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HDR Multiscale, Substitute script, background

Linwood
in PixInsight
Is there some technique to ensuring that the background still matches when you use a preview to isolate the HDR Multiscale, and then substitute it back in. 

Sometimes (maybe most times) it does, but was doing M57 and struggling - getting a good rendition of the ring usually lightens the background slightly so when substituted back in, you can see the square.  I can fix it (sort of) by clipping the background overall afterwards, but that is not a good solution (notably the nearby bared galaxy is then reduced). 

Using a larger preview seems to help as well, but I think by the time it is big enough to match the affect of the HDR process is reduced. 

Is the answer trying to re-stretch the preview (after storing it) to match the original in some way with histogram?  Or is there another approach? 

Thanks, 

Linwood

Comments

  • BlockHeadBlockHead
    Linwood,

    Here is my logic for most objects.

    1. HDRMT is used to compress the dynamic range of bright features.
    2. The background is not bright- it is not what HDRMT is working on (usefully)- especially if you are applying to a fraction of an image. Of course, if you apply to the entire image- everything will correlate. 
    3. The use of a threshold mask (or selective mask) will limit where the HDRMT effect is visible and protect the original background. 

    So I think the use of an appropriate mask is the best approach.
    -the Blockhead
  • Linwood
    Thank you.  I'll give it a try. I was worried about a hard edge on the mask, but (sound of hand slapping forehead) I can just feather it with a convolution before applying the mask. 
  • BlockHeadBlockHead
    Linwood,

    You can feather the edge OR use a threshold type mask. That is only apply to pixels above a certain brightness- this will also protect the background- even if there are dark areas inside the object/area of interest.

    -the Blockhead
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