To use it, you construct a mask image, same size and orientation as your source images, that is pure white everywhere, except black where your source images have dust spots or hot pixels. Then at Options > Preferences > Preprocessing, you check the box for “Use dust and hot pixels mask” and browse to find the file. The mask will then be applied in subsequent stacking operations.
Very briefly, what the feature does is to replace blobs of bad pixel values with a smoothly varying average of surrounding good pixel values. This alone provides a good start at hiding the defects, and then the normal stacking process further improves the result by tending to replace the smoothly varying averaged values with unmodified good values from other frames that happen to have different alignments. The result is that instead of a trail, what typically gets produced is an image that is clean except for one spot of lost or degraded detail where the defect lay in the best focused image.
Here is an example. These crops are from a stack that I shot several years ago, at a time when I think I must have had my camera open during a dust storm.
Left: without the mask; Right: with the mask.
Left: source frame with dust locations marked in yellow; Right: final mask, black on white
Here is another example, this time dealing with hot pixels as a result of long exposure with continuous light.
Left: without the mask; Right: with the mask
Left: cropped mask matching the images shown above; Right: mask for whole frame (dots expanded to be easily visible)
Note that Zerene Stacker makes no attempt to build an appropriate mask. It just applies whatever mask you give to it.
Building the mask is something that you’ll do outside Zerene Stacker, using tools in your favorite image editor such as Photoshop or GIMP.
For the first example, I pulled a source frame into Photoshop, heavily sharpened it to enhance the dust, added a new layer that would become the mask, and used a small brush to manually “dot over” each spot of dust. For brushing I used a contrasting color (bright yellow) to make it simple to see where I had painted. Then to make the final mask, I used Photoshop’s selection and filling tools to turn the yellow spots into black-on-white.
Initially I tried to construct the dust mask automatically from the source images by a combination of sharpening and levels adjustment to bring out the dust spots. But that approach failed because the dust spots had such wide range of appearance, on varied background, that I was not able to get a clean distinction between dust and no dust. It was much faster and better to just dot over the dust spots by hand.
For the second example, I found that it was tedious and unnecessary to make the mask by hand from a source frame of the stack. Instead, I simply made a very long time exposure of a black scene to make the hot pixels easily distinguishable, then used levels-adjustment in Photoshop to isolate hot pixels, selected those, expanded the selection by a couple of pixels to be sure of covering up the “crosses” that are often produced by JPEG compression, and filled the selection to make the mask. Unlike dust, hot pixels don’t move around or accumulate rapidly, so this mask will be usable for a long time in the future with that same camera.
There are a few caveats…
- At this time the process is computationally expensive, roughly doubling the time to run a basic PMax stack in my limited testing to date. Probably I can speed that up with more clever coding, but the value at current cost is high enough that I’m comfortable releasing it as a production feature.
- The mask is applied only during stacking. During retouching, source images will be presented in their original form.
- Error checking is minimal at this time. If the mask is the wrong size, or the wrong orientation, or the mask file can’t be read, or anything else unexpected happens, then either the function won’t work at all, or it’ll do something weird, or you’ll end up with one of the dreaded “Uh-oh, something went wrong” pop-ups.
- It's best if the mask image is TIFF, even when you're stacking JPEG's. This avoids possible issues caused by JPEG compression around the edges of the black areas of the mask.
- The dust & hot pixels mask will be a Pro-only feature, so to trial it with a Student or Personal Edition license, you'll have to click "Yes, start trial" when the notification popup appears.
- If you haven't received an announcement for this beta even though you're already on the Zerene Stacker beta channel, please don't worry about that. I've bundled a bunch of stuff into the current beta, and I decided to post here before pushing out an in-app announcement because of recent questions about hot pixels.