How-to: Blend Images using GIMP

How-to: Blend Images using the GIMP - finished image GIMP (which is the GNU Image Manipulation Program) is an open source alternative to Photoshop for image and photo editing. Available for almost all operating systems (Linux, Microsoft Windows, OS X), it is a powerful alternative to Adobe. Did I mention is it also free?

Image blending is a common task, compositing one image onto another with a soft fade effect. For example, I want to place the BBC's Musketeers logo over a photo of the cast.

The GIMP is a very flexible tool. My way of doing things goes something like this:

    How-to: Blend Images using the GIMP - add layer mask
  • Open the source images From the Menu, select File, Open as Layers...
    Use CTRL + CLICK to select both images. 
  • Resize the canvas and arrange layers as I need them.
  • Select the layer to blend; this is typically my foreground to blend into my background.
  • Make sure the layer to blend has an alpha channel to allow transparency, so that the edges fade into the  background.
  • From the Layers palette, select Add Layer Mask (right click on the layer and choose from the Layers menu).
  • Choose a mask mode - I use White, full opacity.
  • Select Add to add the layer mask in that mode.
  • I'll select the part of the layer I want to blend and protect the rest of it; here I want the logo title to fade at the edges, so in the Toolbox palette, I choose the Elipse selector, then in the Tool Options, anti-alias and feathering - 25px radius - are on.
  • I drag my selection tool around the centre of the logo to define the part I want to protect.
  • In the Select menu, I choose Invert Selection and also Feather Selection to define a softer edge for the blend area.
  • In the Toolbox palette, select the Blend Tool.
  • In the Tool Options palette, choose your blend mode; here I've gone for normal mode, radial pattern, no repeat, no offset, dithering is on.
  • Click and drag the Blend Tool across the layer to blend. This only affects the layer mask, not the layer itself. You can play around with this and try different results. You can also choose foreground-to-background or the other way around depending on how you want to work.
  • I can keep re-applying the blend to play with the results, it doesn't take effect until I apply the layer mask to the parent layer.
  • When I'm happy with my blended layer, right click it in the Layers palette and select Apply Layer Mask. This is the destructive step that applies the blend to the parent layer (I can Undo this using the Undo History to get back to the un-blended layer, or even re-load the source image if the blend goes horribly wrong.
  • I want to clear the selection bounding box, so choose Select None from the Selection menu then switch focus to the background layer in the Layers palette, just so I can see the blended foreground without any clutter.
  • My oval radial blend leaves me a dark top edge in the foreground, so I could reverse out the blend and redo it with an adjusted selection, or perform another blend with a new selection.

There are plenty of settings to play around with in the selection and blending; experimenting with different parameters can give even better results; sometimes serial blends in different areas of the layer, using the same or different selection can yield a better, subtler blend of one layer into another. RC