1. An image generating apparatus which generates a second image that is an image converted from a first image captured by an imaging apparatus placed to be oriented downward at an angle of depression with respect to a horizontal direction, said image generating apparatus comprising: a specifying unit configured to specify a straight line included in the first image and passing through an intersection between an imaging plane and a vertical vector having a start point at the imaging apparatus; and an image extension processing unit configured to generate the second image by sampling pixels of the first image along the specified straight line.