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Computer Graphics1 Geometry Area of polygons & Volume Of Polygonal surfaces

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Computer Graphics2 Calculating Area n Calculation of the area of a polygon can be done in a very simple way. n Assume that the polygon is represented by (P 0, P 1, P 2,..., P n-1 ), P n =P 0 P0 P1 P2 P3 P4 P5 P6

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Computer Graphics3 Step 1 n Consider the problem of calculating the area of the trapezoid that is generated between any 2 consecutive points P i and P i+1 and the X axis. P0 P1 P2 P3 P4 P5 P6 X

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Computer Graphics4 Step 2 n Starting from P0, Calculate the area of the trapezoid which is generated between Pi, P i+1 and the X axis. assume P i =(X i,Y i ) and P i+1 =(X i+1, Y i+1 ) u V = (X i+1 - X i)*( Y i+1 + Y i)/2 n Add this area to the total volume sum.

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Computer Graphics5 Step 3 n Repeat Step 2 for each 2 consecutive points in the polygon. n At the end of the process, the area will be calculated. (up to the sign) n Note the this algorithm works for two positive, two negative, or even two vertices with different y signes!

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Computer Graphics6 Example n Let’s calculate the area of a triangle. V= (x1-x0)(y1+y0)/2+(x2-x1)(y2+y1)/2+(x0-x2)(y0+y2)/2 = |(10+3)(-4-1)/2 + (5-10)(2-4)/2 +(-3-5)(-1+2)/2| = 63/2 X P0=(-3,-1) P1=(10,-4) P2=(5,2)

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Computer Graphics7 Volume Of a Polygonal Surface n A similar method can be used to calculate the volume of polygonal surfaces. n We’ll assume that the polygons in the surface are made of triangles. n We will examine the volume that is generated between a single triangle and the x-z plane.

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Computer Graphics8 Volumes - Cont. p0 p1 p2 p0’ p1’ p2’ A B

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Computer Graphics9 Volumes - Cont. n Calculating the volume is done in 3 stages: u The volume of between A-B-p1 and p2’-p1’- p0’ is simple to calculate. p1 p0’ p1’ p2’ A B

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Computer Graphics10 Volumes - Cont. u Now, it’s also simple to calculate the volume of the pyramid p2-B-p1-A p1 p2 A B

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Computer Graphics11 Volumes - Cont. u And the final volume part is the pyramid p2-p0-B-p1, which is also easy to calculate p0 p2 B p1

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Computer Graphics12 Volumes - Cont. n The sum of these 3 volumes provides us with the total volume between the polygon and the X-Z plane. u This volume may be negative! (depending on the location of the polygon in space) n But hey! when do we add and when do we subtract different volumes? u We can use the normal of the polygon to decide whether it is facing the positive Y direction. u If it does - We add the volume to the total sum u Otherwise we subtract this volume from the total volume sum

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