Straight Line UI proposal
A New Tool For Straight Line Definition
Currently the only one way to specify lines in panorama is to manually create pairs of control points. This approach has multiple disadvantages. From user perspective it's not very intuitive and defining multiple lines can be a pretty tedious work. Also the lines are a part of output space and thus defining them in output space seems to be more appropriate.
Move the straight line definition into the Fast Preview. The line can be drawn in a way similar to what is done in the most vector editors. Control points would be added as the user would click in the preview and the line between two points would be immediately shown (i.e. first click adds first point, second clicks adds point and draws line between the first and second and so on).
There has to be an option allowing the user to specify the type of line – whether it's horizontal, vertical or some general line. It might be handy if there was an option to run optimizer as the lines are added so the user would have an immediate feedback how the lines change the panorama. Note that optimizer shouldn't be run by default, because optimization may take a lot of time rendering proposed tool unusable.
There are several problems that has to be resolved first. If the tool is based upon the contemporary way of defining lines the decision where to place the control points has to be made. Also there is a problem as how the line should be specified when images overlap as noted by kfj in .
The idea is based on the idea from RawTherapee forum .
Before this works the fast preview needs some zooming function to set exactly the points. Also it is necessary to identify the image in which the line should be created in case the point is setting in an overlapping area or using the function for HDR panoramas. See other blueprint .
 http://rawtherapee.com/forum/viewtopic.php?p=17063#17063  https://blueprints.launchpad.net/hugin/+spec/extendedpreview