Put on a Happy Sky

In this lesson you will have the opportunity to learn a procedure to replace an overexposed sky in a photo with a more attractive sky from another image.
Before -

After -

Included in this lesson -

• Decompose HSV filter
• Threshold tool
• Quick mask
• Adjusting a selection
  
• Moving layers

Open the photo with the unattractive sky (foreground image) and the image with the good sky you want to use. It's much easier if these images are the same size, so you may want to scale one or both images before proceeding. For the time being, you may want to minimize the "sky" image to reduce clutter.

Step 1 - Decompose the background image to HSV

Filters - Colors - Decompose ...

Select HSV in the dialog. This will create a new image with the same name as your original foreground image but which has HSV appended.
Be sure the Value layer is active (white border) in the layers dialog

Step 2 - Apply the Threshold tool to the Value layer

Tools - Color Tools - Threshold ... OR Layer - Colors - Threshold ...

Push the middle slider to the right until the sky is white and the foreground is black. Use one of the Paint tools (pencil, brush) to make any adjustments needed.

Step 3 - Apply Quick Mask to the foreground image
Select your original foreground image and toggle the Quick Mask ON by clicking in the bottom left corner of the image window (Shift+Q OR Select - Toggle Quick Mask).
The entire image should now be covered by a translucent red layer.

Step 4 - Copy the Decomposed image and paste it onto the Quick Mask in the foreground image
Select the Decomposed HSV image and copy it.

Edit - Copy (Ctrl+C)

Switch to the foreground image.

Edit - Paste (Ctrl+V)

Step 5 - Define the selection
Toggle Quick Mask OFF by clicking in the bottom left corner of the image window (Shift+Q OR Select - Toggle Quick Mask).
The red layer should be gone and the "marching ants" selection border should appear.

Right now the sky is selected, but you want the foreground to be selected.

Select - Invert (Ctrl+I)
Select - Shrink ... [setting: 1 pixel]
Select - Feather ... (Ctrl+Alt+F) [setting: 2 pixels]

Step 6 - Place the foreground on a new layer of its own

Edit - Copy (Ctrl+C)
Edit - Paste (Ctrl+V)

You should see a floating selection at the top of the layers dialog.


Put this on a new layer of its own.

Layer - New Layer ... (or click the New Layer icon at the bottom of the layers dialog)

Name this layer "foreground".

Step 7 - Copy the new sky from the other image to the foreground image
Activate the image containing the sky you wish to use and copy it.

Edit - Copy (Ctrl+C)

Activate the foreground image

Edit - Paste (Ctrl+V)

Layer - New Layer ... (or click the New Layer icon at the bottom of the layers dialog)

You now have the sky image on its own layer. Name this layer "sky".

Step 8 - Reposition the sky layer
Move the "sky" layer below the "foreground" layer in the stack by either dragging it or using the down button at the bottom of the layers dialog.

Step 9 - Finish up
If you want to save your image in JPG or PNG format you'll need to flatten it to a single layer first.

Image - Flatten Image

Understand, this is NOT the ONLY way to do this. It works well for me and gives you an opportunity to use some functions of the GIMP which may be less familiar to you. The techniques used can be applied in other tasks as well. I hope you find this useful and instructive. Comments and/or donations appreciated.