Filter Module Use

Notes from users, documentation addendums.

Filter Module Use

Postby Guy » Tue Feb 14, 2017 9:50 am

Here are some notes relating to using this module. It is not the only way to use the module and experimentation is encouraged.
Please let me know if anyone sees any errors or has any additional advice they think helpful.
I will update this post as needed.
To see a full alphabetical list of module topics click here.

Filter Module

Purpose:
  • To modify features in the image on the basis of their colour.
Description:
For a general overview see Filter: Feature Manipulation by Colour.
The Filter module can be used to bring out detail of a specific colour (such as faint Ha, Hb, OIII or S2 details), remove artefacts (such as halos, chromatic aberration) or isolate specific features. It functions as an interactive colour filter.

Useful Sources
The Unofficial guide is usually a good source of help. However, it relates to version 1.3.5 so there may have been some changes. The notes below relate to StarTools version 1.4.
The processing tutorial video StarTools: Lifting structures from heavy noise with the Life module has a description of using the Filter module between 6m40s and 8m57s.

When to use:
  • This is best used after the Color module and before the Denoise module (Tracking off).
Example Workflow:
AutoDev-{Band/Lens}-Crop-Bin-Wipe-AutoDev(or Develop)-{As needed: Decon/Sharp/Contrast/HDR/Flux/Life}-Color-{Filter}-Denoise-{If needed: Layer/Magic/Heal/Repair/Synth}
Key: {...} optional modules

Method:
This is a way of using the module which should give good results in most cases:
  1. Specify the action you want to perform when you choose the colour range (spectral line) by setting the Filter Mode.
  2. Optionally - create a mask to select only those elements you want to act on. If you don't the selected action will apply to the selected colour range anywhere in the image.
  3. Select a particular colour on which to do the specified action by clicking on that colour in the image.
  4. Repeat the selection to enhance the chosen effect.
  5. If you make a mistake, the 'Undo' button undoes the last operation, the 'Reset' button discards all the changes since you started using the module.
  6. 'Keep' when the result is right.
What result to look for:
  • The selected colour range should be enhanced or reduced as specified by the Filter Mode setting.
  • Only the areas where the mask is set (green) should be changed.
After Use:
  • If used after the Color module then the usual next step is to turn off Tracking and use the Denoise module.
Special Techniques:

Removing Purple Fringes around stars
For a full description of this technique see Fringe and halo killer.
  1. Put the stars with fringes in a mask - making the stars green. Be sure to include the fringe.
  2. Set the Filter Mode to 'Fringe Killer'.
  3. Set the Filter Width to 1 - the lower value makes the filter more responsive to colours close to (but not exactly the same as) the selected colour.
  4. Click on the coloured star halos in different places to eliminate the fringes completely.
  5. 'Keep the result when you are done.
Description of Controls:

Mask
For general instructions on using masks see Mask.
  • Select the elements that you want to be modified. The action will be applied to areas of the selected colour range within the masked areas.
Filter Mode
Controls what a filter will do with the selected spectral line.
Values are:
  • Conservative Nudge - make the selected colour range more pronounced as long as it doesn't cause over-exposure (clipping).
  • Nudge - make the selected colour range more pronounced - even if it means over-exposure.
  • Pass - keep the selected colour range and reduce all other parts of the spectrum.
  • Reject - reduce the selected spectral band and keep all other parts of the spectrum.
  • Fringe Killer - tries to remove halos and fringes around stars that have the selected colour range.
  • Saturate H-Alpha - tries to make the Hydrogen Alpha (red) spectral line more prominent.
  • Saturate H-Beta - tries to make the Hydrogen Beta (cyan) spectral line more prominent.
  • Default is 'Conservative Nudge'.
Sampling Method
The central colour to filter is selected by clicking a pixel in the image. This setting controls how the colour range will be identified from this:
  • 3 x 3 Average - averages the colour of a 3x3 pixel block around the selected pixel.
  • Single Pixel - uses the colour of the selected pixel only.
  • Default is '3x3 Average'.
Filter Width
Controls how much of neighbouring parts of the colour spectrum that the selected action will be applied to.
  • Values can range from 0 to 10 with 10 being the widest colour range.
  • The width does not have a sharp cutoff - there is a peak and the intensity of the effect fades away as you get further from the peak. The fading is more gradual at higher Filter Width values.
  • Default is 5. Range is 0 to 10.
Mask Fuzz
If a mask is used, Mask Fuzz controls the blending of the transition between masked and non-masked parts of the image.
  • Using this control will allow smooth transitions between filtered and unfiltered parts of the image.
  • Default is 1.0 pixels. Range is 1.0 to 31.0 pixels.
Background Notes:

Chromatic aberration
This can be split into two types:
  • Axial or Longitudinal chromatic aberration - caused by different wavelengths/colours coming into focus at slightly different distances - this is the type that the Filter module addresses.
  • Transverse or Lateral chromatic aberration - colour shifts due to the different channels not quite lining up - this can be mitigated using the Lens module.
Practical Test results:
    With a Filter Mode of 'Reject':
    • A Filter Width of 0 gives a fairly narrow band rejecting a single colour with a steep side to the rejection curve.
    • A Filter Width of 10 gives a wide band rejecting just under half the visible spectrum with a shallow side to the rejection curve.
    With a Filter Mode of 'Pass':
    • With a Filter Width of 0 a fairly narrow pass band showing a single colour and a steep side to the pass curve.
    • With a Filter Width of 10 a wide pass band showing just under half of the visible spectrum and a shallow side to the pass curve.
Guy
 
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Joined: Thu Feb 19, 2015 8:35 am

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