Computer Graphics CC416 Week 04 Mid-point Ellipse algorithm

Ellipse Generation Algorithm • An ellipse is an elongated circle

• Therefore, elliptical curves can be generated by modifying circle-drawing procedures to take into account the different dimensions of an ellipse along the major and minor axes CC416 Computer Graphics

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• Start with the basic equation of a circle

• Divide both sides by

yc

r2

ry (x,y) r x

xc

• The general equation of an ellipse can be stated as CC416 Computer Graphics

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Properties of Ellipses • An ellipse is defined as the set of points such that the sum of the distances from two fixed positions (foci) is the same for all points

d2 d1

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• So d2 d1

• Expressing distances d1, and d2, in terms of the focal coordinates F1 = (x1, y1) and F2 = (x2, y2), we have CC416 Computer Graphics

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• Symmetry considerations can be used to further reduce computations. • An ellipse in standard position is symmetric between quadrants • , but unlike a circle, it is not symmetric between the two octants of a quadrant. • Thus, we must calculate pixel positions along the elliptical arc throughout one quadrant • , then we obtain positions in the remaining three quadrants by symmetry CC416 Computer Graphics

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Mid-Point Ellipse Algorithm • The approach is similar to that used in displaying a circle • Given parameters rx, ry, and (xc, yc), we determine points (x, y) for an ellipse in standard position centered on the origin • Then the points are shifted so the ellipse is centered at (xc, yc) CC416 Computer Graphics

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• The mid-point ellipse method is applied throughout the first quadrant in two parts • This is done according to the slope of the tangent – When slope < -1, take unit steps in the x direction – When slope > -1, take unit steps in the y direction CC416 Computer Graphics

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• From equation

• When (xc, yc) = (0,0)

• the decision parameter will be according to

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• So, the next pixel along the ellipse path according to the sign of the ellipse function evaluated at the midpoint between the two candidate pixels

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• Starting at (0,ry) – take unit steps in the x direction until reaching the boundary between region 1 and region 2 – switch to unit steps in the y direction over the remainder of the curve in the first quadrant – At each step, test the value of the slope using

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• At the boundary between region 1 and region 2, dy/dx = - 1 and

• Therefore, we move out of region 1 whenever CC416 Computer Graphics

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• Recall • Assuming position (xk, yk) has been selected at the previous step

• the next position along the ellipse path is determined by evaluating the decision parameter

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• If p1k < 0, the midpoint is inside the ellipse and the pixel on scan line y, is closer to the ellipse boundary. • Otherwise, the mid position is outside or on the ellipse boundary, and we select the pixel on scan line yk - 1. CC416 Computer Graphics

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• At the next sampling position (xk+1+1 = xk + 2), the decision parameter for region 1 is evaluated as

• where yk+1 is either yk or yk - 1, depending on the sign of p1k CC416 Computer Graphics

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• So decision parameters are incremented by the following amounts:

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• In region 1, the initial value of the decision parameter is obtained by evaluating the ellipse function at the start position (x, y) = (0, ry)

or CC416 Computer Graphics

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• In region 2, we sample at unit steps in the negative y direction • Th midpoint is now taken between horizontal pixels at each step • The decision parameter is evaluated as

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• If p2k > 0, the mid-position is outside the ellipse boundary, and we select the pixel at xk • If p2k ≤ 0, the midpoint is inside or on the ellipse boundary, and we select pixel position xk+1

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• When we enter region 2, ;he initial position (x0, y0) is taken as the last position selected in region 1 and the initial derision parameter in region 2 is then

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• To simplify the calculation of p20, we could select pixel positions in counterclockwise order starting at (rx, 0). • Unit steps would then be taken in the positive y direction up to the last position selected in region 1

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• The midpoint algorithm can be adapted to generate an ellipse in nonstandard position • Where transformation methods can be used to reorient the ellipse

Non Standard position

Standard position CC416 Computer Graphics

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• Midpoint Ellipse Algorithm 1. Input rx, ry and ellipse center (xc, yc), and obtain the first point on an ellipse centered on the origin as

2. Calculate the initial value of the decision parameter in region 1 as CC416 Computer Graphics

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2. At each x position in region 1, starting at k = 0, perform the following test: If p1k < 0, the next point along the ellipse centered on (0, 0) is (xk+1, yk) and Otherwise, the next point along the circle is (xk + 1, yk – 1) CC416 Computer Graphics

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With

and continue until

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4. Calculate the initial value of the decision parameter in region 2 using the last point (x0, y0) calculated in region 1 as

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5. At each yk position in region 2, starting at k = 0, perform the following test: If p2k> 0, the next point along the ellipse centered on (0, 0) is (xk, yk - 1) and

Otherwise, the next point along the circle is (xk + 1, yk - 1) and

using the same incremental calculations for x and y as in region 1. CC416 Computer Graphics

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6. Determine symmetry points in the other three quadrants. 7. Move each calculated pixel position (x, y) onto the elliptical path centered on (xc, yc) and plot the coordinate values: x = x + xc y = y + yc

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• Example – Given rx = 8 and ry = 6 – Initial values and increments for the decision parameter calculations are

– For region 1: • The initial point for the ellipse centered on the origin is (x0, y0) = (0,6), and the initial decision parameter value is CC416 Computer Graphics

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• Successive decision parameter values and positions along the ellipse path are calculated using the midpoint method as

• We now move out of region 1, since CC416 Computer Graphics

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– For region 2 • the initial point is (x0, y0) = (7,3) and the initial decision parameter is

• The remaining positions along the ellipse path in the first quadrant are then calculated as

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More about ellipses • Source: http://en.wikipedia.org/wiki/Ellipse

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