Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 271

3.13 Cross-Fading with Masks and
Selections
Now we will return to learning about the techniques of using select tools for
masking images or for combining images. There may be times you would
like to cross-fade images, or you might want to create a panorama picture
from a series of photos. Both can be achieved by using masking techniques or
selections with soft borders.
3.13.1 Cross-Fading Part 1—Cross-Fading Two
Images with Two Different Motifs
The prerequisite for cross-fading two images is that both images are of the
same image size and resolution. They don't necessarily need to be of the
same format. The images for this example exercise have been prepared for
you accordingly.
The Procedure
• Open the images lido.png and shells.png from the SampleImages folder
on the DVD.
• Duplicate the Background layer of the shells.png image. Name the
duplicated layer shells , for example. Allocate an alpha channel to this
layer using the context menu in the Layers dialog (right-click on the layer
and choose Add Alpha Channel ). This layer will be used in later steps of the
editing process and will be exported. The Background layer can be made
invisible by clicking the eye symbol.
• Switch to Quick Mask mode, by either choosing Select > Toggle Quick Mask
or clicking the Toggle Quick Mask button at the bottom left of the image
window. A red protection layer, a so-called mask, covers the image.
• Fill the mask with a gradient from the foreground color (black) to white
(the standard colors in the color area in the Toolbox). You can find such
a gradient in the gradient settings of the Blend tool . The gradient should
start at the bottom with black and end in white at the top. The image
content at the bottom is to be kept, whereas it will be deleted gradually
toward the lighter areas.
• Switch back to the select mode (marching ants mode) by clicking
the Toggle Quick Mask button. Now a selection appears that fades out
according to the gradient.
• Set the shells layer to active.
• Delete the selected image content on this layer using the Clear command
( Edit > Clear ). The content is deleted, according to the mask's gradient.