admin wrote: normalization of the image, so that the full dynamic range is used. Is that what you are seeing?
Yes, I think so. The reason I'm confused is that when I choose 'as is', it removes any skyglow I've added; the background goes black and we can see the histogram is left clipped again. This is particularly noticeable after tracking has been turned off and we don't then have a choice for 'as is'.
My quer has been prompted by the bell-shaped-histogram stuff on SGL:
I'm finding that without skyglow, the histogram -say in GIMP levels- is sometimes touching the left after a ST process.
Indeed, this is expected behaviour; it takes the image as-is, and then normalizes it in preparation of further stretching (thereby making the first bin of the histogram that contains any values touch the left side of the histogram).
There is/should be no clipping however. The Skyglow parameter simply adds a pedestal value.
E.g. you can simply add back the Skyglow/pedestal to create space between the left side of the histogram and the first bin of the histogram that contains any values.
You can use the Dark Anomaly Headroom to widen the left portion of the "bell curve". In other words, you can keep 100% of the dynamic range allocated to anomalous "darker than real background" values. Or you can choose to reduce this "useless" allocation of dynamic range so other values (containing actual detail) can use it.
If the previous explanations weren't clear, let me put it into an every day analogy;
You run a store named "interesting-detail-n-backgrounds-r-us". You have limited amount of shelf space (histogram bins). You get lots of different RAW materials from your supplier - far more than your shelves can hold - and it's up to you to put these materials on your (limited) shelves in a way that offers a good varied selection. How many different grades of background and interesting detail materials you display on your shelves is up to you.
You noticed that your customers are not really interested in stuff that isn't a nice background or interesting detail. Really, that’s to be expected - it's not the point of your store. Nevertheless, you decide to keep a tiny bit of stuff around that - you're pretty sure - is not really background or interesting detail (e.g. outliers, anomalies, dead pixels). So you decide to allocate a tiny bit of shelf space to it, just to satisfy the few customers that might be looking for it. <--- this is the Dark Anomaly Headroom
Some people are a bit short and can't see the upper shelves. And some just stubbornly refuse to crane their necks. To accommodate these people you can decide to leave those shelves empty and cram everything into the remaining shelve space. <--- Skyglow/pedestal value
Does that help?