CGVR-lab-programs

List of programs for CGVR lab (7th sem)

1. Bresenham Line
2. Bresenham Circle
3. Double buffer (spin rectangle)
4. Cylinder and Parallellopiped
5. 3D Sierpinski gasket
6. 3D sphere approximation
7. Scan-line filling algorithm
8. Cohen Sutherland line clipping
9. Liang-Barsky line clipping
10. Cohen-hedgeman polygon clipping
11. House rotation and reflection
12. Color cube and spinning it

How to run these programs

I followed this tutorial (https://medium.com/swlh/setting-opengl-for-windows-d0b45062caf)

And instead of running on codeblocks, programs are compiled directly with this command

gcc prog.c -I"C:\msys64\mingw64\include" -lopengl32 -lglu32 -lfreeglut

General openGL workings

Some general info to be kept in mind

For 2D stuff camera setup

gluOrtho2D(int left, int right, int bottom, int top)

For 3D stuff camera setup

glOrtho(int left, int right, int bottom, int top, int near, int far)

Never forget to call glFlush()

It is responsinle for pushing whatever is in display buffer onto screen. If not called, rendering might not be seen. Generally, call this whenever anything is drawn or updated onto screen.

Use glEnable(GL_DEPTH_TEST)

When any 3D stuff is happening or some so rt of animation, use this always. In addition to this, add GLUT_DEPTH to glutInitDisplay() and GL_DEPTH_BUFFER_BIT to glClear().

For animations

Never forget glFlush(). Some programs might need glutPostRedisplay(), remember this also.

Geometric Transformations

Use glRotatef / glTranslatef / glScalef. These apply transformations on whole scene. To apply to just one drawing, use this method -

glPushMatrix();
glRotatef(...) or whatever trasformations

... draw stuff ...

glPopMatrix();

Rotation about fixed point

About fixed point, you need to carry out Translation, then Rotation, then inverse Translation

glTranslatef(x,y,0);
glRotatef(angle, x,y,z);
glTranslatef(-x,-y,0);

Reflection about a line

We do Translaton, Rotation, Scale, inverse Rotate, inverse Translate

glTranslatef(0,c,0);  // c is the y-intercept
glRotatef(angle, 0, 0, 1);  // angle = atan(slope) * RADIAN2DEGREE
glScalef(1,-1,1);
glRotatef(angle, 0, 0, 1);
glTranslatef(0,-c,0);

Having more than one window

Keep reference of window_id of each window, and possibly assign different display() functions. Within display(), call glutSetWindow(window_id) first.

For 3D sphere approximation

To view the sphere properly, use these functions

// this makes sure only lines are colored and polygons are not filled
glPolygonMode(GL_FRONT_AND_BACK, GL_LINE); 

// removal of back faces so sphere can be viewed easily
glEnable(GL_CULL_FACE);
glCullFace(GL_BACK); 

For drawing lines with mouse

In case camera is set using gluOrtho2D(-WIDTH/2, WIDTH/2, -HEIGHT/2, HEIGHT/2), use this to draw line

drawLine(xglobal-WIDTH/2, HEIGHT/2-yglobal, x-WIDTH/2, HEIGHT/2-y);

In case camera is set using gluOrtho2D(0, WIDTH, 0, HEIGHT), use this to draw line

drawLine(xglobal, HEIGHT-yglobal, x, HEIGHT-y);