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Prof. Neeraj Bhargava
Pooja Dixit
Department of Computer Science
School of Engineering & System Sciences
MDS, University Ajmer, Rajasthan, India
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 Clipping Basics
 Cohen-Sutherland Line Clipping
 Clipping Polygons
 Sutherland-Hodgman Clipping
 Perspective Clipping
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 Clipping is a process that identifies those portions of a picture that
are either inside or outside of a specified region or space is known as
clipping.
 Clip regions are commonly specified to improve render performance.
A well-chosen clip allows the renderer to save time and energy by
skipping clculations related to pixels that the user cannot seeworld
coordinates
Windowing I
A scene is made up of a collection of
objects specified in world coordinates
View coordinates
Windowing II
When we display a scene only those
objects within a particular window are
displayed.
world coordinates world coordinates
WYmin
WYmax
WYmin
WYmax
WXmin WXmax WXmin WXmax
Windowing III
Because drawing things to a
display takes time we clip
everything outside the window

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 Consider an example in figure that shows
which lines and points should be kept and
which ones should be clipped.

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 What is Point Clipping?
 Point clipping helps in identifying whether a particular point (X, Y) is
within the window and accordingly take appropriate actions for using the
coordinates of the window, either maximum or minimum.
 If x satisfies that Wx1 ≤ X ≤ Wx2, then the coordinate X is inside the
window, and if Y satisfies that Wy1 ≤ Y ≤ Wy2, then Y lies inside the
window.
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 What is Line Clipping?
 The part of the line inside the window is kept and the part of
the line appearing outside is removed in Line Clipping.
 Cohen-Sutherland Line Clippings
 The clip window depicted below is used by this algorithm.
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 The entire region is divided by using 4-bits which represent
Top, Bottom, Right, and Left of the region, As it is the TOP-
LEFT corner, the TOP and LEFT bit is set to 1.
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 For the line, there are three different possibilities:
 Line can be completely inside the window (This line should be
accepted).
 Line can be completely outside of the window (This line will
be completely removed from the region).
 Line can be partially inside the window (We will find
intersection point and draw only that portion of line that is
inside region).
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 Algorithm
Step 1 – For each endpoints, a region code is assigned.
Step 2 – The line is accepted if both endpoints have a region
code 0000.
Step 3 − Else, the logical AND operation is performed for
both region codes.
Step 3.1 − If the result is not 0000, then reject the line.
Step 3.2 − Else clipping is required.
Step 3.2.1 − An endpoint of the line is selected that is
outside the window.
Step 3.2.2 − Find the intersection point at the window
boundary (base on region code).
Step 3.2.3 – The endpoint is replaced with the intersection
point and the region code is updated.
Step 3.2.4 − Repeat step 2 until we find a clipped line either
trivially accepted or trivially rejected.
Step 4 − Repeat step 1 for other lines.
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 What is Polygon Clipping (Sutherland Hodgman Algorithm)?
 Against the edges of the clipping window, the vertices of the
polygon are clipped by using Sutherland Hodman Algorithm.
 New vertices of the polygon are obtained by clipping the left
edge of the polygon window. The polygon is clipped against
right edge by using the new vertices as depicted below.
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 If the edge is not completely inside the clipping window, an intersection
point appears. The portion that is outside is clipped as depicted below:
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 What is Text Clipping?
 Text clipping can be provided in computer graphics by using
different techniques. The characters are generated and the
requirements of a particular application. Text clipping can be done
in three different methods:
 All or none string clipping
 All or none character clipping
 Text clipping
 All or none string clipping is depicted below:
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 Under this method, either the entire string is kept or the
entire string is rejected. From the above figure, STRING2 is
entirely inside the clipping window and so it is kept and
STRING1 is rejected as it is partially inside the window.
 The following figure shows all or none character clipping −
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 It is based on characters rather than on entire string. The string that
is inside the clipping window is kept, and if the string is outside the
window then:
 The portion of the string outside the window is rejected
 The entire character is discarded if the character is on the boundary
of the clipping window.
 Text clipping is depicted below:
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 This method depends on characters method
than on the entire string method. The string
that is inside the clipping window is kept and
the string that is partially outside the window:
 The portion of the string outside the window
is rejected.
 The portion of the character that is outside of
the clipping window is discarded, if the
character is on the boundary of the clipping
window.
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