# OpenGL program for simple Animation (Revolution) in C

OpenGL is a cross-language, cross-platform API for rendering 2D and 3D Vector Graphics. Using this, we can make a lot of design as well as animations. Below is the simple animation made using OpenGL.

Approach :
To make a picture moving, we need to understand the working procedure of a function used to display i.e glClear(GL_COLOR_BUFFER_BIT). Its task is to clear screen with default value after a certain time (normally, after 1/30 sec or 1/60 sec). So, if any change of coordinate happens, then it will appear to be moving as human eye can distinguish image only which is separated by 1/16 second (persistence of vision).

Now, the coordinates of circle are X = r*cos(θ) and Y = r*sin(θ) or for ellipse X = rx*cos(θ) and Y = ry*cos(θ) where rx and ry are radius in X- and Y- direction and θ is the angle.
If we vary θ from 0 to 2*pi (360 degree) at very small increase (say of 1 degree) and draw point on that coordinate, we can make a complete circle or ellipse. We can also make semi-circle or any arc of circle or ellipse by varying the starting and ending value of θ (angle).

These concepts are used to draw the following Animation:

• 7 horizontal parts of ellipse and 3 vertical complete ellipse as well as 1 outer circle and one outer ellipse are used to visualise an orbit drawn by adjusting the θ as well as radius.
• One vertical line is drawn to make the figure. Then to make it move, an oter loop is given where value of j changes with very small amount to make the motion smoother.
• Since, we had to make all point moving in same type of motion to keep the figure together, so equation of motion, that is, glVertex2i(x/2 – 600*cos(j), y/2 – 100*sin(j)) is given inside every inner for loop, so that it can be applied to all points altogether.

For working on Ubuntu operating system:

```gcc filename.c -lGL -lGLU -lglut -lm
where filename.c is the name of the file
with which this program is saved.
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Below is the implementation in C.

 `// C Program to illustrate  ` `// OpenGL animation for revolution ` ` `  `#include ` `#include ` `#include ` ` `  `// global declaration ` `int` `x, y; ` `float` `i, j; ` ` `  `// Initialization function ` `void` `myInit (``void``) ` `{ ` `    ``// Reset background color with black (since all three argument is 0.0) ` `    ``glClearColor(0.0, 0.0, 0.0, 1.0); ` `     `  `    ``// Set picture color to green (in RGB model) ` `    ``// as only argument corresponding to G (Green) is 1.0 and rest are 0.0 ` `    ``glColor3f(0.0, 1.0, 0.0); ` `     `  `    ``// Set width of point to one unit ` `    ``glPointSize(1.0); ` `    ``glMatrixMode(GL_PROJECTION); ` `    ``glLoadIdentity(); ` `     `  `    ``// Set window size in X- and Y- direction ` `    ``gluOrtho2D(-780, 780, -420, 420); ` `} ` ` `  `// Function to display animation ` `void` `display (``void``) ` `{ ` `    ``// Outer loop to make figure moving ` `    ``// loop variable j iterated up to 10000, ` `    ``// indicating that figure will be in motion for large amount of time ` `    ``// around 10000/6.29 = 1590 time it will revolve ` `    ``// j is incremented by small value to make motion smoother ` `    ``for` `(j = 0; j < 10000; j += 0.01) ` `    ``{ ` `        ``glClear(GL_COLOR_BUFFER_BIT); ` `        ``glBegin(GL_POINTS); ` `         `  `        ``// Iterate i up to 2*pi, i.e., 360 degree ` `        ``// plot point with slight increment in angle, ` `        ``// so, it will look like a continuous figure ` ` `  `        ``// Loop is to draw outer circle ` `        ``for` `(i = 0;i < 6.29;i += 0.001) ` `        ``{ ` `            ``x = 200 * ``cos``(i); ` `            ``y = 200 * ``sin``(i); ` `            ``glVertex2i(x, y); ` `             `  `            ``// For every loop, 2nd glVertex function is ` `            ``// to make smaller figure in motion ` `            ``glVertex2i(x / 2 - 600 * ``cos``(j), y / 2 - 100 * ``sin``(j)); ` `        ``} ` `         `  `        ``// 7 loops to draw parallel latitude ` `        ``for` `(i = 1.17; i < 1.97; i += 0.001) ` `        ``{ ` `            ``x = 400 * ``cos``(i); ` `            ``y = -150 + 300 * ``sin``(i); ` `            ``glVertex2i(x, y); ` `            ``glVertex2i(x / 2 - 600 * ``cos``(j), y / 2 - 100 * ``sin``(j)); ` `        ``} ` `         `  `        ``for` `(i = 1.07; i < 2.07; i += 0.001) ` `        ``{ ` `            ``x = 400 * ``cos``(i); ` `            ``y = -200 + 300 * ``sin``(i); ` `            ``glVertex2i(x, y); ` `            ``glVertex2i(x / 2 - 600 * ``cos``(j), y / 2 - 100 * ``sin``(j)); ` `        ``} ` `         `  `        ``for` `(i = 1.05; i < 2.09; i += 0.001) ` `        ``{ ` `            ``x = 400 * ``cos``(i); ` `            ``y = -250 + 300 * ``sin``(i); ` `            ``glVertex2i(x, y); ` `            ``glVertex2i(x / 2 - 600 * ``cos``(j), y / 2 - 100 * ``sin``(j)); ` `        ``} ` `         `  `        ``for` `(i = 1.06; i < 2.08; i += 0.001) ` `        ``{ ` `            ``x = 400 * ``cos``(i); ` `            ``y = -300 + 300 * ``sin``(i); ` `            ``glVertex2i(x, y); ` `            ``glVertex2i(x / 2 - 600 * ``cos``(j), y / 2 - 100 * ``sin``(j)); ` `        ``} ` `         `  `        ``for` `(i = 1.10; i < 2.04; i += 0.001) ` `        ``{ ` `            ``x = 400 * ``cos``(i); ` `            ``y = -350 + 300 * ``sin``(i); ` `            ``glVertex2i(x, y); ` `            ``glVertex2i(x / 2 - 600 * ``cos``(j), y / 2 - 100 * ``sin``(j)); ` `        ``} ` `         `  `        ``for` `(i = 1.16; i < 1.98; i += 0.001) ` `        ``{ ` `            ``x = 400 * ``cos``(i); ` `            ``y = -400 + 300 * ``sin``(i); ` `            ``glVertex2i(x, y); ` `            ``glVertex2i(x / 2 - 600 * ``cos``(j), y / 2 - 100 * ``sin``(j)); ` `        ``} ` `         `  `        ``for` `(i = 1.27; i < 1.87; i += 0.001) ` `        ``{ ` `            ``x = 400 * ``cos``(i); ` `            ``y = -450 + 300 * ``sin``(i); ` `            ``glVertex2i(x, y); ` `            ``glVertex2i(x / 2 - 600 * ``cos``(j), y / 2 - 100 * ``sin``(j)); ` `        ``} ` `         `  `        ``// Loop is to draw vertical line ` `        ``for` `(i = 200; i >=- 200; i--) ` `        ``{ ` `            ``glVertex2i(0, i); ` `            ``glVertex2i(-600 * ``cos``(j), i / 2 - 100 * ``sin``(j)); ` `        ``} ` `         `  `        ``// 3 loops to draw vertical ellipse (similar to longitude) ` `        ``for` `(i = 0;i < 6.29; i += 0.001) ` `        ``{ ` `            ``x = 70 * ``cos``(i); ` `            ``y = 200 * ``sin``(i); ` `            ``glVertex2i(x, y); ` `            ``glVertex2i(x / 2 - 600 * ``cos``(j), y / 2 - 100 * ``sin``(j)); ` `        ``} ` `         `  `        ``for` `(i = 0; i < 6.29; i += 0.001) ` `        ``{ ` `            ``x = 120 * ``cos``(i); ` `            ``y = 200 * ``sin``(i); ` `            ``glVertex2i(x, y); ` `            ``glVertex2i(x / 2 - 600 * ``cos``(j), y / 2 - 100 * ``sin``(j)); ` `        ``} ` `         `  `        ``for` `(i = 0; i < 6.29; i += 0.001) ` `        ``{ ` `            ``x = 160 * ``cos``(i); ` `            ``y = 200 * ``sin``(i); ` `            ``glVertex2i(x, y); ` `            ``glVertex2i(x / 2 - 600 * ``cos``(j), y / 2 - 100 * ``sin``(j)); ` `        ``} ` `         `  `        ``// Loop to make orbit of revolution ` `        ``for` `(i = 0; i < 6.29; i += 0.001) ` `        ``{ ` `            ``x = 600 * ``cos``(i); ` `            ``y = 100 * ``sin``(i); ` `            ``glVertex2i(x, y); ` `        ``} ` `        ``glEnd(); ` `        ``glFlush(); ` `    ``} ` `} ` ` `  `// Driver Program ` `int` `main (``int` `argc, ``char``** argv) ` `{ ` `    ``glutInit(&argc, argv); ` `     `  `    ``// Display mode which is of RGB (Red Green Blue) type ` `    ``glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB); ` `     `  `    ``// Declares window size ` `    ``glutInitWindowSize(1360, 768); ` `     `  `    ``// Declares window position which is (0, 0) ` `    ``// means lower left corner will indicate position (0, 0) ` `    ``glutInitWindowPosition(0, 0); ` ` `  `    ``// Name to window ` `    ``glutCreateWindow(``"Revolution"``); ` ` `  `    ``// Call to myInit() ` `    ``myInit(); ` `    ``glutDisplayFunc(display); ` `    ``glutMainLoop(); ` `} `

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