A model generator computes a first image perimeter color difference value for each of a plurality of first pixels included in a first image that is captured using a first focal length, and selects one of the first image perimeter color difference values that exceeds a perimeter color difference threshold. Next, the model generator computes a second image perimeter color difference value for each of a plurality of second pixels included in a second image that is captured using a second focal length, and selects one of the second image perimeter color difference values that exceeds the perimeter color difference threshold. The model generator then determines that an edge is located at the first focal length by detecting that the selected first image perimeter color difference value is greater than the selected second image perimeter color difference value, and generates an image accordingly.