Application of sutherland hodgman polygon clipping, Computer Graphics

For good understanding of the application of the rules specified above see the following figure, where the shaded region demonstrates the clipped polygon.

801_Application of Sutherland Hodgman Polygon Clipping.png

Figure: Sutherland-Hodgman Polygon Clipping

Pseudo code for Sutherland - Hodgman Algorithm

Define variables

In VertexArray is the array of input polygon vertices

outVerteArray is the array of output polygon vertices

Nin is the number of entries in inVertexArray

Nout is the number of entries in outVertexArray

n is the number of edges of the clip polygon

ClipEdge[x] is the xth edge of clip polygon defined by a pair of vertices

s, p are the start and end point respectively of current polygon edge

i is the intersection point with a clip boundary

j is the vertex loop counter

Define Functions

AddNewVertex(newVertex, Nout, outVertexArray)

: Adds newVertex to outVertexArray and then updates Nout

InsideTest(testVertex, clipEdge[x])

: Checks whether the vertex lies inside the clip edge or not;

  retures         TRUE is inside else returns FALSE

Intersect (first, second, clipEdge[x])

: Clip polygon edge (first, second) against clipEdge[x], outputs the intersection point

{                                        :  begin main

x = 1

while (x ≤ n)                     : Loop through all the n clip edges


Nout = 0                            : Flush the outVertexArray

s = inVertexArray[Nin] : Start with the last vertex in inVertexArray

for j = 1 to Nin do             : Loop through Nin number of polygon vertices (edges)


p = inVertexArrray[j]

if InsideTest(p, clipEdge[x] = = TRUE then                     : Case A

and D

if InsideTest(s, clipEdge[x] = = TRUE then

AddNewVertex(p, Nout, outVertexArray)                        : Case A


i = Intersect(s, p, clipEdge[x])                           :  Case D

AddNewVertex(i, Nout, outVertexArray)

AddNewVertex(p, Nout, outVertexArray)

end if

else      :  i.e. if InsideTest(p, clipEdge[x] = = FALSE

(Cases 2 and 3)

if InsideTest(s, clipEdge[x]) = =TRUE then                     : Case B


 Intersect(s, p, clipEdge[x])

AddNewVertex(i, Nout, outVertexArray)

end if                                        : No action for case C


s = p                                         : Advance to next pair of vertices j = j + 1

end if                                         :  end {for}


x = x + 1                                     : Proceed to the next ClipEdge[x +1]

Nin = Nout

inVertexArray = outVertexArray            :  The output vertex array for the current clip edge becomes the input vertex array for the next clip edge

}                                        : end while

}                                        : end main

Posted Date: 4/3/2013 3:50:42 AM | Location : United States

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