Unit-4�Clipping

PREPARED BY: SUSHANT BHATTARAI

Clipping

SUSHANT BHATTARAI

• Process of discarding the parts of the picture that are outside of a specific region is called clipping
• Any procedure that identifies those portions of a picture that are either inside or outside of a specified region of space is referred to as a clipping algorithm
• The region against which an object is clipped is called a clip window

Prepared By:Sushant Bhattarai (Department of CSIT,MMAMC)

3

Applications of Clipping

• Extracting part of a defined scene for viewing
• Identifying visible surfaces in three-dimensional views
• Antialiasing line segments or object boundaries
• Creating objects using solid-modeling procedures
• Displaying a multi-window environment
• Drawing and painting operations that allow parts of a picture to be selected for copying, moving, erasing, or duplicating

Prepared By:Sushant Bhattarai (Department of CSIT,MMAMC)

4

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

xw_min ≤ x ≤ xw_max

yw_min ≤ y ≤ yw_max

• where the edges of the clip window (xw_min, xw_max, yw_min, yw_max) can be either the world-coordinate window boundaries or viewport boundaries
• If any one of these four inequalities is not satisfied, the point is clipped (not saved for display)

Prepared By:Sushant Bhattarai (Department of CSIT,MMAMC)

5

Line Clipping

• Lines that donot intersect clipping window are either completely inside or outside the window.
• Line clipping can be sub divided on the basis of the alignment of the line segment as:
• Visible
1. Both end point of line segment lie inside the window
2. No clipping necessary
• Non –visible
• Lines are completely outside the window
• Discard the line segment

Prepared By:Sushant Bhattarai (Department of CSIT,MMAMC)

6

Line Clipping

• Partially Visible (Clipping Candidate)
1. A line is partially visible when it lies within a window
2. Here we need to compute the intersection point and clip at intersection point

Prepared By:Sushant Bhattarai (Department of CSIT,MMAMC)

7

Cohen Sutherland Line Clipping Algorithm

• Oldest and most popular line clipping algorithm
• Co-ordinate system is divided into 9 regions. All regions have associated codes.
• Endpoint of every line is assigned a four digit binary code.
• Each bit in the code is set to either 1 or 0.
• Region code for point inside clipping window is 0000.

​

Prepared By:Sushant Bhattarai (Department of CSIT,MMAMC)

8

Cohen Sutherland Line Clipping Algorithm

• For any endpoint(x,y) can be identified as follows:
1. First bit(left most) is set to 1 if the point lies towards left of window
2. Second bit is set to 1 if the point lies towards right of window
3. Third bit is set to 1 if the point lies towards bottom/below of window
4. Fourth bit is set to 1 if the point lies towards top/above of window

​

Prepared By:Sushant Bhattarai (Department of CSIT,MMAMC)

9

• Algorithm
• Step 1 : Assign a region code for two endpoints of given line
• Step 2 : If both endpoints have a region code 0000 then given line is completely inside and we will keep this line
• Step 3 : If step 2 fails, perform the logical AND operation for both region codes.

Step 3.1 : If the result is not 0000, then given line is completely outside.

Step 3.2 : Else line is partially inside.

Step 3.2.a : Choose an endpoint of the line that is outside the given rectangle.

Step 3.2.b : Find the intersection point of the rectangular boundary (based on region code).

Step 3.2.c : Replace endpoint with the intersection point and update the region code.

Step 3.2.d : Repeat step 2 until we find a clipped line either trivially accepted or rejected.

• Step 4 : Repeat step 1 for all lines

Prepared By:Sushant Bhattarai (Department of CSIT,MMAMC)

10

Liang-Barsky Line Clipping

• Taught in class

Prepared By:Sushant Bhattarai (Department of CSIT,MMAMC)

11

Sutherland-Hodgeman Polygon Clipping Algorithm

• Uses divide and conquer strategy.
• Clips polygon successively against the edge of the given clip window.

​

Prepared By:Sushant Bhattarai (Department of CSIT,MMAMC)

12

Sutherland-Hodgeman Polygon Clipping Algorithm

• There are four possible cases for any given edge of given polygon against current clipping edge e.
• Both vertices are inside : Only the second vertex is added to the output list
• First vertex is outside while second one is inside : Both the point of intersection of the edge with the clip boundary and the second vertex are added to the output list
• First vertex is inside while second one is outside : Only the point of intersection of the edge with the clip boundary is added to the output list
• Both vertices are outside : No vertices are added to the output list

​

Prepared By:Sushant Bhattarai (Department of CSIT,MMAMC)

13