Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 190

3.5.4 Transform Tool Options

When selecting the Perspective tool , pay attention to the tool's settings. The
following options are available:
• Transform : As with all transformations (changes in form and size), you
can choose if you want the transformation to be applied to a layer, a
selection, or a path.
• Direction : The Normal (Forward) option performs a transformation
according to the direction of the guides. Corrective (Backward) will
perform the transformation in the reverse direction. This option is used
to correct previously applied perspective deformations.
• Interpolation : This drop-down list allows you to choose the quality for
the recalculation of pixels in the distorted image. Select the Cubic or Sinc
option. Both require more time, but they produce the highest quality.
• Clipping : Depending on which option you select, the result of the
transformation will be automatically adjusted to the size of the layer:
• Adjust : The layer will be adjusted to the canvas size of the layer. The
function either enlarges or reduces the clipping to fit irrespective of
the size of the actual image or layer.
• Clip : The new size of the layer that resulted from the transformation
is enlarged or clipped to the original size of the encompassing
rectangle. If the layer has the size of the image and is reduced in size
during the transformation, the layer size is set to the original image
size again after the transformation.
• Crop to result : The layer and its content will be cropped to
boundaries of the inner rectangle, which is contained within the
transformation frame. Sometimes image contents could be cut off
for that reason.
• Crop with aspect : The layer is cropped or enlarged to a right-angled
image section. It maintains the aspect ratio of the initial layer. The
result of the transformation is limited in size by the transformation
boundary.
• Preview :There are four preview possibilities to support the transformation
process:
• Outline : This option illustrates the transformation with a frame to
mark the outline of the image with handles on each corner.
• Grid : This option illustrates the transformation by putting a grid on
the image with handles at the corners.
• Image : This option illustrates the transformation by superimposing
a copy of the image on the original. The Opacity setting is a great aid.
If you reduce the opacity to about 70%, you can see the underlying
original as well as the image in its transformed state. The same is
valid for Image + Grid .

• NOTE
You can enlarge the layer to the
image size anytime in the editing
process by choosing the Layer >
Layer to Image Size menu item. If
you want to enlarge the size of a
layer to the rectangular boundary of
your pixel content, choose Layer >
Autocrop Layer .