Question about drawing rectangle on opengl - c++

Question

The question asks to draw the rectangle when I press "r" in the keyboard. I try to write a function to draw this rectangle. The character call back function is correct and it gives me feedback that I already press "r". I don't know how to make the triangle appear. I added this line glfwSwapBuffers(window) in the function, but it's still not working. Thanks for help in advance.
#define GLEW_STATIC
#include <GL/glew.h>
#include <GLFW/glfw3.h>
#include <iostream>
#include <vector>
#include <cmath>
#include <iostream>
#define WINDOW_WIDTH 900
#define WINDOW_HEIGHT 600
float frameBuffer[WINDOW_HEIGHT][WINDOW_WIDTH][3];
bool mask[WINDOW_HEIGHT][WINDOW_WIDTH];
GLFWwindow *window;
void display()
{
glColor3f(1.0, 0.0, 0.0);
glBegin(GL_POLYGON);
glVertex2i(50, 90);
glVertex2i(100, 90);
glVertex2i(100, 150);
glVertex2i(50, 150);
glEnd();
glFlush();
}
void CharacterCallback(GLFWwindow* lWindow, unsigned int key)
{
if(char(key) == 'r')
display();
}
void Init()
{
glfwInit();
glfwWindowHint(GLFW_RESIZABLE, GL_FALSE);
window = glfwCreateWindow(WINDOW_WIDTH, WINDOW_HEIGHT, "- <xx>", NULL, NULL);
glfwMakeContextCurrent(window);
glfwSetCharCallback(window, CharacterCallback);
glewExperimental = GL_TRUE;
glewInit();
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
ClearFrameBuffer();
}
int main()
{
Init();
while (glfwWindowShouldClose(window) == 0)
{
glClear(GL_COLOR_BUFFER_BIT);
Display();
glFlush();
glfwSwapBuffers(window);
glfwPollEvents();
}
glfwTerminate();
return 0;
}

Answer 1

When you draw by glBegin/glEnd sequences, then each vertex coordinate is transformed by the current view matrix and current projection matrix.
When you draw a scene, then after the transformations, the coordinates of the geometry have to be in clip space respectively normalized device space. The normalized device space is a cubic volume, with the left bottom front (-1, -1, -1) and right top far (1, 1, 1). All the geometry which is in this cube is "visible" on the viewport. All the geometry which is out of this cube is clipped.
If you want to draw a scene by using window ("pixel") coordinates, then you've to setup an orthographic projection by glOrtho. At Orthographic Projection, the view space coordinates are linearly transformed to the clip space coordinates. This means orthographic projection matrix can be used to scale the view space coordinates. e.g.:
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0, (GLdouble)WINDOW_WIDTH, (GLdouble)WINDOW_HEIGHT, 0.0, -1.0, 1.0);
glMatrixMode(GL_MODELVIEW);
You can do this at the end of Init().