Skewed horizon is a very widespread problem of the modern photos. I am talking about amateur everyday photos, not about skewed on purpose. It is surprising that many people do not even ask «how to level the horizon» because they do not consider this as a problem, but it is really unpleasant to look at these pictures.
Let us teach people how to fix it.
Leveling a horizon
Let’s open one of these photos.
We drag some guidelines from the top ruler to the places of the photo where they need to be smooth.
It often happens that we have problems with optical distortions on photos in addition to leveling the horizon, so we need not only horizontal guidelines, but the vertical too, by means of which we will see that vertical curves didn’t go in different directions.
Now we need to duplicate the layer (Layer — Duplicate Layer). And use the «Rotate Tool».
The tool can be used in two ways: to rotate a layer using the nodes in the corners of the photos or using a modal dialog. Nodes are clear: the horizon is levelled «by eye» when rotating the photo. In a modal window you can enter the degrees of displacement and use the arrow buttons of input fields, which gives a very high precision for leveling the horizon in the photo.
We always align it by horizontal lines, but this example requires the alignment of the center so it would then be easier to work with.
The vertical lines
After levelling the horizon we can deal with vertical lines. They should always be perpendicular to the horizontal part of the canvas and parallel to the vertical parts.
There are many photos of buildings, that we can consider as the wrong example of perpendicular lines. In our case we have a good example of bad vertical lines.
In contrast to the horizon, the vertical lines can be levelled with the «Perspective Tool». This tool also contains nodes at the corners of the layer and a modal dialog, which is simply useless in most cases.
During the application of the «Perspective Tool» it is necessary to watch over horizon, so we wouldn’t have to level it again. However we can level the horizon using only «Perspective Tool» without «Rotate Tool» (I mean small skew, in case of the strong one the rotation cannot be avoided). Afterwards you can remove the guidelines back in ruler and activate the «Fit canvas to layers» option from the «Image» menu.
Correcting the proportions
Objects often lose their proportions after working with the «Perspective Tool». They may become narrower, wider, higher and longer. We need the «Zoom Tool» to fix it.
All distortion tools are working identically in GRIMP: nodes and a modal dialog, but zoom has the icon of the chain between the «Width» and «Height» parameters. This icon is responsible for the aspect ratio.
If you disconnect the chain, the proportions of the image will not be saved while scaling (you can stretch or shrink the picture). In the usual case this is unacceptable, but this is what we need for now.
Change the height or width of the picture.
Any picture loses its sharpness after distortion. The more operations — the more sharpness we lost, so there should be the minimum amount of operations.
Sharpness is lost anyway. Now we will sharpen the picture: duplicate the layer and go to Filters — Enhance — Sharpen.
Adjust the parameters as you need, but make sure that the lines were not too harsh. If this happens after applying the filter — opacity in the layers panel can be lowered to the desired value.
After changing the aspect ratio and sizes visual part of the layer often become less than its actual size and we have to resort to the framing.
For accurate determination of how much layer with a final image was smaller than the original photo — activate the lower background layer on the layers panel and invert the color (Colors — Invert).
Now the difference is obvious: you need to remove redundant empty and inverted space. You can activate the «Crop Tool», select the remaining space…
… and press on the center of the selected area to reduce the image to the desired size.
To save the image go to «File — Export» and choose JPG or PNG format from the list (large file size for saving lossless).