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PHD Thesis - Institute for Computer Graphics and Vision - Graz ...

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A.2. Affine approximation of ellipse transfer 162<br />

Finally the axis lengths a <strong>and</strong> b are<br />

a =<br />

b =<br />

1<br />

√<br />

cc11<br />

1<br />

√ .<br />

cc22<br />

(A.38)<br />

(A.39)<br />

A.2 Affine approximation of ellipse transfer<br />

In this section we discuss the ellipse transfer method used in the evaluation method of Mikolajczyk<br />

<strong>and</strong> Schmid [74]. It approximates the projective trans<strong>for</strong>mation with an affine trans<strong>for</strong>mation.<br />

The method works by trans<strong>for</strong>ming the ellipse shape with an affine trans<strong>for</strong>mation <strong>and</strong><br />

centering the ellipse around a new ellipse center. The new ellipse center is gained by transferring<br />

the ellipse center to the other image by homography trans<strong>for</strong>m. To obtain the new ellipse shape<br />

the second moment matrix of the ellipse is trans<strong>for</strong>med with an affine trans<strong>for</strong>mation which is<br />

an approximate estimate of the true projective trans<strong>for</strong>mation.<br />

Such an approximation was chosen in [74] because the authors were interested in establishing<br />

a corresponding center point. However, one must be aware that there can be quite large approximation<br />

errors. Figure A.2 shows a comparison of the projective <strong>and</strong> affine ellipse transfer. The<br />

ellipse resulting from the projective transfer is drawn in black, the results from the affine transfer<br />

in green. In Figure A.2(b-e) one can see the differences between both methods trans<strong>for</strong>ming the<br />

original ellipse in Figure A.2(a). The ellipse centers <strong>for</strong> the green ones are at the position of the<br />

original ellipse center trans<strong>for</strong>m by the homography.

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