There are two different cases where you will need to rotate an image...

By a Fixed Amount

Rotation can be by a set amount (usually 90o), when it is used to get an image in the correct aspect. That is, when you load a portrait image into an editor it sometimes appears landscape and needs to be rotated by 90o to get the correct aspect.

For example, the image shown on the left below is a size reduced version of the picture as it was after uploading from the camera. Clearly this needs to be rotated so that the column and statue are upright. In this case, we need to apply a rotation of 90o clockwise. In the GIMP this tool can be found on the Image>Transform menu, using the option Rotate 90o clockwise on the menu. In PhotoShop, the rotation tool is found on the Image>Rotate Canvas menu. The result is shown on the right and we can clearly see that the image is correctly oriented!

Original column image Rotate image Upright column

There are a number of other options on the menu that you can explore, depending on the effect that you need to achieve.

Sun Moon LakeRotation by an Arbitrary Amount

You may also need to rotate an image by a small amount to correct the alignment of a horizontal or vertical element such as the horizon. This is especially important in images where the alignment of the feature is an important part of the composition.

Consider the picture on the right - this was taken from a moving boat and the angle of the shore line is not correct. Even though it is only a small amount, it is enough to spoil the composition. If you can't see that the shore is at a slight angle, hover the mouse over the image and you will see a red horizontal line superimposed in front of the shore.

Rotated horizonSo, this image needs to be rotated clockwise by a small amount. In the GIMP the necessary tool can be found in the toolbox, on the Tools>Transform Tools menu, or by pessing Shift+R on the keyboard. In PhotoShop, this tool is found on the Image>Rotate Canvas menu. In GIMP you need to enter a numerical value for the rotation and usually it will take some experimentation to get the correct angle. In this case it was only 0.7o. In recent versions of PhotoShop it is possible to use the mouse to drag the image rotation by eye.

An alternative way to rotate an image, if you prefer to do it freehand, is to use the crop tool, and rotate as your crop.

Notice, once the image has been rotated, although you have corrected to horizontal line, the edges of the image are now squint. Again, if you cannot see this, hover the mouse of the image to see guides.

Horizontal horizonSo, the final stage of this process is to crop the image to ensure that the edges are square (you can find a discussion of cropping here). This final resulting image is shown on the right. Compare this with the original image above. The difference is quite subtle, but the result is definitely a better composition.

Note that this kind of work is best done before any image resizing that you might need to do.

This image would also benefit from some other optimization techniques such as levels and hue and saturation. Roll the mouse over to see the suggested final version. We will look at these other techniques later.

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