How to define the sky mask
In Nebulb there are three types of masks:
- Sky mask: used to let Nebulb know the part of the image that is sky and, therefore, where it should look for stars to align them, compensating for the movement of the sky before stacking (due to Earth rotation).
- Exclusion mask: it is used to define areas of the sky that Nebulb should exclude from the light pollution estimation, since they are objects that are in the sky but should not be taken into account as such. Clear examples of sky objects to be included in the exclusion mask are the moon and clouds (they are in the sky, but should not be taken into account for light pollution estimation).
- Mask for objects above the horizon: this mask is only used after stacking to facilitate the blending between the sky and the ground in the final image, especially when there is some object (like a tower or a tree, for example) that protrude very predominantly above the horizon.
And to be able to define all these masks there are a set of utilities in the top toolbar.
That said, focusing specifically on the sky mask, the usual way to define it is as follows:
1. Select the sky mask
In order to start defining the sky mask you must first select it by pressing the button associated with it (button with stars).
By pressing this button the brushes on the right will be activated so that you can start working on the mask.
2. Use the magic wand
The most convenient way to define the sky mask is to use the magic wand brush and gradually select the area of sky just above the horizon.
That is, the first step in defining the sky mask will always be to select the entire region of the sky that is on the border with the ground.
To do this it is very convenient to zoom in the image to 100% zoom and move along the horizon as you define the mask with the magic wand. With the left mouse button you make the magic wand fill the mask of the sky (not including the ground) and with the right mouse button you can move the image without drawing the mask.
Left mouse button fills the mask, right mouse button moves the image.
3. Adjust the sensitivity of the magic wand (if necessary).
If there are areas where the automatic wand selection is not accurate and goes into the ground, you can reduce its sensitivity by using the scroll bar right next to the wand button.
The further to the left you adjust this slider, the less the magic wand will spread (this is ideal for areas where there is little contrast between the sky and the ground).
And the further to the right you adjust this slider, the more the magic wand's automatic fill will be extended (ideal for areas where there is a lot of contrast between the ground and the sky).
By default, an intermediate sensitivity (slider in the center) is usually adequate for most occasions.
4. Correct incorrectly selected areas
If at any time you see that the sky mask gets inside the ground you can undo the last step (by pressing the key combination "Ctrl + Z") or you can also use the eraser brush to remove the areas of the mask that have not been well defined by including areas of the ground.
5. Define by hand the areas of very low contrast between the sky and the ground
Sometimes there are areas of the horizon that, due to the darkness of the image, have very low contrast and it is very difficult to determine precisely (even with the naked eye) where the ground ends and the sky begins.
In these areas the magic wand is not useful, since the ground/sky boundary is not defined at all.
Therefore, the most useful in these cases is to use a normal brush to define the sky mask in those particular regions.
When using this brush, don't worry too much about the accuracy of the mask selection either, as this is solved by Nebulb's sky/ground blending.
6. Use the paint bucket to finish filling in the sky mask
Once you have completely defined the mask on the entire horizon area (from side to side of the image), the most useful is to use the paint bucket tool to (with a single click on the sky) make the sky mask fill automatically.
In the previous image you can see that the sky mask is defined on the entire horizon strip, so that by simply selecting the paint bucket and clicking on any area of the sky, the sky mask is automatically filled in, as you can see in the following image:
7. Make a final check of the mask
Finally, do a quick visual check of the mask to verify that it correctly covers the entire sky, without including any of the ground.
This check is especially interesting to do on the horizon area, since it is in the transition between the ground and the sky where there may be inaccuracies of selection.
8. What to do when vegetation protrudes over the horizon
In the case that in the area between the ground and the sky there are trees or vegetation do not worry if there are some branches that are included inside the sky mask. This is completely normal and will disappear when Nebulb applies the fusion between the sky and the ground after stacking.
The important thing is that no large branches or fixed objects are included in the sky mask, but if there are small parts of the vegetation that end up inside the sky mask nothing will happen either (in fact it is very likely that the wind will even change their position during the capture).
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