next in the timer callback, then another call to the timer func (exactly as above). In the display func, the triangle is called if the timer callback determined it should.
This describes the relation between drawing and its separation from other logic. You might also want glutSwapBuffers(); at the end of your draw code.