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(?) and Y = r*sin(?) or for ellipse X = rx*cos(?) and Y = ry*cos(?) where rx and ry are radius in X- and Y- direction and ? is the angle.
If we vary ? 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 ? (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 ? as well as radius.
- One vertical line is drawn to make the figure. Then to make it move, an other 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.
Below is the implementation in C.
C
// C Program to illustrate // OpenGL animation for revolution #include<stdio.h> #include<GL/glut.h> #include<math.h> // 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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