Special effects for star trails
Traditionally, star trails are obtained by maximum stacking, i.e., the final image is the result of calculating (pixel by pixel) the maximum of all the captured images.
However, although this is the most common way in which star trails have always been obtained (because it is the simplest), it is not the only possible way. In fact, there are many other (more or less "artistic") ways of stacking the images for a star trails, which result in images of very different looks.
IMPORTANT: In this case, as in all other processes, Nebulb does not invent anything. It simply offers the user various stacking methods for star trails, but all the trails that appear in the final image (as well as the length of the trails) correspond to real captured stars.
That said, the "Special Effect" option, along with the drop-down list to its right, allows you to select the type of stacking that Nebulb performs to obtain the star trails image:
As you can see in the previous image, by default the "Special effect" option is activated with the value of " Fade in & Fade out" (which below you can see which type of stacking it corresponds to).
And below are the different types of star trails stacking that you can perform with Nebulb:
IMPORTANT 1: In order to apply the special effects of star trails it is necessary that the stacking is done in Nebulb, not in the camera. That is, multiple images of a relatively short exposure time (maximum recommended 1 minute) must be captured and then imported into Nebulb to perform the corresponding star trails stacking. Thus, star trails special effects cannot be applied on individual images, nor can they be applied on images already composed (such as those obtained through the Live Composite mode of Olympus cameras).
IMPORTANT 2: The selection of the star trails special effect must always be done before executing the stacking, since, depending on the selected option Nebulb performs a specific stacking for the star trails image. Therefore, in case you want to obtain the final image with different effects you will have to run the stacking again each time (in which case it is interesting to save the project resulting from each stacking with a different name in order to be able to recover each of the obtained versions at any time).
No special effect
When the "Special effect" option is not selected, Nebulb performs a traditional star trails stacking, i.e. a stacking based on the maximum of all imported images. As you can see in the image below, this is the typical appearance of star trails images, where the point at which the movement of each star starts and ends is clearly visible.
IMPORTANT: Although the type of star trails stacking is the traditional one (based on the maximum), this does not stop the rest of the specific processes that Nebulb performs (such as the removal of flying objects or the smoothing and filling of trails). That is, one thing is the type of stacking that is used (which in this case is the traditional one) and quite another is the type of specific functionalities that are executed to obtain an image of star trails of the highest possible quality.
Fade in & Fade out
By selecting this option Nebulb applies a progressive underexposure at both extremes of the star trails during stacking, thus generating a feeling of progressive fading of the trails. In other words, Nebulb automatically and progressively decreases the exposure of the initial and final images of the series before stacking them, which generates this effect by which it seems that some trails merge with others and the image transmits much more continuity, smoothness and harmony (as there is no abrupt transition at the beginning or at the end of the trails).
This is the option that Nebulb selects by default, since it offers a similar result to traditional stacking, but improving it at the extremes of the trails.
ATTENTION: It may seem that with this method of stacking the resulting trails are shorter than with traditional stacking, which is due to the effect generated by the progressive merging of the trails at the ends (which progressively decrease in intensity). Therefore, this effect tends to look better the more photos you capture (longer trails).
Fade in
This effect is similar to the " Fade in & Fade out" effect except that in this case only the progressive underexposure is applied to the images at the beginning of the series. Because of this, the fade is only visible in the initial region of the star trails, which generates an effect of the stars moving in the same direction in which they moved across the sky.
In the following circumpolar image it looks as if the stars are rotating counterclockwise, which is the actual movement they made in the sky.
Fade out
This effect is similar to the " Fade in & Fade out" effect except that in this case only the progressive underexposure is applied to the images at the end of the series. Because of this, the fade is only visible in the final region of the star trails, which generates an effect of the stars moving in the opposite direction to the one they followed in the sky.
In the following circumpolar image it looks as if the stars are rotating clockwise, which is the opposite movement to the one they followed in the sky.
Final sparkle
This stacking method is exactly the same as the " Fade in" method except that in this case, in addition to a progressive underexposure to the images at the beginning of the series, a small constant underexposure is applied to all the images except the last one (which is stacked as is). Because of this, a small sparkle appears at the end of the trails which, together with the sensation of movement caused by the fade in, generates a meteor effect (brighter at the tip and leaving a trail in its wake).
Likewise, this effect also generates a sensation of movement of the stars in the same direction in which they actually moved in the sky.
Initial sparkle
This stacking method is exactly the same as the "Fade out" method except that in this case, in addition to a progressive underexposure to the images at the end of the series, a small constant underexposure is applied to all the images except the first one (which is stacked as is). Because of this, that small sparkle appears at the beginning of the trails which, together with the sensation of movement caused by the fade out, generates a meteor effect (brighter at the tip and leaving a trail in its wake).
Likewise, this effect generates a sensation of movement of the stars in the opposite direction to the one they followed in the sky.
Output vortex
This stacking method is a combination between the "Fade in" effect and a zoom-out centered just at the center of rotation of the stars. That is, the stacking is obtained by applying a progressive underexposure on the initial images of the series and, in addition, a progressive downsizing of the images (zoom-out) from the last image (which is kept at original size) to the first image (which is the one that becomes smaller).
IMPORTANT 1: In order to generate the vortex effect Nebulb automatically detects the point in the image around which all the stars rotate, since that precise position is the one from which the zoom-out should be applied. But this is something that Nebulb does automatically, so you don't have to worry about it at all (you don't have to mark any point in the image or anything like that).
IMPORTANT 2: The vortex effect can only be achieved if the center of rotation of the stars in the sky (the north or south celestial pole) is inside the image, otherwise this visual effect is not generated. In other words, in order to generate a vortex effect the image must be circumpolar. And in case this effect has been selected, but Nebulb detects that the center of rotation of the stars is outside the image, then the "Fade in" effect is applied.
Input vortex
This stacking method is a combination between the "Fade out" and a zoom-in centered just at the center of rotation of the stars. That is, the stacking is obtained by applying a progressive underexposure on the final images of the series and, in addition, a progressive enlargement of the images (zoom-in) from the first image (which is kept at original size) to the last image (which is the most enlarged).
IMPORTANT 1: In order to generate the vortex effect Nebulb automatically detects the point in the image around which all the stars rotate, since that precise position is the one from which the zoom-in should be applied. But this is something that Nebulb does automatically, so you don't have to worry about it at all (you don't have to mark any point in the image or anything like that).
IMPORTANT 2: The vortex effect can only be achieved if the center of rotation of the stars in the sky (the north or south celestial pole) is inside the image, otherwise this visual effect is not generated. In other words, in order to generate a vortex effect the image must be circumpolar. And in case this effect has been selected, but Nebulb detects that the center of rotation of the stars is outside the image, then the "Fade out" effect is applied.
Off-center circumpolar vortexes
Since in order to generate the vortex effect (both input and output) Nebulb automatically detects the point of the image around which the stars are rotating, it is also possible to apply this effect to off-center circumpolar images, i.e., images in which the polar position is not right in the center.
In this way, off-center vortexes are an additional compositional element, since they generate zones of attraction ("input vortex") and radiation ("output vortex") that can be very interesting to achieve a greater visual impact in the final image.
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