Nebulb works internally with one image for the sky and another for the ground, on which, in addition, different adjustments can be applied, such as, for example, light pollution reduction (which is only applied to the sky part).

Because of the latter (different adjustments on the sky than on the ground) it is then necessary to apply a blending that allows to combine these two parts (sky/ground) in a single image, which is what we want to obtain in the end.

And precisely to make this task as easy as possible in Nebulb you have the "Blend" module.

IMPORTANT: For the "Blend" module to be enabled, it is necessary to have defined the sky mask, as this is the way for Nebulb to know which area of the image corresponds to the sky.

Sky/ground transition

As you can see in the image above, the blend module is divided into three sections:

1. Sky transition: This is the transition zone that goes from the horizon to the sky (not including any of the ground).

1. Ground transition: This is the transition zone from the horizon to the foreground of the image (not including any of the sky).

1. Exposure balance: Indicates the exposure variation that can be applied to the horizon area to balance the brightness of the sky with that of the ground, thus achieving a natural sky/ground transition (no brightness imbalance between the two areas).

Below you can see a graphical representation of these 3 zones on a sample image:

1. Blue zone: Sky transition.

1. Red zone: Ground transition.

1. Yellow zone: Region in which the exposure balance takes effect (areas close to the horizon).
   

Controls for adjusting the sky/ground transition

At the top of the "Merge" module there are two selection buttons, since in Nebulb it is possible to independently adjust both the general sky/ground transition (button on the left) and the particular transition between the sky and an object protruding above the horizon (button on the right).

Thus:

1. Left button (the stars button): allows you to adjust the general transition between the sky and the ground for the entire image (this is the default setting).

1. Right button (the lighthouse button): allows you to adjust the transition between the sky and objects protruding above the horizon (in case you have defined the mask for this type of objects).

Therefore, depending on the button you select, you can adjust one transition or the other (general transition vs. particular transition for objects protruding above the horizon).

Also, below you have the function of each of the controls that allow you to adjust the sky/ground transition:

1. Radius: Indicates the extension of the corresponding transition zone. The larger the radius the longer the transition zone and the smaller the radius the shorter it is.

1. Hardness: Indicates how progressive the transition effect is. The lower the hardness the softer the transition, which generally gives more natural results (hence the default hardness value is 0).

1. Opacity: Indicates the transparency of the transition. With 100% opacity the transition is applied completely over the image, while with 0% opacity the transition is not applied at all.

Finally, the "Exposure Balance" adjustment allows you to modify the exposure of the area close to the horizon in order to match the brightness of the sky with that of the ground in that part of the image. Thanks to this adjustment, it is possible to achieve a very natural transition between the sky and the ground, since it allows to balance the brightness in both parts of the image.

Process for adjusting the sky/ground transition

The steps I recommend you follow to adjust the sky/ground transition are these:

1. Start by adjusting the sky transition.

1.1. First modify the "Radius" to see with which value a more natural transition is generated in the sky part (the default value is 50 because it is usually the optimum in many occasions).

1.2. Then modify the "Hardness" to check visually with which value you like more the appearance of the sky area (lower values usually offer more natural transitions).

2. Now try modifying the "Exposure Balance" to balance the brightness of the sky with that of the ground (in the horizon area). The idea here is that you observe that both the sky and the ground are at the same brightness level around the horizon, so that you do not perceive any light discontinuity when visually panning the image.

3. Continue with the adjustment of the ground transition as you have done with the sky: first adjust the "Radius" and then try the "Hardness" so that the ground area is the way you like it.

4. Do a general review of the image to check if there is any area of the image that doesn't look quite natural (because the sky/ground transition is noticeable).

5. If the sky/ground transition still doesn't look natural you can also try reducing the "Opacity" a bit (generally it gives better results to reduce the opacity of the sky than that of the ground). This works especially well when there is vegetation in the horizon area.

6. If the transition still doesn't look as natural as you would like, go back to the starting point and repeat this process (making small changes each time) to try to iteratively fine-tune the transition and get it right.

7. Finally, once the sky/ground transition looks natural, zoom in on the horizon to check in detail that the transition also looks good when you enlarge the image.