Luminance noise reduction
Of the two noise reduction options that Nebulb has (pre-stack) the one that is usually most effective (and convenient) is luminance noise reduction.
Despite its name ("Luminance Noise") this option serves both to slightly reduce the luminance noise of the image and, in addition, it is tremendously effective for hot-pixel reduction.
In fact, this method is so effective for hot-pixel reduction because the luminance noise reduction is applied directly on the original RAW image (before the demosaicing is done to obtain the color image on Bayer-filtered sensors (the vast majority)).
ATTENTION: Luminance noise reduction is available when importing RAW images from Bayer sensors. That is, luminance noise reduction cannot be used when importing images in TIFF or JPEG format and also when importing RAW images from Fuji cameras (the sensor these cameras have (X-Trans) is different from the rest of cameras with Bayer filter). In these cases, the luminance noise reduction option is disabled and cannot be selected.
Below, you can see the effect of luminance noise reduction on a couple of images containing a large number of hot-pixels:
Cropping (at 100% zoom) of a region of an image with many hot-pixels | Result of applying luminance noise reduction |
Cropping (at 100% zoom) of a region of an image with many hot-pixels | Result of applying luminance noise reduction |
As you have seen in the examples above, by simply applying luminance noise reduction the hot-pixels are almost completely eliminated.
However, for difficult cases (where hot-pixels still remain) you can combine luminance noise reduction with color noise reduction (by activating both options), thus achieving an even greater reduction of this type of noise, as you can see below:
Cropping (at 100% zoom) of a region of an image with many hot-pixels | Result when applying luminance noise reduction and color noise reduction |
IMPORTANT: It is always a good idea to try luminance noise reduction first, as it is more effective than color noise reduction and, in addition, maintains the color of the stars much better. The main drawback of color noise reduction is that it causes stars to lose color intensity, since stars are very small objects (comparable in many cases to the size of hot-pixels). However, luminance noise reduction affects the color of stars much less, which is why luminance noise reduction is often preferable to color noise reduction.
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