This is part of a 'How To' series on post process work.
When viewing an image, one of the most off putting of all distractions can be if the horizon line is not straight. This can be true of verticals also, but the horizontal-ness of the horizon is perhaps the greatest distraction.
Most editing programs have some way of correcting the 'level' of an image.
Here, I am simply demonstrating two techniques, in Photoshop, which can be used to straighten the horizon.
This is the original image as opened in Photoshop. As I was more intent on framing the foreground figure, and in relation to the truck, I did not pay attention to the horizon.
In correcting this image, I have two thoughts in mind. The first is I do not want to lose the figure at lower left as he adds dimension and scale to the scene, the second is I wish to preserve as much of the truck as possible, as this provides an anchor point for the activity.
If I removed too much of the truck, the image would have little sense as to what they are doing and where they are headed.
Note : I have the rulers turned on. These are selected from the top menu - View/Rulers. More about that later
Photoshop has some tools which make this sort of work very simple. One of them is the Grid tool. This can be selected from the top menu - View/Show/Grid.
I use this occasionally, but my preference is to use another method, described next. However, if you prefer to use the Grid, it is a very useful tool for this sort of work.
Image 3 - Technique One
My typical method for straightening an image, and how to determine the 'level' of the horizon, is to click and hold the top ruler bar and drag down to the horizon.
What this does is to set a 'Guide' line on the image. This can always be removed later, and is removed by selecting View/Clear Guides from the top menu.
The reason I prefer this approach, is that the horizon does not always need to be perfectly level. There is enough 'clutter' in this image that a slightly non-level horizon would not really be noticed.
Note : The exact same technique can be used for vertical lines by dragging a guide from the left side ruler. Several guides can be placed on the image at any one time
Image 4 - Technique One
With my 'guide' now in place, I have simply selected - Select/All, and then Edit/Transform/Distort.
Clicking and holding on the small 'squares' at the corners, or centre of the image, the image can be dragged to correct the distortion.
Here, I have dragged the top right corner up, the bottom left corner down, and the right side more to the right.
Image 5 - Technique Two
Another very simple way to correct the horizon, and if you want to be far more accurate, is to use the Ruler Tool.
This is selected by right-clicking the Eye-Dropper Tool icon and then selecting Ruler Tool.
Image 6 - Technique Two
Once selected, it is a simple case of clicking on a point on the horizon line, and while holding down the mouse, simply drag the tool across the horizon line.
Here, I have dragged the tool parallel to the horizon line, simply so it is easier for you to see. Normally, I would simply drag it across the actual horizon.
Image 7 - Technique Two
With my Ruler line now in place, I simply select - Image/Image Rotation/Arbitrary.
Image 8 - Technique Two
Once selected, a dialogue window opens and shows the angle of required correction, which has already been determined by our use of the Ruler Tool.
In this case, the image needs to be rotated counter clockwise by 1.68 degrees.
What could be more simple, or accurate - Simply click OK.
Image 9 - Technique Two
Once we have OK'd the pre-determined rotation, it is easy to see how much rotation has actually been applied.
As I am using a white background, the white areas are those I will now need to remove.
Image 10 - Technique Two
All I have done here is to select - Select/All and then Edit/Transform/Distort and then dragged the bottom left corner to the left, the top right corner to the right, and dragged the top and bottom of the image until I lose my white areas.
I have also dragged the right side of the image slightly to the right. I don't want to lose the bolts and shadows which are on the truck though, so I only dragged it slightly right.
That's it. It's quite simple, whichever technique you use.
The original image.
The straightened image using Technique Two described above.
Reality is either technique is fine. If you want absolute accuracy, use Technique Two.
Other posts in this series - Click here for the index page.