A very interesting option, already present in previous versions of GIMP, is the creation of cloud patterns to fill-in burned skies - typical effect when taking a picture of a dark landscape on a cloudy day. In previous versions of GIMP, this filter was applied in a single step, mixing both front and background colours.
However, in the new version it works in a slightly different way: A different filter (Solid Noise...) is applied by default. It will generate the cloud texture - but in a grayscale version. Let’s see how this works.
First step is to use the colour picker to define “your” blue colour, trying to match it with the overall image style. You might prefer a purplish blue, or rather a dark blue.
Of course, next step is to make a selection of the sky area. You may need to use a combination of different selection tools, keys shortcuts and selection modifiers (but you read my book, you know how to do this).
Now, go to "Filters - Render - Clouds - Difference Clouds". GIMP will calculate automatically a random "cloudy" pattern (remember, in grayscale) inside the selection. Every time you apply the filter (with Ctrl + F), a different pattern will be created. If you see one that you like - stop, and save the image...
You may find that GIMP will create the new pattern “on top” of the previous one. Several “Filter” actions may lead to an unnatural effect... If you don’t like the effect, first clear the selected area (fill it with white), then apply the filter again.
With the sky area still selected, create a new layer (right button on the layers window). Fill it in with “your” blue colour. Then, select “Multiply” as fusion mode. As you can remember from previous posts, this is a non-destructive process that allows you to change the colour every time without modifying the image.
If the effect is too strong, you can also play a bit with layer transparency. Finally you will get your cloudy sky. Notice that you may (of course) mix colour and black and white images (provided you work in RGB mode...).
To save the picture as PNG or JPG, you will need to flatten all layers (right-clic on the layer miniatures, then “flatten image” down below...)
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