Thursday 23 February 2017

The Gimp: replace that boring sky

Getting the exposure right for landscape photography can be challenging.

You may end up with your subject just right but the sky burnt out, or an interesting sky but a foreground that is too dark.

Or you could just cheat by combining 2 images using a photo editor.

On my to-do list for this year is to make better use of Gimp, which is probably the best open source image editor.

Finding out how to do something on Gimp is one thing, while remembering the method is (for me) another challenge. Therefore I need to write this stuff down somewhere.

Here is a photo I took recently at Stonehenge;

This is a replica of a sarsen stone, used to demonstrate how a 28 ton piece of rock may have been moved around.

I'm very happy with the detail of the subject, but what has happened to the sky? I could fiddle around trying to darker the sky with the Burn tool, but my results always look like some 6 year old has been let lose with a grey crayon.

It is just so much easier with a shot like this to add a sky from another photo.

I just want the sky from this shot of Stonehenge

As I want the sky from this photo, I need to use it as a background and lay the sarsen image on top. I also need to make sure that the sky image is at least as wide as the sarsen image. So it may be necessary to adjust the size of one or other of the images first.

How to add a sky...

So assuming the image sizes are OK, this is the method for adding the sarsen image to the sky image.

Open the sky and sarsen images in Gimp, and click on the sky in the sarsen image with the Fuzzy Select tool (the icon looks like a magic wand).

Because the sky in this image is almost uniformly white, the whole sky is selected in one go.

Use menu Edit > Cut
This creates a transparent area where the sky used to be.

Now we need to select the subject area (the current selection is the sky) so;
use menu Select > Invert

Use menu Edit > Copy

Select the sky image, then menu Edit > Paste as... > New Layer

Use the Move tool to position the sarsen image.

That is basically how it is done. If we want more sky, then we could increase the height of the sky image canvas (menu Image > Canvas Size...) so that when we overlay the sarsen image, we can slide it down lower on the sky image, revealing more sky.

A more difficult example

I wanted to repeat the process on a photo of Stonehenge.

The problem here is that the sky is not completely bleached out, so when using the Fuzzy Selection tool I only capture part of the sky. So I simply varied the technique.

Instead of removing the sky before over-laying the two photos, I pasted the subject photo over the sky photo first. After the first use of the Fuzzy Selection tool on the top (subject) layer, the image looked like this; I simply did this:-
  1. Fuzzy select a small area of foreground image sky
  2. menu Edit > Cut
  3. repeat, until all foreground sky is removed
This also had to be repeated for the gaps between the standing stones.

The final image (...after a bit more Gimp trickery)


Maybe it is a good idea to take at least two shots when taking photos of landscapes; one with exposure set for the subject, and a second with the camera tilted slightly upwards, exposed for the sky.

That way, the directions of the sun and shadows should look "right" when the two images are used in a composite image like this.

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