Band suppression & threshold sliders

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Astrogate
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Band suppression & threshold sliders

Post by Astrogate » Fri Mar 08, 2019 5:16 pm

Robin, I have not seen this question ask as of yet, so I was wondering are the band suppression & band threshold sliders in the procession section active?

Chris

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Re: Band suppression & threshold sliders

Post by admin » Sun Mar 10, 2019 9:11 am

Hi,

Good question – I thought I'd documented this but I must have missed it out as it was a last-minute feature in version 3.2.

With some CMOS cameras use the horizontal banding in the darker regions of the image when you stretch the image significantly. This shows as fine lighter and darker bands running horizontally typically a pixel or 2 high. Stacking does have some effect in removing these from the image since they vary from frame to frame that tends not to remove the effect completely.

The banding suppression controls in SharpCap can apply a digital filter to the image just after capture that will (with the correct settings) largely remove this horizontal banding. The trick is to stretch the image so that you can see the banding, set the threshold to maximum and then turn up the banding suppression strength until the banding reduces to a level that you feel is a good improvement. Then start turning the threshold back down. At some point the banding will reappear because the threshold is set to low and then you just need to turn the threshold back up a bit until the banding goes again and then your controls are set.

Technically the banding suppression strength controls the amount by which SharpCap smooths out horizontal banding features that run across the whole width of the frame and the threshold controls the brightness level above which the filter doesn't get applied. By adjusting the 2 you can ensure that the right strength of suppression is applied and that is only applied to the darker regions of the image where it seems to be the biggest problem and therefore doesn't degrade the image in brighter regions.

Hope this helps, Robin

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turfpit
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Re: Band suppression & threshold sliders

Post by turfpit » Sun Mar 10, 2019 9:41 am

Robin

Do these controls affect:
  • Just the image on the screen?
  • Just the data being written to disk?
  • Both?
Dave

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Re: Band suppression & threshold sliders

Post by admin » Sun Mar 10, 2019 1:03 pm

Both.

Cheers, Robin

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turfpit
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Re: Band suppression & threshold sliders

Post by turfpit » Sun Mar 10, 2019 2:44 pm

Thanks Robin.
The banding suppression controls in SharpCap can apply a digital filter to the image just after capture that will (with the correct settings) largely remove this horizontal banding
Out of curiosity:
  • What digital filter is being applied?
  • Are there any considerations to be made when processing the previously manipulated data?
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Re: Band suppression & threshold sliders

Post by Astrogate » Mon Mar 11, 2019 2:36 am

Hi Robin

Thank you so much for explaining this feature and I will try it out.

Cheers,

Chris A

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Re: Band suppression & threshold sliders

Post by admin » Mon Mar 11, 2019 10:48 pm

Hi,

The filter applied is as follows (roughly speaking)

* Average the image in the horizontal direction to produce a series of figures representing the average brightness of each row in the image
* apply running mean (length depends on strength selected for the banning suppression) to the row average brightness data
* subtract the smoothed row brightness data from the un-smoothed row brightness data to create an offset for each row which is basically how much brighter or dimmer that row is than its near neighbours
* subtract the offset calculated for each row from each pixel value in the row, correcting for any tendency for the row to be brighter on average or less bright than its neighbours

There are a lot of subtleties such as dealing with raw images and the threshold which ensures that bright features don't throw a spanner in the works. I'm not aware of any special care that should be taken with processing images that have been treated in this way, but is difficult to be sure what might emerge once this is used more. In theory it should be better to correct this on the individual frames where the pattern is purely horizontal than in a final stack where rotation of the frames may have crept in.

Cheers, Robin

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Re: Band suppression & threshold sliders

Post by turfpit » Tue Mar 12, 2019 1:03 am

Thanks Robin.

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