Definitely not a photo pro here - I've tried taking RAW photos via CHDK on Canon S3 IS as .DNG.

I work on Ubuntu 18.04, and when I open these .DNG in photivo, rawtherapee, and I think also darktable, I cannot notice much out of the ordinary.

However, if I try to use dcraw (to produce a .tif), or something that uses dcraw directly like nufraw (whether standalone or as GIMP plugin), I get these colored pixels:


(notice they don't go away with Denoising at 750, and "Hot pixels" checked in nufraw - not even if I move the "Hot pixel sensitivity" slider all the way to the right, to value of 10.000)

So, I wanted to ask:

  • Are these what they call "hot", or "dead", or "stuck" pixels - if so which? If not, what are these?
  • How come I see them only in dcraw-derived processing, and not in others (rawtherapee, photivo)?
  • Can I get rid of them using dcraw or nufraw itself, and if so, how?

EDIT: as per comments, here is a comparison between two photos with the same camera opened in nufraw, with view anchor at top left of image, and zoom 200:


Looks like the pixels are in the same location, but they don't look the same - for example, for the colored pixel indicated by the red arrow, on the left it is blueish and smudges horizontally, on the right it is reddish and smudges vertically.

EDIT02: Getting closer - it turns out, if I click on "Underexposure warning" in nufraw, these pixels start blinking:


Now, why would I even have like one underexposed pixels surrounded by many which are not underexposed, I don't quite understand (maybe it's still related to "hot"/"stuck" pixels?) - however, even if those colored pixels are mere warnings, they still import into Gimp (after you've made the workaround in Bug #1791138 “Cannot edit RAW files in GIMP under Lubuntu 18.04” : Bugs : ufraw package : Ubuntu)

Also, I've noticed that if you do Denoising in nufraw, these colored pixels are not affected - its as if the Denoising is applied only to "underlying" pixels in the image, while the colored pixels are in an "overlay" layer of its own, and remain unaffected...

So they're likely some sort of a warning - except I cannot tell how to get rid of them once I've imported them in, say, Gimp (or converted to tiff as in dcraw -T mypic.DNG)...

  • 1
    Are they in the same place and the same color/intensity in every shot, or do they move around and change in color/intensity across shots? In the first case they might be "stuck", in the second they're more likely to be "hot". "Dead" I would expect to appear as black spots, not colored ones... And if they're only present when using dcraw, then perhaps it's an issue with your particular version of dcraw (try upgrading) or one of their algorithms... – twalberg Jan 31 '19 at 15:01
  • Thanks @twalberg - made edits to post; pixels seem to be in the same place, but not with same intensity/color in every shot (see edit); good to know "dead" are expected to be black. My dcraw is 9.27, and latest dcraw.c says 9.28, so I'm not that off... My guess is the other software likely does some handling/processing of these pixels (which is why I don't see them/find them obvious), which dcraw does not do by default - so it'd be just a matter of knowing how these things are called, and setting up dcraw to handle them. – sdaau Jan 31 '19 at 15:23

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