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Straighten a Horizon with Paint.NET

Paint.NET Digital Photo Editing Tip

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Digital photo editing options cover a range of different faults that can afflict all of our photos. A common error made is failing to keep the camera straight while taking the picture, leading to horizontal or vertical lines within the image being on an angle.

Fortunately it's very easy to correct this problem, whichever pixel-based image editor you use. In this Paint.NET tutorial I'll show you a technique to straighten a horizon in your digital photo editing work flow. I'm using a picture I shot a few weeks ago, but I've deliberately rotated the image for the purpose of this tutorial.

1. Select Your Image

Picture with horizon on an angle
© Ian Pullen

Ideally you will have an image already available that needs a correction to its orientation. Go to File > Open and navigate to your desired image and open it.

It was only when I started writing this digital photo editing tutorial on how to straighten a horizon that I realized that Paint.NET doesn't offer the ability to add guides to an image. Normally, if using Adobe Photoshop or GIMP, I'd drag a guide down onto the image to make it easier to straighten the horizon accurately, but we have to use a different technique with Paint.NET.

2. Mark the Straightened Horizon

Draw a selection
© Ian Pullen

To get around it, I'll add a semi transparent layer and use that as a guide. The first thing to do is go to Layers > Add New Layer and we'll add a fake Paint.NET guide to this layer. Actually this will be a filled selection which is achieved by selecting the Rectangle Select tool from the toolbox and then clicking and drawing a wide rectangle across the top half of the image so that the bottom of the selection crosses the horizon at the middle.

3. Select a Transparent Color

A transparent color is selected
© Ian Pullen

You will now need to choose a contrasting color that will be used to fill the selection, so if your image is very dark you will want to use a very light color. My image is generally quite light, so I'm going to use black as my Primary color.

If you can't see the Colors palette, go to Window > Colors to open it and change the Primary color if necessary. Before filling the selection, we also need to reduce the Transparency – Alpha setting in the Colors palette. If you can't see the Transparency – Alpha slider, click on the More button and you will see the slider at the bottom right. You should move the slider to about the halfway position and, when finished, you can click the Less button.

4. Fill the Selection

Selection filled with color
© Ian Pullen

It's now a simple matter to fill the selection with the semi-transparent color by going to Edit > Fill Selection. This gives a straight horizontal line across the image which can be used to align the horizon with. Before continuing, go to Edit > Deselect to remove the selection as it is no longer needed.

Note: You don't need to use the previous steps when straightening a horizon and you can just follow the next steps, trusting the straightness of the horizon to your eye.

5. Rotate the Image

Horizon is straightened
© Ian Pullen

In the Layers palette (Window > Layers if it isn't visible) click on the Background layer and go to Layers > Rotate / Zoom to open the Rotate / Zoom dialog.

The dialog contains three controls, but for this purpose, only the Roll / Rotate control is used. If you move the cursor over the circular input device, the small black bar turns blue – this is a grab handle and you can click and drag upon that and rotate the circle. As you do so the image also rotates and you can align the horizon with the semi-transparent layer. You can manually change the Angle box in the Fine Tuning section, if necessary, to straighten the horizon more accurately. When the horizon looks straight, click OK.

6. Crop the Image

Selection for cropping is drawn
© Ian Pullen

At this point, the transparent layer is no longer needed and it can be deleted by clicking on the layer in the Layers palette and then clicking the red cross in the bottom bar of the palette.

Rotating the image leads to transparent areas at the edges of the image, so the image needs to be cropped to remove these. This is done by selecting the Rectangle Select tool and drawing a selection over the image that doesn't contain any of the transparent areas. When the selection is positioned correctly, going to Image > Crop to Selection crops the image.

Note: It maybe easier to place the selection if you close any of the palettes that are open.

7. Conclusion

Finished image
© Ian Pullen

Of all the digital photo editing steps that you take, straightening the horizon is one of the simpler one,s but the effect can be surprisingly dramatic. An angled horizon can make an image seem unbalanced even if the viewer doesn't realize why, so taking a few moments to check and straighten the horizon of your photos is a step that you really should try and fit into your digital photo editing work flow.

Finally, do remember that it isn't just the horizon in photos that may need straightening. Vertical lines can also make a photo look odd if they're on an angle. This technique can be used to correct these also.

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