polygon clipping
DESCRIPTION
Computer GraphicsTRANSCRIPT
•Clipping Any procedure which identifies that portion of a picture which is either inside or outside a picture is referred to as a clipping algorithm or clipping.In geometry a polygon is a flat shape consisting of straight lines that are joined to form a circuit. A polygon is traditionally a plane figure that is bounded by a closed path, composed of a finite sequence of straight line segments (i.e., by a closed polygonal chain). These segments are called its edges or sides, and the points where two edges meet are the polygon's vertices. The interior of the polygon is sometimes called its body.
The region against which an object is to be clipped is called clipping window.Clipping is required to specify a localized view along with the convenience and the flexibility of using a window ,because objects in the scene may be completely inside the window, completely outside the window, or partially visible through the window. The Clipping operation eliminates objects or portions of objects that are not visible through the window to ensure the paper construction image.
•Polygon ClippingA polygon boundary processed with a line clipper may be displayed as a series of unconnected line segments,depending on the orientation of the polygon to the clipping window. What we really want to display is a bounded area after clipping. For polygon clipping, we require an algorithm that will generate one or more closed areas that are then scan converted for the appropriate area fill. The output of a polygon clipper should be a sequence of vertices that defines the clipped polygon boundaries. In 2D graphics for example, if the user of an image editing program is modifying an image and has "zoomed in" the view to display only the top half of the image, there is no need for the program to spend any CPU time doing any of the calculations or memory moves needed to display the bottom half. By clipping the bottom half of the image and avoiding these calculations, the program runs faster.
•Sutherland–Hodgeman algorithmThe Sutherland–Hodgeman algorithm is used for clipping polygons. It works by extending each line of the convex clip polygon in turn and selecting only vertices from the subject polygon that are on the visible side
Input each edge (vertex pair) successively.Output is a new list of vertices.Each edge goes through 4 clippers.The rule for each edge for each clipper is:
If first input vertex is outside, and second is inside, output the intersection and the second vertexIf first both input vertices are inside, then just output second vertexIf first input vertex is inside, and second is outside, output is the intersectionIf both vertices are outside, output is nothing
v1
v2v3
v1’
v2v3”
v1”
v3’
Note: Need to consider each of 4 edge boundaries
outside inside
v1v1’
v2
outside inside
v1
v2
outside inside
v1v1’
v2
outside inside
v1
v2
Outside to inside:Output: v1’ and v2
Inside to inside:Output: v2
Inside to outside:Output: v1’
Outside to outside:Output: nothing
v2
v1
v3
Right Clipper
Bottom Clipper
Top Clipper
Left Clipper
v1v2
v2v3
v3v1
v1’
v2
v2’
v2’
v3’v1
v2v2’
v2’v3’
v3’v1
v1v2
v2’
v3’
v1
v2 v2v2’
v2’v3’
v3’v1
v1v2
v3’
Figure 6-27, page 332
v2”
v1’v2
v2’ v2’v2”
v2”v1’
v1’v2
v2v2’
v1’
v2
v2’
v2”
Edges Output Edges Output Edges Output Edges Output
Final
•Weiler- Atherton Polygon ClippingIn this algorithm,the vertex –processing procedures for window boundaries are modified so that concave polygons are displayed correctly. This clipping procedure was developed as a method for identifying visible surfaces,and so it can be applied with arbitrary polygon-clipping regions.The basic idea in this algorithm is that instead of always proceeding around the polygon edges as vertices are processed,we sometimes want to follow the window boundaries .Which path we follow depend on the polygon-processing direction(clockwise or counterclockwise)
Sutherland-Hodgman
Weiler-Atherton
•Importance of clipping in video gamesGood clipping strategy is important in the development of video games in order to maximize the game's frame rate and visual quality. Despite GPU chips that are faster every year, it remains computationally expensive to transform, texture, and shade polygons, especially with the multiple texture and shading passes common today To maximize the game's visual quality, developers prefer to establish the highest possible polygon budget; therefore, every optimization of the graphics pipeline benefits the polygon budget and therefore the game. In video games, then, clipping is a critically important optimization that speeds up the rendering of the current scene, and therefore allows the developer to increase the renderer's polygon budget.
