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Chapter: Graphics and Multimedia - Output Primitives

Two Dimensional Clipping and Viewing

Two Dimensional Geometric Transformations




The Viewing Pipeline



A world-coordinate area selected for display. defines what is to be viewed


An area on a display device to which a window is mapped. defines where it is to be displayed

Viewing transformation

The mapping of a part of a world-coordinate scene to device coordinates.

A window could be a rectangle to have any orientation.


Two-Dimensional Viewing


The Viewing Pipeline

Viewing Effects

Zooming effects

Successively mapping different-sized windows on a fixed-sized viewports.

Panning effects

Moving a fixed-sized window across the various objects in a scene.

Device independent

Viewports are typically defined within the unit square (normalized coordinates)


Viewing Coordinate Reference FrameThe reference frame for specifying the world-coordinate window.

Viewing-coordinate origin: P0 = (x0, y0)

View up vector V: Define the viewing yv direction

Window-to-Viewport Coordinate Transformation

Clipping Operations




        Identify those portions of a picture that are either inside or outside of a specified region of space.


         Clip window

        The region against which an object is to be clipped.

        The shape of clip window

         Applications of clipping

         World-coordinate clipping


Clipping Operations

         Viewport clipping


        It can reduce calculations by allowing concatenation of viewing and geometric transformation matrices.


         Types of clipping


        Point clipping

        Line clipping

        Area (Polygon) clipping

        Curve clipping

        Text clipping

         Point clipping (Rectangular clip window)

Line Clipping

Possible relationships between line positions and a standard rectangular clipping region


         Possible relationships

–   Completely inside the clipping window

–   Completely outside the window

–   Partially inside the window

         Parametric representation of a line

x = x1 + u(x2 - x1) y = y1 + u(y2 - y1)

         The value of u for an intersection with a rectangle boundary edge

–   Outside the range 0 to 1

–   Within the range from 0 to 1


Cohen-Sutherland Line Clipping

         Region code

–   A four-digit binary code assigned to every line endpoint in a picture.

–   Numbering the bit positions in the region code as 1 through 4 from right to left.

         Bit values in the region code

        Determined by comparing endpoint coordinates to the clip boundaries

        A value of 1 in any bit position: The point is in that relative position.

         Determined by the following steps:

        Calculate differences between endpoint coordinates and clipping boundaries.


         Use the resultant sign bit of each difference calculation to set the corresponding bit value.


         The possible relationships:

        Completely contained within the window

        0000 for both endpoints.

        Completely outside the window

        Logical and the region codes of both endpoints, its result is not 0000.



Liang-Barsky Line Clipping

Rewrite the line parametric equation as follows:

         pk = 0, parallel to one of the clipping boundary

        qk < 0, outside the boundary

        qk >= 0, inside the parallel clipping boundary

         pk < 0, the line proceeds from outside to the inside

         pk > 0, the line proceeds from inside to outside

Nicholl-Lee-Nicholl Line Clipping

         Compared to C-S and L-B algorithms

        NLN algorithm performs fewer comparisons and divisions.

        NLN can only be applied to 2D clipping.

         The NLN algorithm

        Clip a line with endpoints P1 and P2


         First determine the position of P1 for the nine possible regions.

        Only three regions need be considered

        The other regions using a symmetry transformation

        Next determine the position of P2 relative to P1.

         To determine the region in which P2 is located

–   Compare the slope of the line to the slopes of the boundaries of the clip region.

–   Example: P1 is left of the clipping rectangle, P2 is in region LT.

Polygon Clipping

Sutherland-Hodgeman Polygon Clipping

         Processing the polygon boundary as a whole against each window edge

         Processing all polygon vertices against each clip rectangle boundary in turn

         Pass each pair of adjacent polygon vertices to a window boundary clipper

There are four cases:

         Intermediate output vertex list

        Once all vertices have been processed for one clip window boundary, it is generated.

        The output list of vertices is clipped against the next window boundary.

        It can be eliminated by a pipeline of clipping routine.

         Convex polygons are correctly clipped.

         If the clipped polygon is concave

        Split the concave polygon


Weiler-Atherton Polygon Clipping

           Developed as a method for identifying visible surfaces

        It can be applied with arbitrary polygon-clipping region.

        Not always proceeding around polygon edges

        Sometimes follows the window boundaries

         For clockwise processing of polygon vertices

        For an outside-to-inside pair of vertices, follow the polygon boundary.

        For an inside-to-outside pair of vertices, follow the window boundary in clockwise direction.

         Curve clipping

        Use bounding rectangle to test for overlap with a rectangular clip window.

         Text clipping

        All-or-none string-clipping

        All-or-none character-clipping

        Clip the components of individual characters

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