Color Module Use

Notes from users, documentation addendums.

Color Module Use

Postby Guy » Mon Jan 30, 2017 10:40 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

Color Module

Purpose:
  • To achieve a good colour balance that accurately describes the colour ratios that were recorded.
Important:
  • To get best results this module should be used on a colour calibrated monitor
  • If you can't do that the 'Max RGB' button helps by showing the dominant channel (R,G or B) for each pixel. This can help you adjust the colour if you have an uncalibrated monitor.
Description:
For a general overview see Color: Advanced Color Correction and Manipulation
Processing can modify the colour balance - in particular stretching the image to bring out detail can have a bad effect. This module helps correct those changes.

Useful Sources
There are very good general instructions for the use of the Color module here
Also here is another detailed description by Ivo: Color module demystified
The Unofficial guide is also a good source of help. It relates to version 1.3.5 so there may have been some changes. The notes below relate to StarTools version 1.4.
The Hangout discussion of StarTools with Ivo discusses the Color module between about 1h14m and 1h47m

When to use:
  • Towards the end of the workflow (after all the stretching) just before turning off Tracking and doing the final Denoise.
  • You can re-use this module if you use one of the 'LRGB Method Emulation' modes that uses CIELab Luminance Retention.
  • Re-use if you want to adjust the colour or saturation in different areas by using a mask.
Example Workflow:
AutoDev-{Band/Lens}-Bin-Crop-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. If you have used a narrow band or light pollution filter then you should look at the Special Techniques section:
  1. At startup, if there is a full mask set, the module auto-calibrates the white point in the image
  2. Select the preferred 'Style' - choose from: Scientific, Artistic Detail Aware, or Artistic Not Detail Aware
  3. Select the 'LRGB Method Emulation' you want to use - usually this is 'Straight CIELab Luminance Retention'.
  4. See if you get what you expected (see below for 'standard' colouring)
  5. If there are problems:
    • Use MaxRGB to look for issues (like unexpected green-dominant areas) - adjust the Red, Green and Blue bias to get a better balance
    • Use Cap Green control to eliminate any unwanted remaining green
  6. If you make a mistake, the 'Reset' button discards all the changes since you started using the module.
  7. 'Keep' the result when you are finished.
Sampling Methods:
Parts of the image can be sampled to establish a good colour balance. The following ways are available:
See also this link and Color Balancing Techniques
  • Sampling overrides the current settings
  • Use Sampling when automatic colour balance is difficult due to, for example, noisy data
  • Sampling doesn't work when a filter (e.g. a narrow band or light pollution filter) has been used as the stars are all missing part of their spectrum - so no good reference white objects, or broad range of spectral types, exist. See 'Special Techniques' for an alternative approach.
  • This colour balance technique is based on the assumption that the sampled object(s) is, on average, a good reference white so we can establish the correct relative balance of the R,G and B channels from this.
  • White light contains all the visible spectrum so we look for objects which contain the same broad range
    • 'White' Galaxies and some Star fields contain a broad range of spectral class stars and so are considered good reference white,
    • Spectral class G2V stars, like our sun, are often considered suitable white references but some people argue that they dont have a
  • Globular Clusters often don't have a good mix - they mostly have very old stars - mainly yellow with some orange and reds - so should be excluded from a star field calibration.
  • For a discussion of these colour balancing techniques see the following references:
    Getting the colors right in astrophotos
    PixInsight color calibration methodology
    See also this Starizona article on True Color Imaging
Single sample
See also Setting white reference by clicking pixel
  • Click on an area in the image that should be white
Sample from Mask
See also Setting white reference by mask sampling
  • Define the elements that make up your sample in the mask (e.g. a galaxy, G2V star, or star field with good mix of star temperatures)
  • Click the 'Sample' button - this uses the mask to define the sample from which the white balance is determined and the Red, Green and Blue bias settings are established.
Star Field Calibration - assumes the star field has a good mix of star temperatures
See also Starfield Colour Calibration
  • Mask - Auto - 'Fat Stars' preset - Do - (optional grow 1 pixel) - Keep
  • Click Sample (uses the star samples and sets RGB bias control settings based on that)
  • Mask - Clear - Invert - Keep
  • The module remembers the RGB settings but now applies them to the whole image (based on the new mask)
What result to look for:
The colour distribution you are looking for depends to a large degree on your preferences.
(See also this link)
If you look at these Thumbnails of Images of M8 you can see the range of colours of the processed M8 images on the internet. Many of them have a red bias and this can be a side-effect of the non-linear stretching causing a colour skew that has not been compensated for.
If using the 'Scientific (Color Constancy)' approach you should look for the following:
  • Good distribution of star colours - Foreground stars should show a good distribution of colour temperatures from red through orange, yellow and white to blue.
  • If a light pollution filter is used the star colours may just be orange and blue with little in between - yellow is often missing.
  • Check for green - This should be rare unless there is an OIII emission region (e.g. M42 core or Tarantula Nebula).
  • The H-alpha should look red, H-beta should look cyan.
  • HII areas (H-alpha + H-beta) should look purplish/pink.
  • Galaxy cores tend to look yellow (older stars) and their outer rims tend to look bluer (younger stars & star formation).
  • Dust tends to let through lower wavelength light (if any) - mainly browns and reds.
If using the 'Artistic, Detail Aware' or 'Artistic, Not Detail Aware' styles you should look for the following:
  • Bright areas will be paler, less colourful than above.
If using the Hubble pallette or similar.
  • Ensure the relative strength of each channel highlights the detail you want.
Light Pollution filter - tends to have a dip where the yellows should be - so galaxy cores lose their yellow.

Ways of getting better results:
  • Try to correct any colour problems (such as those caused by light pollution and gradients) using the Wipe module first.
  • Help the Wipe module by using Flats.
  • Temporarily increase the Saturation Amount control (to say 300%) while working with colour to help when guaging colour balance.
  • If using a light pollution filter visually getting the right balance will be very difficult as there are parts of the spectrum missing. It will often show a lack of yellow and some green when properly colour balanced - There is a way around this. See the Special Techniques section below.
After Use:
  • Normally at this point you are ready to stop Tracking and to use the Denoise module.
Special Techniques:

Colour Balancing data filtered by a Light Pollution Filter
This approach is summarised in the article Colour balancing of data that was filtered by a light pollution filter
  1. Colour balancing by sampling of filtered data will not give meaningful results.
  2. Shoot luminance data with the light pollution filter in place
  3. Shoot colour data without the filter in place
  4. Process both images separately
  5. Combine in Layer module as described here (in this example it combines luminance and colour for Ha and RGB but the techniques are the same)
Colour Balancing data collected using narrow band filters (e.g. Hubble Palette)
For cases where you have used the LRGB module to load the data collected using SII, Ha and OIII narrowband filters to the R,G and B channels respectively.
  1. This is not intended to be true colour so colour balancing becomes a matter of taste.
  2. Optionally create a weighted synthetic luminance frame - As described in the article: Ha,R,G,B -> Synthetic Luminance
  3. Adjust the relative proportions of SII, Ha and OIII using the R, G and B bias sliders until you get a balance that you like.
  4. 'Keep' the result.
Adjusting Colour of Stars and other features separately
Sometimes you want to adjust the colour of the stars separately from the rest of the image:
  1. Create a Star Mask
  2. In Color module click 'Sample' - this sets the colour balance assuming the average star colour is white
  3. Invert the mask - this will apply the white balance to the other (non-star) features
  4. Adjust the saturation of the other features
  5. Make any other changes to the colour balance you want
  6. Invert the mask - so it selects the stars again
  7. Adjust the saturation of the stars
  8. Make any other changes to the colour balance you want
  9. 'Keep' the result.
Description of Controls:

Mask:
For general instructions on using mask see Mask
  • The mask can be used to select areas to sample as a colour reference - see Sampling Methods section above. Click 'Sample' when done.
  • The mask can also be used to selectively adjust the colour of areas in the image. To do this you need to clear the mask before starting the module and set it when prompted at the start.
Max RGB:
For each pixel, shows which channel - R, G or B - is dominant.
http://www.startools.org/modules/color/usage/how-to-determine-a-good-color-balance/maxrgb-mode
  • If your image is too red, pixels that are supposed to be 'neutral' (such as the background) will show mostly red. If your image is too green they will show mostly green. If, however, your image is well calibrated, these neutral pixels will alter between red, green and blue.
Histogram:
The histogram shows the distribution of pixel intensity for the separate R,G and B channels
  • Displays the pixels intensity distribution split into RGB channels
  • Only pixels set in the current mask are counted
Style:
See also the article Tweaking your colors
Options: -
  • Scientific (Color Constancy) - keeps the colour regardless of brightness by separating luminance and colour processing.
  • Artistic, Detail Aware - Emulates much other software where bright areas can look washed out. Tries to compensate for local brightness manipulations during processing (e.g when using HDR), In these areas it will compensate for these changes and show more colour.
  • Artistic, Not Detail Aware - As above but does not try to compensate for local brightness manipulations during processing
  • Only available when Tracking is engaged
Saturation Amount:
Specifies the amount of colour saturation relative to the original image.
  • Default 200%, Range 0-1,000%
  • Reducing to 0% turns the image monochrome
Bright Saturation:
Specifies the colour saturation in the lighter areas
  • Default is Full (10), Range 1.00-10.00 (Full)
  • Reduce this value where there are colour artefacts noticeable in the highlights. For example where there are colour fringes around bright star cores with one side blue and the opposite side red.
Dark Saturation:
Specifies the colour saturation in the darker areas
  • Default 2.00, Range 1-10
  • Reduce this value if there is a lot of colour noise in the dark background.
Cap Green:
  • Green is produced by OIII emission regions (e.g. M42 core or Tarantula Nebula) which are rare.
  • Stretching color data with luminance causes a skew in the colour balance.
  • Very few objects in space are predominatly green when imaged in RGB. So if we find a green pixel, and we are sure the colour balance is right
  • We can assume any pixels that are green are made that way by noise so we convert them to something more natural like yellow or brown.
  • Use as a final change if necessary
LRGB Method Emulation:
See also the article LRGB Method Emulation
With Tracking on, StarTools has from the start separated L and RGB (if the data was linear when imported).
Now is the time to combine them - there are a number of different approaches to combining them to choose from here:
  • Straight CIELab Luminance Retention
  • RGB Ratio, CIELab Luminance Retention
  • 50/50 Layering, CIELab Luminance Retention
  • RGB Ratio
  • 50/50 Layering
  • The default is 'Straight CIELab Luminance Retention'
Bias Slider Mode:
Sets whether the Bias sliders increase or reduce the channel influence
See also the article Setting a colour balance
  • Default is 'Sliders Reduce Color Bias'
Bias Sliders:
These sliders set the colour balance. They can be adjusted manually or by one of the colour sampling techniques described above.
Red Bias Reduce (or Red Bias Increase):
  • Default 1.00 (no reduction), Range 1-20
Green Bias Reduce (or Green Bias Increase):
  • Default 1.00 (no reduction), Range 1-20
Blue Bias Reduce (or Blue Bias Increase):
  • Default 1.00 (no reduction), Range 1-20
Mask Fuzz:
  • If a mask is used, Mask Fuzz controls the blending of the transition between masked and non-masked parts of the image
  • Default 1.0 pixels
Background Notes:
'Real' colours in Astronomical Images
There is an interesting discussion of the issue of 'real' colours in astrophotography in the article by Jerry Lodigruss Color in Astronomical Images
Also there is another related discussion on Starizona True Color Imaging
This discusses ways of getting a proper colour balance including using a white (G2V) star.

G2V vs Other sampling techniques
The article Getting the colors right in your astrophotos discusses other approaches as well as discussing the idea that a sun-centric view of color balance (Anthropocentrism) does not make sense and so methods other than using G2V stars to color balance should be used.
This PixInsight forum post 'About our color calibration methodology'
describes the PixInsight approach to colour balancing - which they call 'spectrum-agnostic' or 'documentary' calibration methods.
They 'try to apply a neutral criterion that pursues a very different goal: to represent a deep sky scene in an unbiased way regarding color, where no particular spectral type or color is being favored over others'.
They advocate using light sources which include a good range of stellar poulations and spectral types. Examples they suggest are
  • a nearby galaxy with negligible red shift.
  • a sampling of a large number of stars - by averaging a sufficiently representative sample of stars there is no bias towards one colour.
Colour Balancing and Light Pollution Filters
Traditional light pollution filters filter out the band of wavelengths that artificial lights (e.g. low pressure sodium lights) typically produce.
This means there is part of the spectrum missing (in the orange-yellow region) which means it is not possible to colour balance using this data.
With the rise of more broadband lighting (LED HPS and Metal Hydride) this approach is also becoming less effective.
Here is and Interesting apprach to using narrow band filters to overcome light pollution Using Photometric Filters to Overcome Light Pollution
Instead of a single LP filter, it uses 3 narrowband filters Ha (656nm x 20nm), sYel (550nm x 19nm) and sV (410nm x 16nm) assigned to R, G and B. Balance the 3 channels by calibrating against a white source.
Guy
 
Posts: 55
Joined: Thu Feb 19, 2015 8:35 am

Return to User Notes

Who is online

Users browsing this forum: No registered users and 1 guest