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Applied Layer Mask
Figure 1

STEP ONE. This is a continuation of my notes on how to Join Two Pictures with the GIMP. I am going to assume that you have read this and followed the example there (SEE STEP SEVEN FROM BLENDING TWO IMAGES WITH THE GIMP).

The blending technique darkened the Shuttle's nose and the white spire at the top of the launch pad tower. Since merging the two photos is accomplished by a gradual reduction in opacity of the Shuttle layer, the Shuttle layer became darker.

To make the Shuttle image a little more defined and dramatic, we will create a special selection (called a Mask) cuts out the background from behind the Shuttle in notes that follow.

NOTE: I assume you know basic operations contained in http://www.gimp.org/tutorials
/Lite_Quickies/

Shuttle Channels Dialog
Figure 2.

STEP TWO. We are going to make a mask (a special selection) of the outline of the Shuttle. Open Figure 1 from the notes on Blending Two Photos with the Gimp. We are going to use the /Dialogs/Channels function. You should get a dialog window that divides the image into three color components: red, green, and blue.

Shuttle Alpha Channel
Figure 2.1

We are going to add a fourth Channel called an "Alpha" Channel using /Layer/Transparency/Add Alpha Channel. Note that all the channels may be active (have a blue background). Click on the Red, Blue and Green Channels to toggle them to be inactive. Now the only active channel should be the Alpha channel as in Figure 2.1 above.

NOTE: The alpha channel controls the opacity / transparency of the other three channels.

Shuttle Mask Screen Shot
Figure 3


STEP THREE. Add a Layer Mask by using /Layer/Mask/Add Layer Mask (just like in Step 4 from my notes on Blending Two Photos with the Gimp). Click OK to accept the default settings of White (Full Opacity). Now this Mask will allow you to do a special selection using the Paint Brush tool.

Anything you paint black will become transparent. If you make a mistake, don't worry. Anything you paint white will become visible again.

NOTE: The GIMP uses a gray "checkered" pattern to indicate a transparency. When you save this mask, the checkered pattern will be replaced by whatever background you have selected.

STEP ELEVEN. When you have the Shuttle isolated apply the Mask using /Layer/Mask/Apply Layer Mask.

NOTE: As long as you don't close your Mask, you can "unapply" the mask and edit it just like before using /Edit/Undo. You may want to save the Shuttle Mask as Shuttle_Mask.xcf. XCF is the Gimp's native file format. Using this format allows you to go back to your mask and correct any flaws.

Placing the Channel Mask
Figure 4

STEP FOUR. Open Figure 1. Now copy and paste your Suttle Mask onto the active image window for Figure 1. Use the /Dialog/Layers window and reduce the opacity (in this example set to 43.4%) so that you can temporarily see through your mask. Now use the Move tool to align the Shuttle on your semi-transparent mask directly over the Shuttle on the Background image. After you have aligned the Shuttle mask with the background image, restore the opacity to 100% and anchor it to the background image.

Channel Mask Applied
Figure 5

STEP FIVE. Open Figure 1. Now copy and paste your Suttle Mask onto the active image window for Figure 1. Use the /Dialog/Layers window and reduce the opacity so that you can temporarily see through your mask. Now use the Move tool to place your semi-transparent mask directly over the shuttle. After you have it positioned the mask, restore the opacity to 100% and anchor it to the background image. Now save your final final product.

NOTE: Compare this result to the Figure 1. Our mask reversed the darkening of the Shuttle's nose and the white spire at the top of the launch pad tower stands out clearly.