StarTools

Hi,

Here are some notes relating to using the Wipe module.
They relate to StarTools version 1.5 and 1.6
Please let me know if anyone sees any errors or has any additional advice they think helpful.
I will update this as needed.
To see a full alphabetical list of module topics click here.
For an index of similar notes on the other StarTools modules see StarTools Main Window Use.

Guy

Wipe Module

The description below is intended to help in getting good results. Please feel free to experiment - and provide feed back.

Purpose:

• To remove background colour casts and gradients such as unwanted light (light pollution, moon, street lights), vignetting, amp glow and colour casts.

Description
For a general overview see Wipe: Light Pollution, Vignetting and Gradient Removal.
Wipe flattens the field and removes gradients, Wipe also calibrates the background and removes bias in the colour channels. Note that this does not necessarily mean that the background becomes a nice neutral grey. Rather it makes sure that the background is luminance bias-free across all colour channels. Subsequent colour calibration will modify and neutralize the chrominance (colour) information.

Useful Sources
There is a good general description of the Wipe module in Wipe Usage.
The processing tutorial video M8 in Color with modest data describes the Wipe module between 1m45s and 3m40s.
The video StarTools: Removing vignetting and complex gradients describes a more complicated use of the Wipe module.
The Hangout discussion of StarTools with Ivo discusses the Wipe module between about 0h20m and 0h32m.

When to use:

• After using Initial AutoDev, Lens, Crop and Bin as needed.
• Only use Wipe once - although you can use 'Undo' on main screen to immediately go back and redo the Wipe Module if you aren't happy with the results.

Example Workflow (v1.5):
AutoDev-{Band/Lens}-Bin-Crop-Wipe-AutoDev(or Develop)-{Decon/Sharp/Contrast/HDR/Flux/Life}-Color-{Entropy/Filter}-Denoise-{Layer/Shrink/Heal/Repair/Synth}
Key: {...} optional modules

Example Workflow (v1.6):
AutoDev-{Band/Lens}-Bin-Crop-Wipe-AutoDev (or Develop)-{Contrast/HDR/Sharp/Decon/Flux/Life}-Color-{Entropy/Filter}-Denoise (or Denoise 2)-{Layer/Shrink/Heal/Repair/Synth/Stereo 3D}
Key: {...} optional modules

Method:
This is a way of using the module which should give good results in most cases:

1. When doing the Initial AutoDev (initial global stretch):
   • Look at the image and see what sort of gradients you have. If the brightness drops away at the corners you have vignetting, If there is a general lightness in the background choose 'Gradient' If there is an intensity in a particular area - especially at the edge - you have 'Amp glow'.
   • Look for any stacking artefacts which need to be cropped out before using the Wipe module.
   • Look for any dust specks, dead pixels, trees, mountains etc. which will need to be masked out.
2. After using the Crop module to remove any stacking artefacts, load the Wipe module.
3. Set the mask to exclude the target, dust specks and other dark anomalies as described below:
   Mask - Clear - Lassoo around Dark Anomalies and Target as needed - Invert - Keep.

   

   KurtM_M8_WipeMask_qs.jpg (22.84 KiB) Viewed 7643 times

4. Adjust the Dark Anomaly Filter to mask the effect of small Dark Anomalies (see below).
5. Choose a preset which best describes what you are trying to achieve (see below). If there are multiple issues choose the most aggressive (i.e. Vignetting > Gradient > Color Cast).
6. Default values often work fine. Press 'Do'.
7. Check the results - Use the 'Color/Luminance' button to check both results if in 'Compose' mode
8. If the result is not right, ensure the subject, dust specks, dead pixels, trees etc. are not included in the analysis by using a combination of Mask and Dark Anomaly Filter controls as described below.
9. Adjust the 'Corner Aggressiveness' and 'Drop Off Point' only when there is Vignetting.
10. Press 'Keep' when done.

What result to look for:

• After applying Wipe the image is temporarily stretched automatically so you can see the results better - the stretched image should have a uniform and dark background.
• There should be no remaining background hue or colour cast. If there is check the masking and redo, and try increasing the Aggressiveness.
• Look at the stretched image background for halos and other casts - this may be due to dark features such as dead pixels, dust, trees etc. which have not been removed. These can be removed using the Mask or Dark Anomaly Filter as described below.
• If some detail has been wiped reduce the Aggressiveness.
• If casts exist try increasing the Aggressiveness.

After use:

• The Wipe module should make the background darker. It is important to redo the global stretch (using Develop or AutoDev) to take advantage of the extra dynamic range made available.

Ways of getting better results:

• Use Flats when stacking so that the Vignetting, dust specks, etc. will have already been accurately compensated for. This will simplify the Wipe to work specifically on light gradients.
• If Wipe does not reduce the colour cast well it may be that the image has a lot of noise. In this case, increasing the Dark Anomaly filter can make a difference.

Special Techniques:

Masking out flaws (e.g. dust or artefacts) that were hidden before running Wipe
See the full description in the topic Wipe module - editing mask.

• Use the Wipe module as normal and 'Keep' the flawed result.
• Use the 'Mask' button on the main screen to edit the mask to exclude the newly highlighted flaws.
• Crop any remaining stacking artifacts.
• Use the 'Restore' button on the main screen - select 'Original' - this reverts to the Original (Linear, Binned & Cropped) image.
• Use the Wipe module as normal - which now uses the updated mask.

Description of Controls:
Presets:
The following buttons pre-set the control settings to values most suited to the intended purpose:

• Gradient - gradual changes in background light level across the image - e.g. from light pollution or the moon.
• Vignetting - for removing any background light level that falls away at the corners - use flats instead if you can.
• NrwBand - optimised wipe for narrow band images - Light pollution and gradients are less of a problem with narrow band data.
• Amp Glow - a brightness normally at one edge of the image due to an uneven heating effect from the camera electronics.

Mask
For general instructions on using masks see Mask.

• Mask out (make non-green) the main subject and any dust specks, mountains, trees etc, using the 'Lassoo' Brush Mode.
  This will stop Wipe trying to use them in analysing how to wipe the image - the whole image will still be wiped though.
• If using the 'Amp Glow' preset - select (make green) the pixels that are affected by amp glow.
• You must use Develop or AutoDev to bring out the image details before using Wipe if you want to see them in the Mask.

Color/Luminance
In 'Compose' mode the Wipe module works on the luminance and colour datasets separately but in parallel. You can see the results of each by pressing this button. You will be reminded if you haven't checked the color view.

Mode
This specifies the kind of gradients removed by Wipe:

• Correct Color and Brightness - use for dealing with Vignetting, Gradient, Narrow Band and Amp glow.
• Correct Color Only - select if only interested in removing uniform colour casts.
• Correct Brightness Only - can be used if color absolutely needs to be retained. This can be useful when processing narrowband data, or data that was not acquired on earth.
• Default is 'Correct Color and Brightness'.

Aggressiveness
Sets how aggressive Wipe should be:

• Wipe searches for a constant background level of brightness around each pixel in the image - the search area is governed by the 'Aggressiveness' setting - the higher the value the smaller the area searched to establish the background level. See the description in the topic Wipe feature - when to use Mask.
• Default is 75% for Gradient and Vignetting preset, 0% for NrwBand, and 98% for Amp Glow presets.
• Change if gradients remain - up to 95% in extreme cases. Reduce from the default if detail is being wiped.

Corner Aggressiveness
Sets how much to increase gradient removal aggressiveness in the corners.

• Range is 0% to 100%.
• Defaults to 100% - comes into effect When 'Drop off Point' is less than 100% - i.e. use for removing vignetting.
• Normally the default (100%) is fine. Change from default (100%) if the background level in the corners is different from that in the centre. Should need only to reduce it by 1 or 2%.

Drop Off Point:
Defines the point from the centre at which the 'Corner Aggressiveness' factor comes into effect.

• Range is 0% to 100%.
• Defaults is 100% unless using the 'Vignetting' preset when the default is 0%.
• Change from default (0% when removing vignetting) if the background levels show uneven rings about the centre.
• Leave at 100% except when removing vignetting - Reduce from 100% (to 0% or thereabouts) to allow the 'Corner Aggressiveness' setting to apply as you move out to the corners.

Precision:
Defines the image 'resolution' that Wipe uses when analysing the image prior to wiping.

• Range is 128x128, 256x256, 512x512, 1024x1024 or 2048x2048.
• Default is 256x256 pixels for Gradient and Vignetting presets, 128x128 for Color Cast and Narrow Band, and 512x512 for Amp Glow.
• Change from the default to a larger number if the image has rapidly changing gradients and some background gradient has not been compensated for.

Dark Anomaly Filter:
Use this to stop dark spots in the image (such as dead pixels, small dust specks and small scratches) affecting the Wipe.

• Default is 1 pixel. Range is 'Off'(0) to 30 pixels.
• Set to the size of the dark area. Larger dark areas (including larger dust specks) should be deselected using the mask instead.
• Try Increasing to 2-10 pixels - look at the effect - continue until no further benefit.

Background Notes

Dark Anomalies
Every night sky image that is still linear has a minimum background light level caused by the sky glow.
This is represented by a certain pixel value. If there are pixels with values below this then they don't represent something in the sky.
These values are either caused by hot or stuck pixels, dust or scratches, noise, or something terrestrial like a tree or mountain.
StarTools calls these Dark Anomalies.

Dark Anomaly Filter and Mask
Wipe tries to find this background level so as to identify the minimum and maximum pixel values that represent light from the sky.
Dark Anomalies are dips below the minimum level which would cause Wipe to set an artificially low background level.
In order for StarTools to find the correct background level it uses two methods to identify and ignore Dark Anomalies

• Mask - Larger Dark Anomalies, such as mountains or trees, as well as dust bunnies can be identified using a Mask.
• Dark Anomaly Filter - The Dark Anomaly Filter lets Wipe automatically ignore small clusters of pixels with very low values when deciding the background level. The value of the Dark Anomaly Filter sets the maximum pixel cluster size that will be ignored. Set to 1 pixel to ignore single pixels such as cold or stuck pixels. Set higher to ignore clusters caused by noise or small dust specks or scratches.

Dark Anomaly Headroom
The Wipe module looks for the global background level and subtracts that from the signal to reveal more detail.
The only time this is a problem is when there is no global background - i.e. when you are not looking at the sky (trees & mountains or dust) or when there is a faulty pixel.
In this case the subtraction would produce a negative value and so clip to 0.
To avoid this, Dark Anomalies are identified - using the Dark Anomaly Filter and (optionally) Mask - and some dynamic range is allocated for these.
The background level is raised for the whole image so that when the signal level is subtracted from it, the result is always greater than 0.
The amount the background level is raised by is called the Dark Anomaly Headroom. See the diagram below.
In the Wipe module the Dark Anomaly Headroom allows all the dark anomalies to be mapped directly- without modification.
The Contrast module allows some compression of the Dark Anomaly Headroom.