Clipping is a technique that identifies parts of an image that are inside or outside a defined clipping region or window. There are different types of clipping including point, line, polygon, curve, and text clipping. The Cohen-Sutherland algorithm is commonly used for line clipping. It assigns 4-bit codes to line endpoints to determine if they are fully inside, outside, or intersect the clipping window boundary. Intersecting line segments are then subdivided and clipped. Midpoint subdivision is another algorithm that divides partially visible lines at their midpoint into shorter segments.

1. Clipping
Procedure that identifies those portions of a
picture that are either inside or outside a
specified region of space is referred to as a
clipping algorithm, or clipping. A region
against which a object is to clipped is called
a clip window. Application of clipping
includes extracting a portion of a scene,
drawing and painting operations.
In clipping only those elements will be
displayed which comes inside the window.

2. Everything outside the window is discarded.
Sheilding: Determines those elements of the
picture which will not be displayed.
Clipping is basically of following types:-
1.) Point Clipping
2.) Line Clipping
3.) Polygon Clipping
4.) Curve Clipping
5.) Text Clipping

3. Point Clipping
Assuming that the clip window is a rectangle
in standard position, we save a point P = (x,y)
for display if the following inequalities are
satisfied: xwmin < x < xwmax
ywmin < y < ywmax
where the edges of the clip window(xwmin,
xwmax, xymin, xymax) can be either the world-
coordinate window boundaries or viewport
boundaries.

4. If any one of these four inequalities is not
satisfied, the point is clipped( not saved for
display).
Point clipping can be applied to scenes
involving explosions or sea foam that are
modeled with particles (points) distributed
in some region of the scene.

5. Line Clipping
A Clipping line procedure involves two parts:
1.)If we cannot identify a line as completely
inside or outside the clipping window then
we must perform calculation of those lines
which intersect the clipping window.
2.) Then we start clipping.
We process lines by checking their endpoints
& test which points of the lines comes inside
or outside the clip window.

6. P6 P2
P8
P1 P4
P5
P3
Before Clipping P7 Clipping Window
P2
P1 P4
P8’
P3’ P7’
After Clipping

7. In Line Clipping some lines may be invisible
some may be visible & some may be
partially visible.
Line P1P2 is visible b’coz both endpoints
comes inside the clipping window.
Line P5P6 is invisible b’coz there endpoints
are completely exterior to the window.
Line P3P4 & P7P8 are partially visible lines.

8. COHEN- SUTHERLAND ALGORITHM
Steps for finding out the clipping of lines are:
1.) We use 4-bit code for every endpoint of
the line. Bits are set to 1 depending upon
the following conditions:
1st bit (left most bit)- will set to 1 if end
point is above the top edge of window.
2nd bit – set to 1 if endpoint is below the
bottom edge of the window.
3rd bit – set to 1 if endpoint is to the right
of the right edge of the window.

9. 4th bit – set to 1 if endpoint is to the left of
the left edge of the window.
Left Edge Top Edge
1001 1000 1010
Clip Window 0010
0001
0000
0101 0100 0110
Right Edge
Bottom Edge

10. P6 P2 P8
P1
P4
P5
P3 P7

11. 2.) Using coding scheme we can say that:
Line EP1 EP2 Logical Results
AND
P1P2 0000 0000 0000 Totally visible
P3P4 0001 0000 0000 Partially visible
P5P6 0001 0001 0001 Totally invisible
P7P8 0000 0000 0000 Totally visible
3.) After selecting those lines which are
partially visible, we determine those points

12. of the line which intersect with the Clipping
window boundaries. Then we subdivide the
intersection points into various smaller line
segments in visible & not visible categories.
The line segment which comes under visible
category will be clipped.
4.) Using the bit values of the endpoints of
intersecting lines we choose those endpoints
of segments which have no intersecting

13. Bit value (any bit value should not be 1) i.e
which have bit values equivalent to the
boundary of Clipping window(0000).
MIDPOINT SUBDIVISION ALGORIT:-
For Clipping any Partially visible line we use
this algorithm. It is based on the bisection
method.The line is divided at its midpoint into
two shorter line segments using the endpoint
values.

14. The midpoint coordinates (xm, ym) of aline
joining (x1,y1) & (x2,y2) are given by
xm = x1 + x2 ym = y1 + y2
2 2