What is OpenGL and why needed

OpenGL (Open Graphics Library) is a cross-platform, language-independent API (a library of about 250 functions) for rendering 2D and 3D graphics. It communicates with the graphics hardware (GPU) and provides functions to draw primitives, apply transformations, lighting, texturing and so on. It is used from C/C++ (and other languages), with GLUT/GLFW for windows and input.

Why it is required

1. Hardware independence: the same program runs on many platforms and graphics cards (Windows, Linux, macOS).
2. Easy programming: high-level calls (glVertex, glRotate, glLight) replace low-level programming of pixels and hardware.
3. Hardware acceleration: the commands are executed by the GPU, giving fast rendering.
4. Rich features: modelling and viewing transformations, hidden surface removal (depth buffer), lighting and shading, texture mapping, blending, anti-aliasing.
5. Standard and open: widely supported, with good documentation, and used by industry.

Example: drawing a coloured triangle

#include <GL/glut.h>

void display(void) {
    glClear(GL_COLOR_BUFFER_BIT);      // clear the window
    glBegin(GL_TRIANGLES);
        glColor3f(1, 0, 0); glVertex2f(-0.5f, -0.5f);
        glColor3f(0, 1, 0); glVertex2f( 0.5f, -0.5f);
        glColor3f(0, 0, 1); glVertex2f( 0.0f,  0.5f);
    glEnd();
    glFlush();
}

int main(int argc, char **argv) {
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutCreateWindow("Triangle");
    glutDisplayFunc(display);
    glutMainLoop();
    return 0;
}

The output is a triangle with red, green and blue corners and colour blending in between.