Sky/ground blending in star trails images with trees above the horizon

In the case that in a star trails image there are trees (vegetation) protruding above the horizon it is very convenient to select the maximum image for the ground (to get the most natural result possible).

Below you can see the difference between selecting the sigma-clipping and the maximum image for the ground when there is a tree protruding above the horizon:

Final star trails image in which the sigma-clipping image has been selected for the ground. As you can see, the sky/ground blending looks a bit weird, since the star trails are not visible between the tree branches.

Same star trails image as above, but having selected the maximum image for the ground. In this case the sky/ground blending is much more natural because the star trails are also visible between the tree branches.

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How to adjust the sky/ground blending when there are trees above the horizon

These are the steps to follow to achieve the most natural sky/ground blending possible when there are trees (vegetation) protruding above the horizon in a star trails image:

1. Select maximum image for the ground

Star trails image obtained by selecting the maximum image for the ground. The initial result is quite good, except that a plane trail (yellow arrows) appears near the tree and the horizon.

Enlargement of the previous image in which you can better see the plane trail next to the tree and the horizon.

2. Define the mask for objects protruding above the horizon

To include the tree in the mask for objects above the horizon you have to: (1) activate the mask for objects above the horizon, (2) draw the mask on the image and (3) refresh the masks. The only thing you have to be careful with is to define the mask for the part of the tree trunk that is not above the sky (yellow arrow), because that part of the mask has to be defined more precisely (the rest of the mask is fine, even if part of the sky is included).

3. Adjusting the general sky/ground blending

The idea here is to adjust the sky/ground transition of the horizon in general (regardless of how the blending of objects protruding above the horizon looks).

The general blending adjustments (red arrow) are applied in order to get the blending between the sky and the ground to look good for the horizon (without taking into account the tree). In this case, to avoid that the plane trail appears in the horizon area, the radius of the sky transition has been reduced a lot and a little bit its opacity.

4. Adjust sky/ground blending for objects protruding above the horizon

Finally, blending adjustments are applied for objects protruding above the horizon (the tree). To prevent the trail of the plane from appearing, the radius and opacity of the sky transition are greatly reduced, thus achieving the desired final image.

Enlargement of the previous image in which the final result can be seen in more detail.

ADVICE: When there is vegetation on the ground it works very well to reduce the opacity of the sky transition. In this way, the trails of planes and satellites do not appear next to the vegetation, and at the same time a very good and natural sky/ground blending is achieved.

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