Color: Advanced Color Correction and Manipulation

2 images of a globular cluster side-by-side
Left: traditional processing, Right: StarTools color constancy showing star temperatures evenly until well into the core.

Thanks to StarTools' Tracking feature the Color module provides you with unparalleled flexibility when it comes to colour presentation in your image.

Whereas other software without Tracking data mining, destroys colour and colour saturation in bright parts of the image as the data gets stretched, StarTools allows you to retain colour and saturation throughout the image with its 'Color Constancy' feature. This ability allows you to display all colours in the scene as if it were evenly illuminated, meaning that even very bright cores of galaxies and nebulas retain the same colour throughout, irrespective of their local brightness, or indeed acquisition methods and parameters.

This ability is important in scientific representation of your data, as it allows the viewer to compare similar objects or areas like-for-like, since colour in outer space very often correlates with chemical signatures or temperature.

Two images of M63 Sunflower Galaxy
Top: traditional processing, Bottom: StarTools color constancy showing true color of the core, regardless of brightness. (image acquisition by Jim Misti)

The same is true for star temperatures across the image, even in bright, dense star clusters. This mode allows the viewer of your image to objectively compare different parts and objects in the image without suffering from reduced saturation in bright areas. It allows the viewer to explore the universe that you present in full colour, adding another dimension of detail, irrespective of the exposure time and subsequent stretching of the data.

For example, StarTools enables you to keep M42's colour constant throughout, even in its bright core. No fiddling with different exposure times, masked stretching or saturation curves needed. You are able to show M31's true colours instead of a milky white, or resolve star temperatures to well within a globular cluster's bright core. All that said, if you're a fan of the traditional 'handicapped' way of colour processing in other software, then StarTools can emulate this type of processing as well.

The Color module's abilities don't stop there, however. It is also capable of emulating a range of complex LRGB color compositing methods that have been invented over the years. And it does it at the click of a button! Even if you acquired data with an OSC or DSLR, you will still be able to use these compositing methods; the Color module will generate synthetic luminance from your RGB on the fly and re-composite the image in your desired compositing style.

The Color module allows for various ways to calibrate the image, including by star field, sampling G2V star, galaxy sampling and - unique to StarTools - the MaxRGB calibration view. The latter allows for objective colour calibration, even on poorly calibrated screens.

Aside from Color calibration (thanks to Tracking data mining carried out on a linear version of your data, no matter whether you have stretched it or not), the Color module comes with a number of ways to control colour saturation in your image. A green removal algorithm rounds out the feature set.


Usage

The Color module is very powerful - offering capabilities surpassing most other software - yet it is simple to use.

The primary goal that the Color module was designed to accomplish, is achieving a good colour balance that accurately describes the colour ratios that were recorded. In accomplishing that goal, the Color module goes further than other software by offering a way to negate the adverse effects of non-linear dynamic range manipulations on the data (thanks to Tracking data mining). In simple terms, this means that colouring can be reproduced (and compared!) in a consistent manner regardless of how bright or dim a part of the scene is shown.


Launching the Color module

Screenshot of Color module asking to set full mask.
If a full mask is not set, the Color modules allows you to set it now, as colour balancing is typically applied to the full image (requiring a full mask).

Upon launch, the colour module blinks the mask three times in the familiar way. If a full mask is not set, the Color modules allows you to set it now, as colour balancing is typically applied to the full image (requiring a full mask).

An automatically colour balanced M101.
StarTools tends to come up with a reasonable colour balance by default.

In addition to blinking the mask, the Color module also analyses the image and sets the Red Bias Reduce, Green Bias Reduce and Blue Bias Reduce factors to an value which it deems the most appropriate for your image. This behaviour is identical to manually clicking the 'Sample' button.


Setting a colour balance

The Red, Green and Blue Bias controls.
The Red, Green and Blue Bias controls.

The Red Bias Reduce, Green Bias Reduce and Blue Bias Reduce factors are the most important settings in the Color module. They directly determine the colour balance in your image. Their operation is intuitive; too much red in your image? Pump up the 'Red Bias Reduce' value. Too little red in your image? Reduce the 'Red Bias Reduce' value.

If you'd rather operate on these values in terms of Bias Increase, then simply switch the 'Bias Slider Mode' setting to 'Sliders Increase Color Bias'.

Switching between these two modes you can see that, for example, a RedBias Reduce of 8.00 is the same as a Green and Blue Bias Increase of 8.00. It makes intuitive sense when you think about it - a relative decrease of red makes blue and green more prevalent and vice versa.


Color balancing techniques

Now that we know how to change the colour balance, how do we know what to actually set it to?

There are a great number of tools and techniques that can be applied in StarTools that let you home in on a good colour balance. Before delving into them, It is highly recommended to switch 'Style' to 'Scientific (Color Constancy)' during color balancing, even if that is not the preferred style of rendering the colour of the end result, this is because the Color Constancy feature makes it much easier to colour balance by eye in some instances due to its ability to show continuous, constant colour throughout the image. Once a satisfactory colour balance is achieved, of course, feel free to switch to any alternative style of colour rendering.


White reference by clicking a pixel

If you know that a particular pixel or area in your image is supposed to be a shade of neutral white or gray, simply clicking on it is sufficient to let StarTools compute the right Red, Green and Blue bias settings to make that pixel appear neutral. This technique is particularly useful if you have a star of spectral type G2V (sun-like) in your image. The reasoning is that the sun is the perfect daylight white reference, and so any similar star elsewhere in the galaxy should be too.


White point reference by mask sampling

Some stars put in a mask.
We can calibrate against a big enough population of non-associated foreground stars, by putting them in a mask, clicking 'Sample' in the Color module and applying the found bias values to the whole image again.

Upon launch, or upon clicking the Sample button, the Color module samples whatever mask is set (note also that the set mask also ensures the Color module only applies any changes to the masked-in pixels!) and sets the Red, Green and Blue bias settings accordingly.

We can use this same behaviour to sample larger parts of the image that we know should be white. This method mostly exploits the fact that stars come in all sorts of sizes and temperatures (and thus colours!) and that this distribution is completely random. Therefore if we sample a large enough population, we should find the average star to be somewhere in the middle. Our sun is a very average star and is the white balance that we're after. Therefore, if we sample a large enough number of pixels containing a large enough number of stars, we should find a good colour balance.

A color balanced M101.
A reasonably good color balance achieved by putting all stars in a mask using the Auto feature and sampling them.

We can accomplish that in two ways; we either sample all stars (but only stars!) in a wide enough field, or we sample a whole galaxy that happens to be in the image (note that the galaxy must be of a certain type to be a good candidate and be reasonably close - preferably a barred spiral galaxy much like our own Milkyway).

Whichever you choose, we need to create a mask, so we launch the Mask editor. Here we can use the Auto feature to select a suitable selection of stars, or we can us the Flood Fill Brighter or Lassoo tool to select a galaxy. Once selected, return to the Color module and click Sample. StarTools will now determine the correct Red, Green and Blue bias to match the white reference pixels in the mask so that they come out neutral.

To apply the new colour balance to the whole image, launch the Mask editor once more and click Clear, then click Invert to select the whole image. Upon return to the Color module, the whole image will now be balanced by the Red, Green and Blue bias values we determined earlier with just the white reference pixels selected.


White balancing in MaxRGB mode

MaxRGB mode showing green channel dominance.
Major green channel dominance in the core points to color imbalance in that area.

StarTools comes with a unique colour balancing aid called MaxRGB. This mode of colour balancing is exceptionally useful if trying to colour balance by eye, but the user suffers from colour blindness or uses a screen that is not colour calibrated very well.

MaxRGB showing green dominance removed.
Reducing the green bias has removed green dominance in the core, leaving only spurious/random green dominance due to noise.

The MaxRGB aid allows you to view which channel is dominant per-pixel. If a pixel is mostly red, that pixel is shown red, if a pixel is mostly green, that pixel is shown green, and if a pixel is mostly blue, that pixel is shown blue.

Green corrected image.
Switching from MaxRGB mode to Normal mode confirms the image still looks good.

By cross referencing the normal image with the MaxRGB image, it is possible to find deficiencies in the colour balance. For example, the colour green is very rarely dominant in space (with the exception of highly dominant OIII emission areas in, for example the Trapezium in M42).

Therefore, if we see large areas of green, we know that we have too much green in our image and we should adjust the bias accordingly. Similarly if we have too much red or blue in our image, the MaxRGB mode will show many more red than blue pixels in areas that should show an even amount (for example the background). Again we then know we should adjust red or green accordingly.


White balancing by known features and processes

A color balanced M101.
M101 exhibiting a nice yellow core, bluer outer regions, red/brown dust lanes and purple HII knots, while the foreground stars show a good distribution of color temperatures from red to orange, yellow, white to blue.

StarTools' Color Constancy feature makes it much easier to see colours and spot processes, interactions, emissions and chemical composition in objects. In fact, the Color Constancy feature makes colouring comparable between different exposure lengths and different gear. This allows for the user to start spotting colours repeating in different features of comparable objects. Such features are, for example, the yellow cores of galaxies (due to the relative over representation of older stars as a result of gas depletion), the bluer outer rims of galaxies (due to the relative over representation of bright blue young stars as a result of the abundance of gas) and the pink/purplish HII area 'blobs' in their discs. Red/brown (white light filtered by dust) dust lanes complement a typical galaxy's rendering.

Similarly, HII areas in our own galaxy (e.g. most nebulae), while in StarTools Color Constancy Style mode, display the exact same colour signature found in the galaxies; a pink/purple as a result of predominantly deep red Hydrogen-alpha emissions mixed with much weaker blue/green
emissions of Hydrogen-beta and Oxygen-III emissions and (more dominantly) reflected blue star light from bright young blue giants who are often born in these areas, and shape the gas around them.

Dusty areas where the bright blue giants have 'boiled away' the Hydrogen through radiation pressure (for example the Pleiades) reflect the blue star light of any surviving stars, becoming distinctly blue reflection nebulae. Sometimes gradients can be spotted where (gas-rich) purple gives away to (gas-poor) blue (for example the Rosette core) as this process is caught in the act.

Diffraction spikes, while artefacts, also can be of great help when calibrating colours; the "rainbow" patterns (though skewed by the dominant colour of the star whose light is being diffracted) should show a nice continuum of colouring.

Finally, star temperatures, in a wide enough field, should be evenly distributed; the amount of red, orange, yellow, white and blue stars should be roughly equal. If any of these colors are missing or are over-represented we know the colour balance is off.


Colour balancing of data that was filtered by a light pollution filter

Colour balancing of data that was filtered by a light pollution filter is fundamentally impossible; narrow (or wider) bands of the spectrum are missing and no amount of colour balancing is going to bring them back and achieve proper colouring. A typical filtered data set will show a distinct lack in yellow and some green when properly colour balanced. It's by no means the end of the world - it's just something to be mindful of.

Correct colouring may be achieved however by shooting deep luminance data with light pollution filter in place, while shooting colour data without filter in place, after which both are processed separately and finally combined. Colour data is much more forgiving in terms of quality of signal and noise; the human eye is much more sensitive to noise in the luminance data that it is in the colour data. By making clever use of that fact and performing some trivial light pollution removal in Wipe, the best of both worlds can be achieved.


Tweaking your colors

A version of M101 with more colour in the shadows.
Increasing saturation makes colours more vivid, while increasing the Dark Saturation response parameter introduces more colour in the shadows.

Once you have achieved a color balance you are happy with, the StarTools Color module offers a great number of ways to change the presentation of your colours.

Style

The parameter with the biggest impact is the 'Style' parameter. StarTools is renowned for its Color Constancy feature, rendering colours in objects regardless of how the luminance data was stretched, the reasoning being that colours in outer space don't magically change depending on how we stretch our image. Other software sadly lets the user stretch the colour information along with the luminance information, warping, distorting and destroying hue and saturation in the process. The 'Scientific (Color Constancy)' setting for Style undoes these distortions using Tracking information, arriving at the colours as recorded.

To emulate the way other software renders colours, two other settings are available for the Style parameter. These settings are "Artistic, Detail Aware" and "Artistic, Not Detail Aware". The former still uses some Tracking information to better recover colours in areas whose dynamic range was optimised locally, while the latter does not compensate for any distortions whatsoever.

LRGB Method Emulation

The LRGB Method Emulation allows you to emulate a number of colour compositing methods that have been invented over the years. Even if you acquired data with an OSC or DSLR, you will still be able to use these compositing methods; the Color module will generate synthetic luminance from your RGB on the fly and re-composite the image in your desired compositing style.

The difference in colouring can be subtle or more pronounced. Much depends on the data and the method chosen.

'Straight CIELab Luminance Retention' manipulates all colours in a psychovisually optimal way in CIELab space, introducing colour without affecting apparent brightness.

M101 with desaturated core like other software renders it.
A more 'handicapped' way of showing colours is also available, emulating the way other software distorts and destroys hues and saturation along with stretching the luminance data.

'RGB Ratio, CIELab Luminance Retention' uses a method first proposed by Till Credner of the Max-Planck-Institut and subsequently rediscovered by Paul Kanevsky, using RGB ratios multiplied by luminance in order to better preserve star colour. Luminance retention in CIELab color space is applied afterwards.

'50/50 Layering, CIELab Luminance Retention' uses a method proposed by Robert Gendler, where luminance is layered on top of the colour information with a 50% opacity. Luminance retention in CIELab color space is applied afterwards. The inherent loss of 50% in saturation is compensated for, for your convenience, in order to allow for easier comparison with other methods.

'RGB Ratio' uses a method first proposed by Till Credner of the Max-Planck-Institut and subsequently rediscovered by Paul Kanevsky, using RGB ratios multiplied by luminance in order to better preserve star colour. No further luminance retention is attempted.

'50/50 Layering, CIELab Luminance Retention' uses a method proposed by Robert Gendler, where luminance is layered on top of the colour information with a 50% opacity. No further luminance retention is attempted. The inherent loss of 50% in saturation is compensated for, for your convenience, in order to allow for easier comparison with other methods.

Note that the LRGB Emulation Method feature is only available when Tracking is engaged.

Saturation

The 'Saturation' parameter allows colours to be rendered more, or less vividly, whereby Bright Saturation and Dark Saturation control how much colour and saturation is introduced in the highlights and shadows respectively. It is important to note that introducing colour in the shadows may exacerbate colour noise, though Tracking will make sure any such noise exacerbations are recorded and dealt with during the final denoising stage.

Cap Green

The 'Cap Green' parameter, finally, removes spurious green pixels if needed, reasoning that green dominant colours in outer space are rare and must therefore be caused by noise. Use of this feature should be considered a last resort if colour balancing does not yield adequate results and the green noise is severe. The final denoising stage should, thanks to Tracking data mining, pin pointed the green channel noise already and should be able to adequately mitigate it.


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