Preserve aspect ratio in keystone correction (perspective control)
Please improve perspective control so that it preserves the aspect ratio. http://wilson.bronger.org/perspective/ compares DT and Hugin and shows the problem clearly. Hugin takes the focal length and crop factor and projects the image on the full sphere. Then, it optimizes the yaw, pitch, and roll angles so that -- when projected back on the plane -- horizonals and verticals are preserved as such. It needs two verticals and one horizontal lines for this.
I think that there is a more straightforward way to achieve that than going via the projection on the sphere. I haven't yet understood the mathematics behind this fully. Anyway, I think this would be a valuable improvement.