GIMP's Rotate Tool is used to rotate layers within an image and the Tool Options offers a number of features that affect the way that the tool functions.
The Rotate Tool is quite easy to use and once the Tool Options have been set, clicking on the image opens the Rotate dialog. In the dialog, you can use the slider to adjust the angle of the rotation or click directly on the image and rotate it by dragging. The cross hairs that appear on the layer show the center point of the rotation and you can drag this as desired.
Do remember that you need to ensure that the layer you want to rotate is selected in the layers palette.
The Tool Options for GIMP's Rotate Tool, many of which are common to all the transform tools, are as follows.
Transform
By default, the
Rotate Tool will operate upon the active layer and this option will be set to
Layer. The
Transform option in the GIMP
Rotate Tool can also be set to
Selection or
Path. Before using the
Rotate Tool, you should check in the
Layers or
Paths palette, which is active as this will be what you apply the rotation to. When rotating a selection, the selection will be obvious on the screen because of the selection's outline. If there is an active selection and
Transform is set to
Layer, only the part of the active layer within the selection will be rotated.
Direction
The default setting is
Normal (Forward) and when you apply the GIMP
Rotate Tool it will rotate the layer in the direction that you would expect. The other option is
Corrective (Backward) and at first glance this seems to make little practical sense. However this is an incredibly useful setting when you need to adjust horizontal or vertical lines in a photo, such as to straighten a horizon where the camera wasn't held straight. To make use of the
Corrective setting, set the
Preview option to
Grid. Now, when you click on the layer with the
Rotate Tool, you just need to rotate the grid until the horizontal lines of the grid are parallel with the horizon. When the rotation is applied, the layer will be rotated in the reverse direction and the horizon will be straightened.
•
Step-by-Step: Straighten a Horizon with GIMP
Interpolation
There are four
Interpolation options for the GIMP
Rotate Tool and these affect the quality of the rotated image. It defaults to
Cubic, which generally offers the highest quality of the options, and is usually the best option. On lower spec machines, the
None option will speed up the rotation if the other options are unacceptably slow, but edges may appear visibly jagged.
Linear offers a reasonable balance of speed and quality when using less powerful machines. The final option,
Sinc (Lanzos3), offers a high quality interpolation and when quality is especially important, it maybe worth experimenting with this.
Clipping
This only becomes relevant if parts of the area of the layer being rotated will fall outside of the existing borders of the image. When set to
Adjust, the parts of the layer outside of the image borders will not be visible, but will continue to exist. Therefore if you move the layer, parts of the layer outside the image border can be moved back within the image and become visible. When set to
Clip, the layer is cropped to the image border and if the layer is moved, there will be no areas outside of the image that will become visible.
Crop to result and
Crop with aspect both crop the layer after rotation so that all corners are right angles and the edges of the layer are either horizontal or vertical.
Crop with aspect differs in that the resulting layer's proportions will match the layer before the rotation.
Preview
This allows you to set how the rotation is displayed to you while you are making the transformation. The default is
Image and this shows an overlaid version of the layer so that you can see the changes as they are made. This may be a little slow on less powerful computers. The
Outline option just shows a border outline which can be quicker, but less accurate, on slower machines. The
Grid option is best when direction is set to
Corrective and
Image + Grid allows you to preview the image being rotated with an overlaid grid.
Opacity
This slider allows you to reduce the opacity of the preview so that layers below are visible to varying degrees which may be useful in some circumstances when rotating a layer.
Grid Options
Below the
Opacity slider is a drop down and input box that allow you to alter the number of grid lines that are displayed when either of the
Preview options that display a grid are selected. You can choose to alter by the
Number of grid lines or
Grid line spacing and the actual alteration is made by using the slider below the drop down.
15 Degrees
This check box allows you to constrain the angle of rotation to 15 degree increments. Holding down the
Ctrl key while using the
Rotate Tool will also constrain the rotation to 15 degree increments on the fly.
