Paint.net is a really nifty little program, ideal for those that just working on jpeg files. However on first look it doesn’t seem to have a straighten tool (ie something to level the horizon), the image menu seen to have rotates (only 90° left and right or a 180° twist and horizontal and vertical flips, no obvious small and definable rotation! However a deeper look around will show you there is such a rotation tool under the layers menu item. Yet there is an even bigger omission, no simple levelling tool to help pick the horizon and overlay guides (like aftershot or picasa) of a drag to mark a second point to define the rotation (like lightroom or perfect photo). Trying to straighten a horizon is starting to look tedious, but it isn’t!
No need to panic, it is easy to use the layers to create your own temporary guides. Starting with the Layer item on the menu pick the add new layer item on the menu to create an empty and transparent second layer. Now mow to that layer by clicking on layer 2 in the layers window. Next using the shape tool from the main drawing tool bar and use and open rectangle, mark a suitable large rectangle on the screen. It will be easier to judge if you have this rectangle cross the horizon somewhere near the middle of the image. This rectangle becomes your level guide.
Now you can return to the background layer and this time select the rotate/zoom tool.It looks a bit complex (and it can do a lot) but we are only going to use the little rotation ring on the round shape tool on the upper left) As you rotate this by clicking the mouse on the reference mark on the outer ring and moving you mouse up and down. you will see the photo rotate, while the rectangle your created on layer 2 remains steady. Gently rotate until the horizon is parallel with the rectangle edge. Simple. If you want more control of the rotation you can enter an angle or use the little up and down adjusted beside the angle dialogue box..
To finish you will need to delete your temporary layer 2 (go back into layer 2 and select delete layer from the layers menu) and then crop the image to avoid the undefined edges, but that's a subject of a future post!