How to define the sky mask when the horizon is flat
Feature available from version 1.5.0
When the image has a completely flat horizon (because the sea or ocean is in the background) it is much easier to define the sky mask using straight lines.
In order to define the mask using straight lines you just have to select the normal brush, click on a point of the image and then click on another point (also pressing the Shift key). This draws a straight line (the thickness of the brush) from the previous point to the current one.
In this way, it is also possible to define the mask using polylines (a succession of chained straight lines), by simply clicking on the desired points of the image, keeping the Shift key pressed.
Below you can see a real example of the application of this utility:
The first thing to do in this case is to select the sky mask (1) and the normal brush (2). Once that is done, the most convenient thing to do is to zoom in on one edge of the image (the left one in this case), reduce the radius of the brush (with the mouse wheel) and draw a point on the horizon (3).
Then just move to the right side (using the right mouse button) and, with the Shift key pressed, click on the right boundary of the sea. When you do this Nebulb automatically draws a line (the thickness of the brush) that goes from one end of the horizon to the other.
Once this is done, the only thing left to do is to define the mask on the rightmost part of the horizon (the part that is not the sea). For this purpose the most useful tool is usually the magic wand, because it allows you to define very easily the area of the sky just above the horizon.
And finally, once the mask has been defined in the entire horizon area (from one end of the image to the other), all that remains is to select the paint bucket and click on a point in the sky to fill the entire mask completely.
In this way, it is possible to define the sky mask very quickly and easily on an image with a flat horizon.
Feature available from version 1.5.0
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